Add pragma: no cover to make coverage pass

[skip ci]

docs/en/docs/release-notes.md

@@ -7,6 +7,12 @@ hide:
 
 ## Latest Changes
 
+### Translations
+
+* 🔨 Update docs and translations scripts, enable Turkish. PR [#14824](https://github.com/fastapi/fastapi/pull/14824) by [@tiangolo](https://github.com/tiangolo).
+
+## 0.128.1
+
 ### Features
 
 * ✨ Add `viewport` meta tag to improve Swagger UI on mobile devices. PR [#14777](https://github.com/fastapi/fastapi/pull/14777) by [@Joab0](https://github.com/Joab0).
@@ -70,6 +76,7 @@ hide:
 
 ### Internal
 
+* ⬇️ Downgrade LLM translations model to GPT-5 to reduce mistakes. PR [#14823](https://github.com/fastapi/fastapi/pull/14823) by [@tiangolo](https://github.com/tiangolo).
 * 🐛 Fix translation script commit in place. PR [#14818](https://github.com/fastapi/fastapi/pull/14818) by [@tiangolo](https://github.com/tiangolo).
 * 🔨 Update translation script to retry if LLM-response doesn't pass validation with Translation Fixer tool. PR [#14749](https://github.com/fastapi/fastapi/pull/14749) by [@YuriiMotov](https://github.com/YuriiMotov).
 * 👷 Run tests only on relevant code changes (not on docs). PR [#14813](https://github.com/fastapi/fastapi/pull/14813) by [@tiangolo](https://github.com/tiangolo).

docs/en/mkdocs.yml

@@ -325,6 +325,8 @@ extra:
     name: pt - português
   - link: /ru/
     name: ru - русский язык
+  - link: /tr/
+    name: tr - Türkçe
   - link: /uk/
     name: uk - українська мова
 extra_css:

fastapi/__init__.py

@@ -1,6 +1,6 @@
 """FastAPI framework, high performance, easy to learn, fast to code, ready for production"""
 
-__version__ = "0.128.0"
+__version__ = "0.128.1"
 
 from starlette import status as status
 

scripts/docs.py

@@ -20,14 +20,18 @@ from slugify import slugify as py_slugify
 logging.basicConfig(level=logging.INFO)
 
 SUPPORTED_LANGS = {
-    "en",
     "de",
+    "en",
     "es",
+    # "fr",
     "ja",
     "ko",
     "pt",
     "ru",
+    "tr",
     "uk",
+    # "zh",
+    # "zh-hant",
 }
 
 

scripts/translate.py

@@ -86,7 +86,7 @@ def translate_page(
         print(f"Found existing translation: {out_path}")
         old_translation = out_path.read_text(encoding="utf-8")
     print(f"Translating {en_path} to {language} ({language_name})")
-    agent = Agent("openai:gpt-5.2")
+    agent = Agent("openai:gpt-5")
 
     prompt_segments = [
         general_prompt,

tests/test_request_params/test_cookie/test_nullable_and_defaults.py

@@ -0,0 +1,402 @@
+from typing import Annotated, Any, Union
+from unittest.mock import Mock, patch
+
+import pytest
+from dirty_equals import IsList, IsOneOf
+from fastapi import Cookie, FastAPI
+from fastapi.testclient import TestClient
+from pydantic import BaseModel, BeforeValidator, field_validator
+
+app = FastAPI()
+
+
+def convert(v: Any) -> Any:
+    return v
+
+
+# =====================================================================================
+# Nullable required
+
+
+@app.get("/nullable-required")
+async def read_nullable_required(
+    int_val: Annotated[
+        Union[int, None],
+        Cookie(),
+        BeforeValidator(lambda v: convert(v)),
+    ],
+    str_val: Annotated[
+        Union[str, None],
+        Cookie(),
+        BeforeValidator(lambda v: convert(v)),
+    ],
+):
+    return {
+        "int_val": int_val,
+        "str_val": str_val,
+        "fields_set": None,
+    }
+
+
+class ModelNullableRequired(BaseModel):
+    int_val: Union[int, None]
+    str_val: Union[str, None]
+
+    @field_validator("*", mode="before")
+    @classmethod
+    def convert_fields(cls, v):
+        return convert(v)
+
+
+@app.get("/model-nullable-required")
+async def read_model_nullable_required(
+    params: Annotated[ModelNullableRequired, Cookie()],
+):
+    return {
+        "int_val": params.int_val,
+        "str_val": params.str_val,
+        "fields_set": params.model_fields_set,
+    }
+
+
+@pytest.mark.parametrize(
+    "path",
+    [
+        "/nullable-required",
+        "/model-nullable-required",
+    ],
+)
+def test_nullable_required_schema(path: str):
+    assert app.openapi()["paths"][path]["get"]["parameters"] == [
+        {
+            "required": True,
+            "schema": {
+                "title": "Int Val",
+                "anyOf": [{"type": "integer"}, {"type": "null"}],
+            },
+            "name": "int_val",
+            "in": "cookie",
+        },
+        {
+            "required": True,
+            "schema": {
+                "title": "Str Val",
+                "anyOf": [{"type": "string"}, {"type": "null"}],
+            },
+            "name": "str_val",
+            "in": "cookie",
+        },
+    ]
+
+
+@pytest.mark.parametrize(
+    "path",
+    [
+        "/nullable-required",
+        "/model-nullable-required",
+    ],
+)
+def test_nullable_required_missing(path: str):
+    client = TestClient(app)
+    with patch(f"{__name__}.convert", Mock(wraps=convert)) as mock_convert:
+        response = client.get(path)
+
+    assert mock_convert.call_count == 0, (
+        "Validator should not be called if the value is missing"
+    )
+    assert response.status_code == 422
+    assert response.json() == {
+        "detail": [
+            {
+                "type": "missing",
+                "loc": ["cookie", "int_val"],
+                "msg": "Field required",
+                "input": IsOneOf(None, {}),
+            },
+            {
+                "type": "missing",
+                "loc": ["cookie", "str_val"],
+                "msg": "Field required",
+                "input": IsOneOf(None, {}),
+            },
+        ]
+    }
+
+
+@pytest.mark.parametrize(
+    "path",
+    [
+        "/nullable-required",
+        "/model-nullable-required",
+    ],
+)
+def test_nullable_required_pass_value(path: str):
+    client = TestClient(app)
+    client.cookies.set("int_val", "1")
+    client.cookies.set("str_val", "test")
+
+    with patch(f"{__name__}.convert", Mock(wraps=convert)) as mock_convert:
+        response = client.get(path)
+
+    assert mock_convert.call_count == 2, "Validator should be called for each field"
+    assert response.status_code == 200, response.text
+    assert response.json() == {
+        "int_val": 1,
+        "str_val": "test",
+        "fields_set": IsOneOf(None, IsList("int_val", "str_val", check_order=False)),
+    }
+
+
+# =====================================================================================
+# Nullable with default=None
+
+
+@app.get("/nullable-non-required")
+async def read_nullable_non_required(
+    int_val: Annotated[
+        Union[int, None],
+        Cookie(),
+        BeforeValidator(lambda v: convert(v)),
+    ] = None,
+    str_val: Annotated[
+        Union[str, None],
+        Cookie(),
+        BeforeValidator(lambda v: convert(v)),
+    ] = None,
+):
+    return {
+        "int_val": int_val,
+        "str_val": str_val,
+        "fields_set": None,
+    }
+
+
+class ModelNullableNonRequired(BaseModel):
+    int_val: Union[int, None] = None
+    str_val: Union[str, None] = None
+
+    @field_validator("*", mode="before")
+    @classmethod
+    def convert_fields(cls, v):
+        return convert(v)
+
+
+@app.get("/model-nullable-non-required")
+async def read_model_nullable_non_required(
+    params: Annotated[ModelNullableNonRequired, Cookie()],
+):
+    return {
+        "int_val": params.int_val,
+        "str_val": params.str_val,
+        "fields_set": params.model_fields_set,
+    }
+
+
+@pytest.mark.parametrize(
+    "path",
+    [
+        "/nullable-non-required",
+        "/model-nullable-non-required",
+    ],
+)
+def test_nullable_non_required_schema(path: str):
+    assert app.openapi()["paths"][path]["get"]["parameters"] == [
+        {
+            "required": False,
+            "schema": {
+                "title": "Int Val",
+                "anyOf": [{"type": "integer"}, {"type": "null"}],
+                # "default": None, # `None` values are omitted in OpenAPI schema
+            },
+            "name": "int_val",
+            "in": "cookie",
+        },
+        {
+            "required": False,
+            "schema": {
+                "title": "Str Val",
+                "anyOf": [{"type": "string"}, {"type": "null"}],
+                # "default": None, # `None` values are omitted in OpenAPI schema
+            },
+            "name": "str_val",
+            "in": "cookie",
+        },
+    ]
+
+
+@pytest.mark.parametrize(
+    "path",
+    [
+        "/nullable-non-required",
+        "/model-nullable-non-required",
+    ],
+)
+def test_nullable_non_required_missing(path: str):
+    client = TestClient(app)
+
+    with patch(f"{__name__}.convert", Mock(wraps=convert)) as mock_convert:
+        response = client.get(path)
+
+    assert mock_convert.call_count == 0, (
+        "Validator should not be called if the value is missing"
+    )
+    assert response.status_code == 200
+    assert response.json() == {
+        "int_val": None,
+        "str_val": None,
+        "fields_set": IsOneOf(None, []),
+    }
+
+
+@pytest.mark.parametrize(
+    "path",
+    [
+        "/nullable-non-required",
+        "/model-nullable-non-required",
+    ],
+)
+def test_nullable_non_required_pass_value(path: str):
+    client = TestClient(app)
+    client.cookies.set("int_val", "1")
+    client.cookies.set("str_val", "test")
+
+    with patch(f"{__name__}.convert", Mock(wraps=convert)) as mock_convert:
+        response = client.get(path)
+
+    assert mock_convert.call_count == 2, "Validator should be called for each field"
+    assert response.status_code == 200, response.text
+    assert response.json() == {
+        "int_val": 1,
+        "str_val": "test",
+        "fields_set": IsOneOf(None, IsList("int_val", "str_val", check_order=False)),
+    }
+
+
+# =====================================================================================
+# Nullable with not-None default
+
+
+@app.get("/nullable-with-non-null-default")
+async def read_nullable_with_non_null_default(
+    *,
+    int_val: Annotated[
+        Union[int, None],
+        Cookie(),
+        BeforeValidator(lambda v: convert(v)),
+    ] = -1,
+    str_val: Annotated[
+        Union[str, None],
+        Cookie(),
+        BeforeValidator(lambda v: convert(v)),
+    ] = "default",
+):
+    return {
+        "int_val": int_val,
+        "str_val": str_val,
+        "fields_set": None,
+    }
+
+
+class ModelNullableWithNonNullDefault(BaseModel):
+    int_val: Union[int, None] = -1
+    str_val: Union[str, None] = "default"
+
+    @field_validator("*", mode="before")
+    @classmethod
+    def convert_fields(cls, v):
+        return convert(v)
+
+
+@app.get("/model-nullable-with-non-null-default")
+async def read_model_nullable_with_non_null_default(
+    params: Annotated[ModelNullableWithNonNullDefault, Cookie()],
+):
+    return {
+        "int_val": params.int_val,
+        "str_val": params.str_val,
+        "fields_set": params.model_fields_set,
+    }
+
+
+@pytest.mark.parametrize(
+    "path",
+    [
+        "/nullable-with-non-null-default",
+        "/model-nullable-with-non-null-default",
+    ],
+)
+def test_nullable_with_non_null_default_schema(path: str):
+    assert app.openapi()["paths"][path]["get"]["parameters"] == [
+        {
+            "required": False,
+            "schema": {
+                "title": "Int Val",
+                "anyOf": [{"type": "integer"}, {"type": "null"}],
+                "default": -1,
+            },
+            "name": "int_val",
+            "in": "cookie",
+        },
+        {
+            "required": False,
+            "schema": {
+                "title": "Str Val",
+                "anyOf": [{"type": "string"}, {"type": "null"}],
+                "default": "default",
+            },
+            "name": "str_val",
+            "in": "cookie",
+        },
+    ]
+
+
+@pytest.mark.parametrize(
+    "path",
+    [
+        "/nullable-with-non-null-default",
+        "/model-nullable-with-non-null-default",
+    ],
+)
+@pytest.mark.xfail(
+    reason="Missing parameters are pre-populated with default values before validation"
+)
+def test_nullable_with_non_null_default_missing(path: str):
+    client = TestClient(app)
+
+    with patch(f"{__name__}.convert", Mock(wraps=convert)) as mock_convert:
+        response = client.get(path)
+
+    assert mock_convert.call_count == 0, (
+        "Validator should not be called if the value is missing"
+    )
+    assert response.status_code == 200  # pragma: no cover
+    assert response.json() == {  # pragma: no cover
+        "int_val": -1,
+        "str_val": "default",
+        "fields_set": IsOneOf(None, []),
+    }
+    # TODO: Remove 'no cover' when the issue is fixed
+
+
+@pytest.mark.parametrize(
+    "path",
+    [
+        "/nullable-with-non-null-default",
+        "/model-nullable-with-non-null-default",
+    ],
+)
+def test_nullable_with_non_null_default_pass_value(path: str):
+    client = TestClient(app)
+    client.cookies.set("int_val", "1")
+    client.cookies.set("str_val", "test")
+
+    with patch(f"{__name__}.convert", Mock(wraps=convert)) as mock_convert:
+        response = client.get(path)
+
+    assert mock_convert.call_count == 2, "Validator should be called for each field"
+    assert response.status_code == 200, response.text
+    assert response.json() == {
+        "int_val": 1,
+        "str_val": "test",
+        "fields_set": IsOneOf(None, IsList("int_val", "str_val", check_order=False)),
+    }

tests/test_request_params/test_file/test_nullable_and_defaults.py

@@ -0,0 +1,480 @@
+from typing import Annotated, Any, Union
+from unittest.mock import Mock, patch
+
+import pytest
+from dirty_equals import IsOneOf
+from fastapi import FastAPI, File, UploadFile
+from fastapi.testclient import TestClient
+from pydantic import BeforeValidator
+from starlette.datastructures import UploadFile as StarletteUploadFile
+
+from .utils import get_body_model_name
+
+app = FastAPI()
+
+
+def convert(v: Any) -> Any:
+    return v
+
+
+# =====================================================================================
+# Nullable required
+
+
+@app.post("/nullable-required-bytes")
+async def read_nullable_required_bytes(
+    file: Annotated[
+        Union[bytes, None],
+        File(),
+        BeforeValidator(lambda v: convert(v)),
+    ],
+    files: Annotated[
+        Union[list[bytes], None],
+        File(),
+        BeforeValidator(lambda v: convert(v)),
+    ],
+):
+    return {
+        "file": len(file) if file is not None else None,
+        "files": [len(f) for f in files] if files is not None else None,
+    }
+
+
+@app.post("/nullable-required-uploadfile")
+async def read_nullable_required_uploadfile(
+    file: Annotated[
+        Union[UploadFile, None],
+        File(),
+        BeforeValidator(lambda v: convert(v)),
+    ],
+    files: Annotated[
+        Union[list[UploadFile], None],
+        File(),
+        BeforeValidator(lambda v: convert(v)),
+    ],
+):
+    return {
+        "file": file.size if file is not None else None,
+        "files": [f.size for f in files] if files is not None else None,
+    }
+
+
+@pytest.mark.parametrize(
+    "path",
+    [
+        "/nullable-required-bytes",
+        "/nullable-required-uploadfile",
+    ],
+)
+def test_nullable_required_schema(path: str):
+    openapi = app.openapi()
+    body_model_name = get_body_model_name(openapi, path)
+
+    assert app.openapi()["components"]["schemas"][body_model_name] == {
+        "properties": {
+            "file": {
+                "title": "File",
+                "anyOf": [{"type": "string", "format": "binary"}, {"type": "null"}],
+            },
+            "files": {
+                "title": "Files",
+                "anyOf": [
+                    {"type": "array", "items": {"type": "string", "format": "binary"}},
+                    {"type": "null"},
+                ],
+            },
+        },
+        "required": ["file", "files"],
+        "title": body_model_name,
+        "type": "object",
+    }
+
+
+@pytest.mark.parametrize(
+    "path",
+    [
+        "/nullable-required-bytes",
+        "/nullable-required-uploadfile",
+    ],
+)
+def test_nullable_required_missing(path: str):
+    client = TestClient(app)
+
+    with patch(f"{__name__}.convert", Mock(wraps=convert)) as mock_convert:
+        response = client.post(path)
+
+    assert mock_convert.call_count == 0, (
+        "Validator should not be called if the value is missing"
+    )
+    assert response.status_code == 422
+    assert response.json() == {
+        "detail": [
+            {
+                "type": "missing",
+                "loc": ["body", "file"],
+                "msg": "Field required",
+                "input": IsOneOf(None, {}),
+            },
+            {
+                "type": "missing",
+                "loc": ["body", "files"],
+                "msg": "Field required",
+                "input": IsOneOf(None, {}),
+            },
+        ]
+    }
+
+
+@pytest.mark.parametrize(
+    "path",
+    [
+        "/nullable-required-bytes",
+        "/nullable-required-uploadfile",
+    ],
+)
+def test_nullable_required_pass_empty_file(path: str):
+    client = TestClient(app)
+
+    with patch(f"{__name__}.convert", Mock(wraps=convert)) as mock_convert:
+        response = client.post(
+            path,
+            files=[("file", b""), ("files", b""), ("files", b"")],
+        )
+
+    assert mock_convert.call_count == 2, "Validator should be called for each field"
+    call_args = [call_args_item.args for call_args_item in mock_convert.call_args_list]
+    file_call_arg_1 = call_args[0][0]
+    files_call_arg_1 = call_args[1][0]
+
+    assert (
+        (file_call_arg_1 == b"")  # file as bytes
+        or isinstance(file_call_arg_1, StarletteUploadFile)  # file as UploadFile
+    )
+    assert (
+        (files_call_arg_1 == [b"", b""])  # files as bytes
+        or all(  # files as UploadFile
+            isinstance(f, StarletteUploadFile) for f in files_call_arg_1
+        )
+    )
+
+    assert response.status_code == 200, response.text
+    assert response.json() == {
+        "file": 0,
+        "files": [0, 0],
+    }
+
+
+@pytest.mark.parametrize(
+    "path",
+    [
+        "/nullable-required-bytes",
+        "/nullable-required-uploadfile",
+    ],
+)
+def test_nullable_required_pass_file(path: str):
+    client = TestClient(app)
+
+    with patch(f"{__name__}.convert", Mock(wraps=convert)) as mock_convert:
+        response = client.post(
+            path,
+            files=[
+                ("file", b"test 1"),
+                ("files", b"test 2"),
+                ("files", b"test 3"),
+            ],
+        )
+
+    assert mock_convert.call_count == 2, "Validator should be called for each field"
+    assert response.status_code == 200, response.text
+    assert response.json() == {"file": 6, "files": [6, 6]}
+
+
+# =====================================================================================
+# Nullable with default=None
+
+
+@app.post("/nullable-non-required-bytes")
+async def read_nullable_non_required_bytes(
+    file: Annotated[
+        Union[bytes, None],
+        File(),
+        BeforeValidator(lambda v: convert(v)),
+    ] = None,
+    files: Annotated[
+        Union[list[bytes], None],
+        File(),
+        BeforeValidator(lambda v: convert(v)),
+    ] = None,
+):
+    return {
+        "file": len(file) if file is not None else None,
+        "files": [len(f) for f in files] if files is not None else None,
+    }
+
+
+@app.post("/nullable-non-required-uploadfile")
+async def read_nullable_non_required_uploadfile(
+    file: Annotated[
+        Union[UploadFile, None],
+        File(),
+        BeforeValidator(lambda v: convert(v)),
+    ] = None,
+    files: Annotated[
+        Union[list[UploadFile], None],
+        File(),
+        BeforeValidator(lambda v: convert(v)),
+    ] = None,
+):
+    return {
+        "file": file.size if file is not None else None,
+        "files": [f.size for f in files] if files is not None else None,
+    }
+
+
+@pytest.mark.parametrize(
+    "path",
+    [
+        "/nullable-non-required-bytes",
+        "/nullable-non-required-uploadfile",
+    ],
+)
+def test_nullable_non_required_schema(path: str):
+    openapi = app.openapi()
+    body_model_name = get_body_model_name(openapi, path)
+
+    assert app.openapi()["components"]["schemas"][body_model_name] == {
+        "properties": {
+            "file": {
+                "title": "File",
+                "anyOf": [{"type": "string", "format": "binary"}, {"type": "null"}],
+                # "default": None, # `None` values are omitted in OpenAPI schema
+            },
+            "files": {
+                "title": "Files",
+                "anyOf": [
+                    {"type": "array", "items": {"type": "string", "format": "binary"}},
+                    {"type": "null"},
+                ],
+                # "default": None, # `None` values are omitted in OpenAPI schema
+            },
+        },
+        "title": body_model_name,
+        "type": "object",
+    }
+
+
+@pytest.mark.parametrize(
+    "path",
+    [
+        "/nullable-non-required-bytes",
+        "/nullable-non-required-uploadfile",
+    ],
+)
+def test_nullable_non_required_missing(path: str):
+    client = TestClient(app)
+
+    with patch(f"{__name__}.convert", Mock(wraps=convert)) as mock_convert:
+        response = client.post(path)
+
+    assert mock_convert.call_count == 0, (
+        "Validator should not be called if the value is missing"
+    )
+    assert response.status_code == 200
+    assert response.json() == {
+        "file": None,
+        "files": None,
+    }
+
+
+@pytest.mark.parametrize(
+    "path",
+    [
+        "/nullable-non-required-bytes",
+        "/nullable-non-required-uploadfile",
+    ],
+)
+def test_nullable_non_required_pass_empty_file(path: str):
+    client = TestClient(app)
+
+    with patch(f"{__name__}.convert", Mock(wraps=convert)) as mock_convert:
+        response = client.post(
+            path,
+            files=[("file", b""), ("files", b""), ("files", b"")],
+        )
+
+    assert mock_convert.call_count == 2, "Validator should be called for each field"
+    call_args = [call_args_item.args for call_args_item in mock_convert.call_args_list]
+    file_call_arg_1 = call_args[0][0]
+    files_call_arg_1 = call_args[1][0]
+
+    assert (
+        (file_call_arg_1 == b"")  # file as bytes
+        or isinstance(file_call_arg_1, StarletteUploadFile)  # file as UploadFile
+    )
+    assert (
+        (files_call_arg_1 == [b"", b""])  # files as bytes
+        or all(  # files as UploadFile
+            isinstance(f, StarletteUploadFile) for f in files_call_arg_1
+        )
+    )
+
+    assert response.status_code == 200, response.text
+    assert response.json() == {"file": 0, "files": [0, 0]}
+
+
+@pytest.mark.parametrize(
+    "path",
+    [
+        "/nullable-non-required-bytes",
+        "/nullable-non-required-uploadfile",
+    ],
+)
+def test_nullable_non_required_pass_file(path: str):
+    client = TestClient(app)
+
+    with patch(f"{__name__}.convert", Mock(wraps=convert)) as mock_convert:
+        response = client.post(
+            path,
+            files=[("file", b"test 1"), ("files", b"test 2"), ("files", b"test 3")],
+        )
+
+    assert mock_convert.call_count == 2, "Validator should be called for each field"
+    assert response.status_code == 200, response.text
+    assert response.json() == {"file": 6, "files": [6, 6]}
+
+
+# =====================================================================================
+# Nullable with not-None default
+
+
+@app.post("/nullable-with-non-null-default-bytes")
+async def read_nullable_with_non_null_default_bytes(
+    *,
+    file: Annotated[
+        Union[bytes, None],
+        File(),
+        BeforeValidator(lambda v: convert(v)),
+    ] = b"default",
+    files: Annotated[
+        Union[list[bytes], None],
+        File(default_factory=lambda: [b"default"]),
+        BeforeValidator(lambda v: convert(v)),
+    ],
+):
+    return {
+        "file": len(file) if file is not None else None,
+        "files": [len(f) for f in files] if files is not None else None,
+    }
+
+
+# Note: It seems to be not possible to create endpoint with UploadFile and non-None default
+
+
+@pytest.mark.parametrize(
+    "path",
+    [
+        "/nullable-with-non-null-default-bytes",
+    ],
+)
+def test_nullable_with_non_null_default_schema(path: str):
+    openapi = app.openapi()
+    body_model_name = get_body_model_name(openapi, path)
+
+    assert app.openapi()["components"]["schemas"][body_model_name] == {
+        "properties": {
+            "file": {
+                "title": "File",
+                "anyOf": [{"type": "string", "format": "binary"}, {"type": "null"}],
+                "default": "default",  # <= Default value for file looks strange to me
+            },
+            "files": {
+                "title": "Files",
+                "anyOf": [
+                    {"type": "array", "items": {"type": "string", "format": "binary"}},
+                    {"type": "null"},
+                ],
+            },
+        },
+        "title": body_model_name,
+        "type": "object",
+    }
+
+
+@pytest.mark.parametrize(
+    "path",
+    [
+        pytest.param(
+            "/nullable-with-non-null-default-bytes",
+            marks=pytest.mark.xfail(
+                reason="AttributeError: 'bytes' object has no attribute 'read'",
+            ),
+        ),
+    ],
+)
+def test_nullable_with_non_null_default_missing(path: str):
+    client = TestClient(app)
+
+    with patch(f"{__name__}.convert", Mock(wraps=convert)) as mock_convert:
+        response = client.post(path)
+
+    assert mock_convert.call_count == 0, (  # pragma: no cover
+        "Validator should not be called if the value is missing"
+    )
+    assert response.status_code == 200  # pragma: no cover
+    assert response.json() == {"file": None, "files": None}  # pragma: no cover
+    # TODO: Remove 'no cover' when the issue is fixed
+
+
+@pytest.mark.parametrize(
+    "path",
+    [
+        "/nullable-with-non-null-default-bytes",
+    ],
+)
+def test_nullable_with_non_null_default_pass_empty_file(path: str):
+    client = TestClient(app)
+
+    with patch(f"{__name__}.convert", Mock(wraps=convert)) as mock_convert:
+        response = client.post(
+            path,
+            files=[("file", b""), ("files", b""), ("files", b"")],
+        )
+
+    assert mock_convert.call_count == 2, "Validator should be called for each field"
+    call_args = [call_args_item.args for call_args_item in mock_convert.call_args_list]
+    file_call_arg_1 = call_args[0][0]
+    files_call_arg_1 = call_args[1][0]
+
+    assert (
+        (file_call_arg_1 == b"")  # file as bytes
+        or isinstance(file_call_arg_1, StarletteUploadFile)  # file as UploadFile
+    )
+    assert (
+        (files_call_arg_1 == [b"", b""])  # files as bytes
+        or all(  # files as UploadFile
+            isinstance(f, StarletteUploadFile) for f in files_call_arg_1
+        )
+    )
+
+    assert response.status_code == 200, response.text
+    assert response.json() == {"file": 0, "files": [0, 0]}
+
+
+@pytest.mark.parametrize(
+    "path",
+    [
+        "/nullable-with-non-null-default-bytes",
+    ],
+)
+def test_nullable_with_non_null_default_pass_file(path: str):
+    client = TestClient(app)
+
+    with patch(f"{__name__}.convert", Mock(wraps=convert)) as mock_convert:
+        response = client.post(
+            path,
+            files=[("file", b"test 1"), ("files", b"test 2"), ("files", b"test 3")],
+        )
+
+    assert mock_convert.call_count == 2, "Validator should be called for each field"
+    assert response.status_code == 200, response.text
+    assert response.json() == {"file": 6, "files": [6, 6]}

tests/test_request_params/test_form/test_nullable_and_defaults.py

@@ -0,0 +1,609 @@
+from typing import Annotated, Any, Union
+from unittest.mock import Mock, patch
+
+import pytest
+from dirty_equals import IsList, IsOneOf, IsPartialDict
+from fastapi import FastAPI, Form
+from fastapi.testclient import TestClient
+from pydantic import BaseModel, BeforeValidator, field_validator
+
+from .utils import get_body_model_name
+
+app = FastAPI()
+
+
+def convert(v: Any) -> Any:
+    return v
+
+
+# =====================================================================================
+# Nullable required
+
+
+@app.post("/nullable-required")
+async def read_nullable_required(
+    int_val: Annotated[
+        Union[int, None],
+        Form(),
+        BeforeValidator(lambda v: convert(v)),
+    ],
+    str_val: Annotated[
+        Union[str, None],
+        Form(),
+        BeforeValidator(lambda v: convert(v)),
+    ],
+    list_val: Annotated[
+        Union[list[int], None],
+        Form(),
+        BeforeValidator(lambda v: convert(v)),
+    ],
+):
+    return {
+        "int_val": int_val,
+        "str_val": str_val,
+        "list_val": list_val,
+        "fields_set": None,
+    }
+
+
+class ModelNullableRequired(BaseModel):
+    int_val: Union[int, None]
+    str_val: Union[str, None]
+    list_val: Union[list[int], None]
+
+    @field_validator("*", mode="before")
+    def convert_fields(cls, v):
+        return convert(v)
+
+
+@app.post("/model-nullable-required")
+async def read_model_nullable_required(
+    params: Annotated[ModelNullableRequired, Form()],
+):
+    return {
+        "int_val": params.int_val,
+        "str_val": params.str_val,
+        "list_val": params.list_val,
+        "fields_set": params.model_fields_set,
+    }
+
+
+@pytest.mark.parametrize(
+    "path",
+    [
+        "/nullable-required",
+        "/model-nullable-required",
+    ],
+)
+def test_nullable_required_schema(path: str):
+    openapi = app.openapi()
+    body_model_name = get_body_model_name(openapi, path)
+
+    assert app.openapi()["components"]["schemas"][body_model_name] == {
+        "properties": {
+            "int_val": {
+                "title": "Int Val",
+                "anyOf": [{"type": "integer"}, {"type": "null"}],
+            },
+            "str_val": {
+                "title": "Str Val",
+                "anyOf": [{"type": "string"}, {"type": "null"}],
+            },
+            "list_val": {
+                "title": "List Val",
+                "anyOf": [
+                    {"type": "array", "items": {"type": "integer"}},
+                    {"type": "null"},
+                ],
+            },
+        },
+        "required": ["int_val", "str_val", "list_val"],
+        "title": body_model_name,
+        "type": "object",
+    }
+
+
+@pytest.mark.parametrize(
+    "path",
+    [
+        "/nullable-required",
+        "/model-nullable-required",
+    ],
+)
+def test_nullable_required_missing(path: str):
+    client = TestClient(app)
+
+    with patch(f"{__name__}.convert", Mock(wraps=convert)) as mock_convert:
+        response = client.post(path)
+
+    assert mock_convert.call_count == 0, (
+        "Validator should not be called if the value is missing"
+    )
+    assert response.status_code == 422
+    assert response.json() == {
+        "detail": [
+            {
+                "type": "missing",
+                "loc": ["body", "int_val"],
+                "msg": "Field required",
+                "input": IsOneOf(None, {}),
+            },
+            {
+                "type": "missing",
+                "loc": ["body", "str_val"],
+                "msg": "Field required",
+                "input": IsOneOf(None, {}),
+            },
+            {
+                "type": "missing",
+                "loc": ["body", "list_val"],
+                "msg": "Field required",
+                "input": IsOneOf(None, {}),
+            },
+        ]
+    }
+
+
+@pytest.mark.parametrize(
+    "path",
+    [
+        pytest.param(
+            "/nullable-required",
+            marks=pytest.mark.xfail(
+                reason="Empty str is replaced with None, but then None gets dropped"
+            ),
+        ),
+        pytest.param(
+            "/model-nullable-required",
+            marks=pytest.mark.xfail(
+                reason="Empty strings are not replaced with None for models"
+            ),
+        ),
+    ],
+)
+def test_nullable_required_pass_empty_str(path: str):
+    client = TestClient(app)
+
+    with patch(f"{__name__}.convert", Mock(wraps=convert)) as mock_convert:
+        response = client.post(
+            path,
+            data={
+                "int_val": "",
+                "str_val": "",
+                "list_val": "0",  # Empty strings are not treated as null for lists. It's Ok
+            },
+        )
+
+    assert mock_convert.call_count == 3, "Validator should be called for each field"
+    assert mock_convert.call_args_list == [
+        (""),  # int_val
+        (""),  # str_val
+        (["0"]),  # list_val
+    ]
+    assert response.status_code == 200, response.text  # pragma: no cover
+    assert response.json() == {  # pragma: no cover
+        "int_val": None,
+        "str_val": None,
+        "list_val": [0],
+        "fields_set": IsOneOf(
+            None, IsList("int_val", "str_val", "list_val", check_order=False)
+        ),
+    }
+    # TODO: Remove 'no cover' when the issue is fixed
+
+
+@pytest.mark.parametrize(
+    "path",
+    [
+        "/nullable-required",
+        "/model-nullable-required",
+    ],
+)
+def test_nullable_required_pass_value(path: str):
+    client = TestClient(app)
+
+    with patch(f"{__name__}.convert", Mock(wraps=convert)) as mock_convert:
+        response = client.post(
+            path, data={"int_val": "1", "str_val": "test", "list_val": ["1", "2"]}
+        )
+
+    assert mock_convert.call_count == 3, "Validator should be called for each field"
+    assert response.status_code == 200, response.text
+    assert response.json() == {
+        "int_val": 1,
+        "str_val": "test",
+        "list_val": [1, 2],
+        "fields_set": IsOneOf(
+            None, IsList("int_val", "str_val", "list_val", check_order=False)
+        ),
+    }
+
+
+# =====================================================================================
+# Nullable with default=None
+
+
+@app.post("/nullable-non-required")
+async def read_nullable_non_required(
+    int_val: Annotated[
+        Union[int, None],
+        Form(),
+        BeforeValidator(lambda v: convert(v)),
+    ] = None,
+    str_val: Annotated[
+        Union[str, None],
+        Form(),
+        BeforeValidator(lambda v: convert(v)),
+    ] = None,
+    list_val: Annotated[
+        Union[list[int], None],
+        Form(),
+        BeforeValidator(lambda v: convert(v)),
+    ] = None,
+):
+    return {
+        "int_val": int_val,
+        "str_val": str_val,
+        "list_val": list_val,
+        "fields_set": None,
+    }
+
+
+class ModelNullableNonRequired(BaseModel):
+    int_val: Union[int, None] = None
+    str_val: Union[str, None] = None
+    list_val: Union[list[int], None] = None
+
+    @field_validator("*", mode="before")
+    def convert_fields(cls, v):
+        return convert(v)
+
+
+@app.post("/model-nullable-non-required")
+async def read_model_nullable_non_required(
+    params: Annotated[ModelNullableNonRequired, Form()],
+):
+    return {
+        "int_val": params.int_val,
+        "str_val": params.str_val,
+        "list_val": params.list_val,
+        "fields_set": params.model_fields_set,
+    }
+
+
+@pytest.mark.parametrize(
+    "path",
+    [
+        "/nullable-non-required",
+        "/model-nullable-non-required",
+    ],
+)
+def test_nullable_non_required_schema(path: str):
+    openapi = app.openapi()
+    body_model_name = get_body_model_name(openapi, path)
+
+    assert app.openapi()["components"]["schemas"][body_model_name] == {
+        "properties": {
+            "int_val": {
+                "title": "Int Val",
+                "anyOf": [{"type": "integer"}, {"type": "null"}],
+                # "default": None, # `None` values are omitted in OpenAPI schema
+            },
+            "str_val": {
+                "title": "Str Val",
+                "anyOf": [{"type": "string"}, {"type": "null"}],
+                # "default": None, # `None` values are omitted in OpenAPI schema
+            },
+            "list_val": {
+                "title": "List Val",
+                "anyOf": [
+                    {"type": "array", "items": {"type": "integer"}},
+                    {"type": "null"},
+                ],
+                # "default": None, # `None` values are omitted in OpenAPI schema
+            },
+        },
+        "title": body_model_name,
+        "type": "object",
+    }
+
+
+@pytest.mark.parametrize(
+    "path",
+    [
+        "/nullable-non-required",
+        "/model-nullable-non-required",
+    ],
+)
+def test_nullable_non_required_missing(path: str):
+    client = TestClient(app)
+
+    with patch(f"{__name__}.convert", Mock(wraps=convert)) as mock_convert:
+        response = client.post(path)
+
+    assert mock_convert.call_count == 0, (
+        "Validator should not be called if the value is missing"
+    )
+    assert response.status_code == 200
+    assert response.json() == {
+        "int_val": None,
+        "str_val": None,
+        "list_val": None,
+        "fields_set": IsOneOf(None, []),
+    }
+
+
+@pytest.mark.parametrize(
+    "path",
+    [
+        pytest.param(
+            "/nullable-non-required",
+            marks=pytest.mark.xfail(
+                reason="Empty str is replaced with None, but then None gets dropped"
+            ),
+        ),
+        pytest.param(
+            "/model-nullable-non-required",
+            marks=pytest.mark.xfail(
+                reason="Empty strings are not replaced with None for models"
+            ),
+        ),
+    ],
+)
+def test_nullable_non_required_pass_empty_str(path: str):
+    client = TestClient(app)
+
+    with patch(f"{__name__}.convert", Mock(wraps=convert)) as mock_convert:
+        response = client.post(
+            path,
+            data={
+                "int_val": "",
+                "str_val": "",
+                "list_val": "0",  # Empty strings are not treated as null for lists. It's Ok
+            },
+        )
+
+    assert mock_convert.call_count == 3, "Validator should be called for each field"
+    assert mock_convert.call_args_list == [
+        (""),  # int_val
+        (""),  # str_val
+        (["0"]),  # list_val
+    ]
+    assert response.status_code == 200, response.text  # pragma: no cover
+    assert response.json() == {  # pragma: no cover
+        "int_val": None,
+        "str_val": None,
+        "list_val": [0],
+        "fields_set": IsOneOf(
+            None, IsList("int_val", "str_val", "list_val", check_order=False)
+        ),
+    }
+    # TODO: Remove 'no cover' when the issue is fixed
+
+
+@pytest.mark.parametrize(
+    "path",
+    [
+        "/nullable-non-required",
+        "/model-nullable-non-required",
+    ],
+)
+def test_nullable_non_required_pass_value(path: str):
+    client = TestClient(app)
+
+    with patch(f"{__name__}.convert", Mock(wraps=convert)) as mock_convert:
+        response = client.post(
+            path, data={"int_val": "1", "str_val": "test", "list_val": ["1", "2"]}
+        )
+
+    assert mock_convert.call_count == 3, "Validator should be called for each field"
+    assert response.status_code == 200, response.text
+    assert response.json() == {
+        "int_val": 1,
+        "str_val": "test",
+        "list_val": [1, 2],
+        "fields_set": IsOneOf(
+            None, IsList("int_val", "str_val", "list_val", check_order=False)
+        ),
+    }
+
+
+# =====================================================================================
+# Nullable with not-None default
+
+
+@app.post("/nullable-with-non-null-default")
+async def read_nullable_with_non_null_default(
+    *,
+    int_val: Annotated[
+        Union[int, None],
+        Form(),
+        BeforeValidator(lambda v: convert(v)),
+    ] = -1,
+    str_val: Annotated[
+        Union[str, None],
+        Form(),
+        BeforeValidator(lambda v: convert(v)),
+    ] = "default",
+    list_val: Annotated[
+        Union[list[int], None],
+        Form(default_factory=lambda: [0]),
+        BeforeValidator(lambda v: convert(v)),
+    ],
+):
+    return {
+        "int_val": int_val,
+        "str_val": str_val,
+        "list_val": list_val,
+        "fields_set": None,
+    }
+
+
+class ModelNullableWithNonNullDefault(BaseModel):
+    int_val: Union[int, None] = -1
+    str_val: Union[str, None] = "default"
+    list_val: Union[list[int], None] = [0]
+
+    @field_validator("*", mode="before")
+    def convert_fields(cls, v):
+        return convert(v)
+
+
+@app.post("/model-nullable-with-non-null-default")
+async def read_model_nullable_with_non_null_default(
+    params: Annotated[ModelNullableWithNonNullDefault, Form()],
+):
+    return {
+        "int_val": params.int_val,
+        "str_val": params.str_val,
+        "list_val": params.list_val,
+        "fields_set": params.model_fields_set,
+    }
+
+
+@pytest.mark.parametrize(
+    "path",
+    [
+        "/nullable-with-non-null-default",
+        "/model-nullable-with-non-null-default",
+    ],
+)
+def test_nullable_with_non_null_default_schema(path: str):
+    openapi = app.openapi()
+    body_model_name = get_body_model_name(openapi, path)
+    body_model = app.openapi()["components"]["schemas"][body_model_name]
+
+    assert body_model == {
+        "properties": {
+            "int_val": {
+                "title": "Int Val",
+                "anyOf": [{"type": "integer"}, {"type": "null"}],
+                "default": -1,
+            },
+            "str_val": {
+                "title": "Str Val",
+                "anyOf": [{"type": "string"}, {"type": "null"}],
+                "default": "default",
+            },
+            "list_val": IsPartialDict(
+                {
+                    "title": "List Val",
+                    "anyOf": [
+                        {"type": "array", "items": {"type": "integer"}},
+                        {"type": "null"},
+                    ],
+                }
+            ),
+        },
+        "title": body_model_name,
+        "type": "object",
+    }
+
+    if path == "/model-nullable-with-non-null-default":
+        # Check default value for list_val param for model-based Body parameters only.
+        # default_factory is not reflected in OpenAPI schema
+        assert body_model["properties"]["list_val"]["default"] == [0]
+
+
+@pytest.mark.parametrize(
+    "path",
+    [
+        "/nullable-with-non-null-default",
+        "/model-nullable-with-non-null-default",
+    ],
+)
+@pytest.mark.xfail(
+    reason="Missing parameters are pre-populated with default values before validation"
+)
+def test_nullable_with_non_null_default_missing(path: str):
+    client = TestClient(app)
+
+    with patch(f"{__name__}.convert", Mock(wraps=convert)) as mock_convert:
+        response = client.post(path)
+
+    assert mock_convert.call_count == 0, (
+        "Validator should not be called if the value is missing"
+    )
+    assert response.status_code == 200  # pragma: no cover
+    assert response.json() == {  # pragma: no cover
+        "int_val": -1,
+        "str_val": "default",
+        "list_val": [0],
+        "fields_set": IsOneOf(None, []),
+    }
+    # TODO: Remove 'no cover' when the issue is fixed
+
+
+@pytest.mark.parametrize(
+    "path",
+    [
+        pytest.param(
+            "/nullable-with-non-null-default",
+            marks=pytest.mark.xfail(
+                reason="Empty str is replaced with default value, not with None"  # Is this correct ???
+            ),
+        ),
+        pytest.param(
+            "/model-nullable-with-non-null-default",
+            marks=pytest.mark.xfail(
+                reason="Empty strings are not replaced with None for models"
+            ),
+        ),
+    ],
+)
+def test_nullable_with_non_null_default_pass_empty_str(path: str):
+    client = TestClient(app)
+
+    with patch(f"{__name__}.convert", Mock(wraps=convert)) as mock_convert:
+        response = client.post(
+            path,
+            data={
+                "int_val": "",
+                "str_val": "",
+                "list_val": "0",  # Empty strings are not treated as null for lists. It's Ok
+            },
+        )
+
+    assert mock_convert.call_count == 3, "Validator should be called for each field"
+    assert mock_convert.call_args_list == [
+        (""),  # int_val
+        (""),  # str_val
+        (["0"]),  # list_val
+    ]
+    assert response.status_code == 200, response.text  # pragma: no cover
+    assert response.json() == {  # pragma: no cover
+        "int_val": None,
+        "str_val": None,
+        "list_val": [0],
+        "fields_set": IsOneOf(
+            None, IsList("int_val", "str_val", "list_val", check_order=False)
+        ),
+    }
+    # TODO: Remove 'no cover' when the issue is fixed
+
+
+@pytest.mark.parametrize(
+    "path",
+    [
+        "/nullable-with-non-null-default",
+        "/model-nullable-with-non-null-default",
+    ],
+)
+def test_nullable_with_non_null_default_pass_value(path: str):
+    client = TestClient(app)
+
+    with patch(f"{__name__}.convert", Mock(wraps=convert)) as mock_convert:
+        response = client.post(
+            path, data={"int_val": "1", "str_val": "test", "list_val": ["1", "2"]}
+        )
+
+    assert mock_convert.call_count == 3, "Validator should be called for each field"
+    assert response.status_code == 200, response.text
+    assert response.json() == {
+        "int_val": 1,
+        "str_val": "test",
+        "list_val": [1, 2],
+        "fields_set": IsOneOf(
+            None, IsList("int_val", "str_val", "list_val", check_order=False)
+        ),
+    }

tests/test_request_params/test_header/test_nullable_and_defaults.py

@@ -0,0 +1,526 @@
+from typing import Annotated, Any, Union
+from unittest.mock import Mock, patch
+
+import pytest
+from dirty_equals import AnyThing, IsList, IsOneOf, IsPartialDict
+from fastapi import FastAPI, Header
+from fastapi.testclient import TestClient
+from pydantic import BaseModel, BeforeValidator, field_validator
+
+app = FastAPI()
+
+
+def convert(v: Any) -> Any:
+    return v
+
+
+# =====================================================================================
+# Nullable required
+
+
+@app.get("/nullable-required")
+async def read_nullable_required(
+    int_val: Annotated[
+        Union[int, None],
+        Header(),
+        BeforeValidator(lambda v: convert(v)),
+    ],
+    str_val: Annotated[
+        Union[str, None],
+        Header(),
+        BeforeValidator(lambda v: convert(v)),
+    ],
+    list_val: Annotated[
+        Union[list[int], None],
+        Header(),
+        BeforeValidator(lambda v: convert(v)),
+    ],
+):
+    return {
+        "int_val": int_val,
+        "str_val": str_val,
+        "list_val": list_val,
+        "fields_set": None,
+    }
+
+
+class ModelNullableRequired(BaseModel):
+    int_val: Union[int, None]
+    str_val: Union[str, None]
+    list_val: Union[list[int], None]
+
+    @field_validator("*", mode="before")
+    @classmethod
+    def convert_fields(cls, v):
+        return convert(v)
+
+
+@app.get("/model-nullable-required")
+async def read_model_nullable_required(
+    params: Annotated[ModelNullableRequired, Header()],
+):
+    return {
+        "int_val": params.int_val,
+        "str_val": params.str_val,
+        "list_val": params.list_val,
+        "fields_set": params.model_fields_set,
+    }
+
+
+@pytest.mark.parametrize(
+    "path",
+    [
+        pytest.param(
+            "/nullable-required",
+            marks=pytest.mark.xfail(
+                reason="Title contains hyphens for single Header parameters"
+            ),
+        ),
+        "/model-nullable-required",
+    ],
+)
+def test_nullable_required_schema(path: str):
+    assert app.openapi()["paths"][path]["get"]["parameters"] == [
+        {
+            "required": True,
+            "schema": {
+                "title": "Int Val",
+                "anyOf": [{"type": "integer"}, {"type": "null"}],
+            },
+            "name": "int-val",
+            "in": "header",
+        },
+        {
+            "required": True,
+            "schema": {
+                "title": "Str Val",
+                "anyOf": [{"type": "string"}, {"type": "null"}],
+            },
+            "name": "str-val",
+            "in": "header",
+        },
+        {
+            "required": True,
+            "schema": {
+                "title": "List Val",
+                "anyOf": [
+                    {"type": "array", "items": {"type": "integer"}},
+                    {"type": "null"},
+                ],
+            },
+            "name": "list-val",
+            "in": "header",
+        },
+    ]
+
+
+@pytest.mark.parametrize(
+    "path",
+    [
+        "/nullable-required",
+        pytest.param(
+            "/model-nullable-required",
+            marks=pytest.mark.xfail(
+                reason="With Header model fields use underscores in error locs for headers"
+            ),
+        ),
+    ],
+)
+def test_nullable_required_missing(path: str):
+    client = TestClient(app)
+    with patch(f"{__name__}.convert", Mock(wraps=convert)) as mock_convert:
+        response = client.get(path)
+
+    assert mock_convert.call_count == 0, (
+        "Validator should not be called if the value is missing"
+    )
+    assert response.status_code == 422
+    assert response.json() == {
+        "detail": [
+            {
+                "type": "missing",
+                "loc": ["header", "int-val"],
+                "msg": "Field required",
+                "input": AnyThing(),
+            },
+            {
+                "type": "missing",
+                "loc": ["header", "str-val"],
+                "msg": "Field required",
+                "input": AnyThing(),
+            },
+            {
+                "type": "missing",
+                "loc": ["header", "list-val"],
+                "msg": "Field required",
+                "input": AnyThing(),
+            },
+        ]
+    }
+
+
+@pytest.mark.parametrize(
+    "path",
+    [
+        "/nullable-required",
+        "/model-nullable-required",
+    ],
+)
+def test_nullable_required_pass_value(path: str):
+    client = TestClient(app)
+
+    with patch(f"{__name__}.convert", Mock(wraps=convert)) as mock_convert:
+        response = client.get(
+            path,
+            headers=[
+                ("int-val", "1"),
+                ("str-val", "test"),
+                ("list-val", "1"),
+                ("list-val", "2"),
+            ],
+        )
+
+    assert mock_convert.call_count == 3, "Validator should be called for each field"
+    assert response.status_code == 200, response.text
+    assert response.json() == {
+        "int_val": 1,
+        "str_val": "test",
+        "list_val": [1, 2],
+        "fields_set": IsOneOf(
+            None, IsList("int_val", "str_val", "list_val", check_order=False)
+        ),
+    }
+
+
+# =====================================================================================
+# Nullable with default=None
+
+
+@app.get("/nullable-non-required")
+async def read_nullable_non_required(
+    int_val: Annotated[
+        Union[int, None],
+        Header(),
+        BeforeValidator(lambda v: convert(v)),
+    ] = None,
+    str_val: Annotated[
+        Union[str, None],
+        Header(),
+        BeforeValidator(lambda v: convert(v)),
+    ] = None,
+    list_val: Annotated[
+        Union[list[int], None],
+        Header(),
+        BeforeValidator(lambda v: convert(v)),
+    ] = None,
+):
+    return {
+        "int_val": int_val,
+        "str_val": str_val,
+        "list_val": list_val,
+        "fields_set": None,
+    }
+
+
+class ModelNullableNonRequired(BaseModel):
+    int_val: Union[int, None] = None
+    str_val: Union[str, None] = None
+    list_val: Union[list[int], None] = None
+
+    @field_validator("*", mode="before")
+    @classmethod
+    def convert_fields(cls, v):
+        return convert(v)
+
+
+@app.get("/model-nullable-non-required")
+async def read_model_nullable_non_required(
+    params: Annotated[ModelNullableNonRequired, Header()],
+):
+    return {
+        "int_val": params.int_val,
+        "str_val": params.str_val,
+        "list_val": params.list_val,
+        "fields_set": params.model_fields_set,
+    }
+
+
+@pytest.mark.parametrize(
+    "path",
+    [
+        pytest.param(
+            "/nullable-non-required",
+            marks=pytest.mark.xfail(
+                reason="Title contains hyphens for single Header parameters"
+            ),
+        ),
+        "/model-nullable-non-required",
+    ],
+)
+def test_nullable_non_required_schema(path: str):
+    assert app.openapi()["paths"][path]["get"]["parameters"] == [
+        {
+            "required": False,
+            "schema": {
+                "title": "Int Val",
+                "anyOf": [{"type": "integer"}, {"type": "null"}],
+                # "default": None, # `None` values are omitted in OpenAPI schema
+            },
+            "name": "int-val",
+            "in": "header",
+        },
+        {
+            "required": False,
+            "schema": {
+                "title": "Str Val",
+                "anyOf": [{"type": "string"}, {"type": "null"}],
+                # "default": None, # `None` values are omitted in OpenAPI schema
+            },
+            "name": "str-val",
+            "in": "header",
+        },
+        {
+            "required": False,
+            "schema": {
+                "title": "List Val",
+                "anyOf": [
+                    {"type": "array", "items": {"type": "integer"}},
+                    {"type": "null"},
+                ],
+                # "default": None, # `None` values are omitted in OpenAPI schema
+            },
+            "name": "list-val",
+            "in": "header",
+        },
+    ]
+
+
+@pytest.mark.parametrize(
+    "path",
+    [
+        "/nullable-non-required",
+        "/model-nullable-non-required",
+    ],
+)
+def test_nullable_non_required_missing(path: str):
+    client = TestClient(app)
+
+    with patch(f"{__name__}.convert", Mock(wraps=convert)) as mock_convert:
+        response = client.get(path)
+
+    assert mock_convert.call_count == 0, (
+        "Validator should not be called if the value is missing"
+    )
+    assert response.status_code == 200
+    assert response.json() == {
+        "int_val": None,
+        "str_val": None,
+        "list_val": None,
+        "fields_set": IsOneOf(None, []),
+    }
+
+
+@pytest.mark.parametrize(
+    "path",
+    [
+        "/nullable-non-required",
+        "/model-nullable-non-required",
+    ],
+)
+def test_nullable_non_required_pass_value(path: str):
+    client = TestClient(app)
+
+    with patch(f"{__name__}.convert", Mock(wraps=convert)) as mock_convert:
+        response = client.get(
+            path,
+            headers=[
+                ("int-val", "1"),
+                ("str-val", "test"),
+                ("list-val", "1"),
+                ("list-val", "2"),
+            ],
+        )
+
+    assert mock_convert.call_count == 3, "Validator should be called for each field"
+    assert response.status_code == 200, response.text
+    assert response.json() == {
+        "int_val": 1,
+        "str_val": "test",
+        "list_val": [1, 2],
+        "fields_set": IsOneOf(
+            None, IsList("int_val", "str_val", "list_val", check_order=False)
+        ),
+    }
+
+
+# =====================================================================================
+# Nullable with not-None default
+
+
+@app.get("/nullable-with-non-null-default")
+async def read_nullable_with_non_null_default(
+    *,
+    int_val: Annotated[
+        Union[int, None],
+        Header(),
+        BeforeValidator(lambda v: convert(v)),
+    ] = -1,
+    str_val: Annotated[
+        Union[str, None],
+        Header(),
+        BeforeValidator(lambda v: convert(v)),
+    ] = "default",
+    list_val: Annotated[
+        Union[list[int], None],
+        Header(default_factory=lambda: [0]),
+        BeforeValidator(lambda v: convert(v)),
+    ],
+):
+    return {
+        "int_val": int_val,
+        "str_val": str_val,
+        "list_val": list_val,
+        "fields_set": None,
+    }
+
+
+class ModelNullableWithNonNullDefault(BaseModel):
+    int_val: Union[int, None] = -1
+    str_val: Union[str, None] = "default"
+    list_val: Union[list[int], None] = [0]
+
+    @field_validator("*", mode="before")
+    @classmethod
+    def convert_fields(cls, v):
+        return convert(v)
+
+
+@app.get("/model-nullable-with-non-null-default")
+async def read_model_nullable_with_non_null_default(
+    params: Annotated[ModelNullableWithNonNullDefault, Header()],
+):
+    return {
+        "int_val": params.int_val,
+        "str_val": params.str_val,
+        "list_val": params.list_val,
+        "fields_set": params.model_fields_set,
+    }
+
+
+@pytest.mark.parametrize(
+    "path",
+    [
+        pytest.param(
+            "/nullable-with-non-null-default",
+            marks=pytest.mark.xfail(
+                reason="Title contains hyphens for single Header parameters"
+            ),
+        ),
+        "/model-nullable-with-non-null-default",
+    ],
+)
+def test_nullable_with_non_null_default_schema(path: str):
+    parameters = app.openapi()["paths"][path]["get"]["parameters"]
+    assert parameters == [
+        {
+            "required": False,
+            "schema": {
+                "title": "Int Val",
+                "anyOf": [{"type": "integer"}, {"type": "null"}],
+                "default": -1,
+            },
+            "name": "int-val",
+            "in": "header",
+        },
+        {
+            "required": False,
+            "schema": {
+                "title": "Str Val",
+                "anyOf": [{"type": "string"}, {"type": "null"}],
+                "default": "default",
+            },
+            "name": "str-val",
+            "in": "header",
+        },
+        {
+            "required": False,
+            "schema": IsPartialDict(
+                {
+                    "title": "List Val",
+                    "anyOf": [
+                        {"type": "array", "items": {"type": "integer"}},
+                        {"type": "null"},
+                    ],
+                }
+            ),
+            "name": "list-val",
+            "in": "header",
+        },
+    ]
+
+    if path == "/model-nullable-with-non-null-default":
+        # Check default value for list_val param for model-based Body parameters only.
+        # default_factory is not reflected in OpenAPI schema
+        assert parameters[2]["schema"]["default"] == [0]
+
+
+@pytest.mark.parametrize(
+    "path",
+    [
+        "/nullable-with-non-null-default",
+        "/model-nullable-with-non-null-default",
+    ],
+)
+@pytest.mark.xfail(
+    reason="Missing parameters are pre-populated with default values before validation"
+)
+def test_nullable_with_non_null_default_missing(path: str):
+    client = TestClient(app)
+
+    with patch(f"{__name__}.convert", Mock(wraps=convert)) as mock_convert:
+        response = client.get(path)
+
+    assert mock_convert.call_count == 0, (
+        "Validator should not be called if the value is missing"
+    )
+    assert response.status_code == 200  # pragma: no cover
+    assert response.json() == {  # pragma: no cover
+        "int_val": -1,
+        "str_val": "default",
+        "list_val": [0],
+        "fields_set": IsOneOf(None, []),
+    }
+    # TODO: Remove 'no cover' when the issue is fixed
+
+
+@pytest.mark.parametrize(
+    "path",
+    [
+        "/nullable-with-non-null-default",
+        "/model-nullable-with-non-null-default",
+    ],
+)
+def test_nullable_with_non_null_default_pass_value(path: str):
+    client = TestClient(app)
+
+    with patch(f"{__name__}.convert", Mock(wraps=convert)) as mock_convert:
+        response = client.get(
+            path,
+            headers=[
+                ("int-val", "1"),
+                ("str-val", "test"),
+                ("list-val", "1"),
+                ("list-val", "2"),
+            ],
+        )
+
+    assert mock_convert.call_count == 3, "Validator should be called for each field"
+    assert response.status_code == 200, response.text
+    assert response.json() == {
+        "int_val": 1,
+        "str_val": "test",
+        "list_val": [1, 2],
+        "fields_set": IsOneOf(
+            None, IsList("int_val", "str_val", "list_val", check_order=False)
+        ),
+    }

tests/test_request_params/test_path/test_nullable_and_defaults.py

@@ -0,0 +1,2 @@
+# Not appllicable for Path parameters
+# Path parameters cannot have default values or be nullable

tests/test_request_params/test_query/test_nullable_and_defaults.py

@@ -0,0 +1,483 @@
+from typing import Annotated, Any, Union
+from unittest.mock import Mock, patch
+
+import pytest
+from dirty_equals import IsList, IsOneOf, IsPartialDict
+from fastapi import FastAPI, Query
+from fastapi.testclient import TestClient
+from pydantic import BaseModel, BeforeValidator, field_validator
+
+app = FastAPI()
+
+
+def convert(v: Any) -> Any:
+    return v
+
+
+# =====================================================================================
+# Nullable required
+
+
+@app.get("/nullable-required")
+async def read_nullable_required(
+    int_val: Annotated[
+        Union[int, None],
+        BeforeValidator(lambda v: convert(v)),
+    ],
+    str_val: Annotated[
+        Union[str, None],
+        BeforeValidator(lambda v: convert(v)),
+    ],
+    list_val: Annotated[
+        Union[list[int], None],
+        Query(),
+        BeforeValidator(lambda v: convert(v)),
+    ],
+):
+    return {
+        "int_val": int_val,
+        "str_val": str_val,
+        "list_val": list_val,
+        "fields_set": None,
+    }
+
+
+class ModelNullableRequired(BaseModel):
+    int_val: Union[int, None]
+    str_val: Union[str, None]
+    list_val: Union[list[int], None]
+
+    @field_validator("*", mode="before")
+    @classmethod
+    def convert_all(cls, v: Any) -> Any:
+        return convert(v)
+
+
+@app.get("/model-nullable-required")
+async def read_model_nullable_required(
+    params: Annotated[ModelNullableRequired, Query()],
+):
+    return {
+        "int_val": params.int_val,
+        "str_val": params.str_val,
+        "list_val": params.list_val,
+        "fields_set": params.model_fields_set,
+    }
+
+
+@pytest.mark.parametrize(
+    "path",
+    [
+        "/nullable-required",
+        "/model-nullable-required",
+    ],
+)
+def test_nullable_required_schema(path: str):
+    assert app.openapi()["paths"][path]["get"]["parameters"] == [
+        {
+            "required": True,
+            "schema": {
+                "title": "Int Val",
+                "anyOf": [{"type": "integer"}, {"type": "null"}],
+            },
+            "name": "int_val",
+            "in": "query",
+        },
+        {
+            "required": True,
+            "schema": {
+                "title": "Str Val",
+                "anyOf": [{"type": "string"}, {"type": "null"}],
+            },
+            "name": "str_val",
+            "in": "query",
+        },
+        {
+            "in": "query",
+            "name": "list_val",
+            "required": True,
+            "schema": {
+                "anyOf": [
+                    {"items": {"type": "integer"}, "type": "array"},
+                    {"type": "null"},
+                ],
+                "title": "List Val",
+            },
+        },
+    ]
+
+
+@pytest.mark.parametrize(
+    "path",
+    [
+        "/nullable-required",
+        "/model-nullable-required",
+    ],
+)
+def test_nullable_required_missing(path: str):
+    client = TestClient(app)
+
+    with patch(f"{__name__}.convert", Mock(wraps=convert)) as mock_convert:
+        response = client.get(path)
+
+    assert mock_convert.call_count == 0, (
+        "Validator should not be called if the value is missing"
+    )
+    assert response.status_code == 422
+    assert response.json() == {
+        "detail": [
+            {
+                "type": "missing",
+                "loc": ["query", "int_val"],
+                "msg": "Field required",
+                "input": IsOneOf(None, {}),
+            },
+            {
+                "type": "missing",
+                "loc": ["query", "str_val"],
+                "msg": "Field required",
+                "input": IsOneOf(None, {}),
+            },
+            {
+                "type": "missing",
+                "loc": ["query", "list_val"],
+                "msg": "Field required",
+                "input": IsOneOf(None, {}),
+            },
+        ]
+    }
+
+
+@pytest.mark.parametrize(
+    "path",
+    [
+        "/nullable-required",
+        "/model-nullable-required",
+    ],
+)
+def test_nullable_required_pass_value(path: str):
+    client = TestClient(app)
+
+    with patch(f"{__name__}.convert", Mock(wraps=convert)) as mock_convert:
+        response = client.get(
+            path, params={"int_val": "1", "str_val": "test", "list_val": ["1", "2"]}
+        )
+
+    assert mock_convert.call_count == 3, "Validator should be called for each field"
+    assert response.status_code == 200, response.text
+    assert response.json() == {
+        "int_val": 1,
+        "str_val": "test",
+        "list_val": [1, 2],
+        "fields_set": IsOneOf(
+            None, IsList("int_val", "str_val", "list_val", check_order=False)
+        ),
+    }
+
+
+# =====================================================================================
+# Nullable with default=None
+
+
+@app.get("/nullable-non-required")
+async def read_nullable_non_required(
+    int_val: Annotated[
+        Union[int, None],
+        BeforeValidator(lambda v: convert(v)),
+    ] = None,
+    str_val: Annotated[
+        Union[str, None],
+        BeforeValidator(lambda v: convert(v)),
+    ] = None,
+    list_val: Annotated[
+        Union[list[int], None],
+        Query(),
+        BeforeValidator(lambda v: convert(v)),
+    ] = None,
+):
+    return {
+        "int_val": int_val,
+        "str_val": str_val,
+        "list_val": list_val,
+        "fields_set": None,
+    }
+
+
+class ModelNullableNonRequired(BaseModel):
+    int_val: Union[int, None] = None
+    str_val: Union[str, None] = None
+    list_val: Union[list[int], None] = None
+
+    @field_validator("*", mode="before")
+    @classmethod
+    def convert_all(cls, v: Any) -> Any:
+        return convert(v)
+
+
+@app.get("/model-nullable-non-required")
+async def read_model_nullable_non_required(
+    params: Annotated[ModelNullableNonRequired, Query()],
+):
+    return {
+        "int_val": params.int_val,
+        "str_val": params.str_val,
+        "list_val": params.list_val,
+        "fields_set": params.model_fields_set,
+    }
+
+
+@pytest.mark.parametrize(
+    "path",
+    [
+        "/nullable-non-required",
+        "/model-nullable-non-required",
+    ],
+)
+def test_nullable_non_required_schema(path: str):
+    assert app.openapi()["paths"][path]["get"]["parameters"] == [
+        {
+            "required": False,
+            "schema": {
+                "title": "Int Val",
+                "anyOf": [{"type": "integer"}, {"type": "null"}],
+                # "default": None, # `None` values are omitted in OpenAPI schema
+            },
+            "name": "int_val",
+            "in": "query",
+        },
+        {
+            "required": False,
+            "schema": {
+                "title": "Str Val",
+                "anyOf": [{"type": "string"}, {"type": "null"}],
+                # "default": None, # `None` values are omitted in OpenAPI schema
+            },
+            "name": "str_val",
+            "in": "query",
+        },
+        {
+            "in": "query",
+            "name": "list_val",
+            "required": False,
+            "schema": {
+                "anyOf": [
+                    {"items": {"type": "integer"}, "type": "array"},
+                    {"type": "null"},
+                ],
+                "title": "List Val",
+                # "default": None, # `None` values are omitted in OpenAPI schema
+            },
+        },
+    ]
+
+
+@pytest.mark.parametrize(
+    "path",
+    [
+        "/nullable-non-required",
+        "/model-nullable-non-required",
+    ],
+)
+def test_nullable_non_required_missing(path: str):
+    client = TestClient(app)
+
+    with patch(f"{__name__}.convert", Mock(wraps=convert)) as mock_convert:
+        response = client.get(path)
+
+    assert mock_convert.call_count == 0, (
+        "Validator should not be called if the value is missing"
+    )
+    assert response.status_code == 200
+    assert response.json() == {
+        "int_val": None,
+        "str_val": None,
+        "list_val": None,
+        "fields_set": IsOneOf(None, []),
+    }
+
+
+@pytest.mark.parametrize(
+    "path",
+    [
+        "/nullable-non-required",
+        "/model-nullable-non-required",
+    ],
+)
+def test_nullable_non_required_pass_value(path: str):
+    client = TestClient(app)
+
+    with patch(f"{__name__}.convert", Mock(wraps=convert)) as mock_convert:
+        response = client.get(
+            path, params={"int_val": "1", "str_val": "test", "list_val": ["1", "2"]}
+        )
+
+    assert mock_convert.call_count == 3, "Validator should be called for each field"
+    assert response.status_code == 200, response.text
+    assert response.json() == {
+        "int_val": 1,
+        "str_val": "test",
+        "list_val": [1, 2],
+        "fields_set": IsOneOf(
+            None, IsList("int_val", "str_val", "list_val", check_order=False)
+        ),
+    }
+
+
+# =====================================================================================
+# Nullable with not-None default
+
+
+@app.get("/nullable-with-non-null-default")
+async def read_nullable_with_non_null_default(
+    *,
+    int_val: Annotated[
+        Union[int, None],
+        BeforeValidator(lambda v: convert(v)),
+    ] = -1,
+    str_val: Annotated[
+        Union[str, None],
+        BeforeValidator(lambda v: convert(v)),
+    ] = "default",
+    list_val: Annotated[
+        Union[list[int], None],
+        Query(default_factory=lambda: [0]),
+        BeforeValidator(lambda v: convert(v)),
+    ],
+):
+    return {
+        "int_val": int_val,
+        "str_val": str_val,
+        "list_val": list_val,
+        "fields_set": None,
+    }
+
+
+class ModelNullableWithNonNullDefault(BaseModel):
+    int_val: Union[int, None] = -1
+    str_val: Union[str, None] = "default"
+    list_val: Union[list[int], None] = [0]
+
+    @field_validator("*", mode="before")
+    @classmethod
+    def convert_all(cls, v: Any) -> Any:
+        return convert(v)
+
+
+@app.get("/model-nullable-with-non-null-default")
+async def read_model_nullable_with_non_null_default(
+    params: Annotated[ModelNullableWithNonNullDefault, Query()],
+):
+    return {
+        "int_val": params.int_val,
+        "str_val": params.str_val,
+        "list_val": params.list_val,
+        "fields_set": params.model_fields_set,
+    }
+
+
+@pytest.mark.parametrize(
+    "path",
+    [
+        "/nullable-with-non-null-default",
+        "/model-nullable-with-non-null-default",
+    ],
+)
+def test_nullable_with_non_null_default_schema(path: str):
+    parameters = app.openapi()["paths"][path]["get"]["parameters"]
+    assert parameters == [
+        {
+            "required": False,
+            "schema": {
+                "title": "Int Val",
+                "anyOf": [{"type": "integer"}, {"type": "null"}],
+                "default": -1,
+            },
+            "name": "int_val",
+            "in": "query",
+        },
+        {
+            "required": False,
+            "schema": {
+                "title": "Str Val",
+                "anyOf": [{"type": "string"}, {"type": "null"}],
+                "default": "default",
+            },
+            "name": "str_val",
+            "in": "query",
+        },
+        {
+            "in": "query",
+            "name": "list_val",
+            "required": False,
+            "schema": IsPartialDict(
+                {
+                    "anyOf": [
+                        {"items": {"type": "integer"}, "type": "array"},
+                        {"type": "null"},
+                    ],
+                    "title": "List Val",
+                }
+            ),
+        },
+    ]
+
+    if path == "/model-nullable-with-non-null-default":
+        # Check default value for list_val param for model-based Body parameters only.
+        # default_factory is not reflected in OpenAPI schema
+        assert parameters[2]["schema"]["default"] == [0]
+
+
+@pytest.mark.parametrize(
+    "path",
+    [
+        "/nullable-with-non-null-default",
+        "/model-nullable-with-non-null-default",
+    ],
+)
+@pytest.mark.xfail(
+    reason="Missing parameters are pre-populated with default values before validation"
+)
+def test_nullable_with_non_null_default_missing(path: str):
+    client = TestClient(app)
+
+    with patch(f"{__name__}.convert", Mock(wraps=convert)) as mock_convert:
+        response = client.get(path)
+
+    assert mock_convert.call_count == 0, (
+        "Validator should not be called if the value is missing"
+    )
+    assert response.status_code == 200  # pragma: no cover
+    assert response.json() == {  # pragma: no cover
+        "int_val": -1,
+        "str_val": "default",
+        "list_val": [0],
+        "fields_set": IsOneOf(None, []),
+    }
+    # TODO: Remove 'no cover' when the issue is fixed
+
+
+@pytest.mark.parametrize(
+    "path",
+    [
+        "/nullable-with-non-null-default",
+        "/model-nullable-with-non-null-default",
+    ],
+)
+def test_nullable_with_non_null_default_pass_value(path: str):
+    client = TestClient(app)
+
+    with patch(f"{__name__}.convert", Mock(wraps=convert)) as mock_convert:
+        response = client.get(
+            path, params={"int_val": "1", "str_val": "test", "list_val": ["1", "2"]}
+        )
+
+    assert mock_convert.call_count == 3, "Validator should be called for each field"
+    assert response.status_code == 200, response.text
+    assert response.json() == {
+        "int_val": 1,
+        "str_val": "test",
+        "list_val": [1, 2],
+        "fields_set": IsOneOf(
+            None, IsList("int_val", "str_val", "list_val", check_order=False)
+        ),
+    }