class DefaultPlaceholder:
    """
    You shouldn't use this class directly.

    It's used internally to recognize when a default value has been overwritten, even
    if the overridden default value was truthy.
    """

    def __init__(self, value: Any):
        self.value = value

    def __bool__(self) -> bool:
        return bool(self.value)

        "include-optional-dependencies", []
    )
    # Override main [project] configs with custom configs for this package
    for key, value in project_config.items():
        metadata[key] = value
    # Get custom build config for the current package
    build_config: Dict[str, Any] = (
        config.get("tool", {}).get("pdm", {}).get("build", {})
    )
    # Override PDM build config with custom build config for this package

```python
from fastapi import FastAPI
```

::: fastapi.FastAPI
    options:
        members:
            - openapi_version
            - webhooks
            - state
            - dependency_overrides
            - openapi
            - websocket
            - include_router
            - get
            - put
            - post
            - delete
            - options
            - head

    ],
)
def test_override_simple(url, status_code, expected):
    app.dependency_overrides[common_parameters] = overrider_dependency_simple
    response = client.get(url)
    assert response.status_code == status_code
    assert response.json() == expected
    app.dependency_overrides = {}


def test_override_with_sub_main_depends():
    app.dependency_overrides[common_parameters] = overrider_dependency_with_sub

常见实例是调用外部第三方身份验证应用。

向第三方应用发送令牌，然后返回经验证的用户。

但第三方服务商处理每次请求都可能会收费，并且耗时通常也比调用写死的模拟测试用户更长。

一般只要测试一次外部验证应用就够了，不必每次测试都去调用。

此时，最好覆盖调用外部验证应用的依赖项，使用返回模拟测试用户的自定义依赖项就可以了。

### 使用 `app.dependency_overrides` 属性

对于这些用例，**FastAPI** 应用支持 `app.dependcy_overrides` 属性，该属性就是**字典**。

要在测试时覆盖原有依赖项，这个字典的键应当是原依赖项（函数），值是覆盖依赖项（另一个函数）。

这样一来，**FastAPI** 就会调用覆盖依赖项，不再调用原依赖项。

```Python hl_lines="26-27  30"

    "pydantic-extra-types >=2.0.0",
]

[tool.mypy]
strict = true

[[tool.mypy.overrides]]
module = "fastapi.concurrency"
warn_unused_ignores = false
ignore_missing_imports = true

[[tool.mypy.overrides]]
module = "fastapi.tests.*"
ignore_missing_imports = true
check_untyped_defs = true

[[tool.mypy.overrides]]
module = "docs_src.*"
disallow_incomplete_defs = false
disallow_untyped_defs = false

# Custom Request and APIRoute class

In some cases, you may want to override the logic used by the `Request` and `APIRoute` classes.

In particular, this may be a good alternative to logic in a middleware.

For example, if you want to read or manipulate the request body before it is processed by your application.

!!! danger
    This is an "advanced" feature.

    If you are just starting with **FastAPI** you might want to skip this section.

    * PR [#292](https://github.com/tiangolo/fastapi/pull/292).

* Implement dependency overrides for testing.
    * This allows using overrides/mocks of dependencies during tests.
    * New docs: [Testing Dependencies with Overrides](https://fastapi.tiangolo.com/advanced/testing-dependencies/).
    * PR [#291](https://github.com/tiangolo/fastapi/pull/291).

## 0.27.2

```python
from fastapi import FastAPI
```

::: fastapi.FastAPI
    options:
        members:
            - openapi_version
            - webhooks
            - state
            - dependency_overrides
            - openapi
            - websocket
            - include_router
            - get
            - put
            - post
            - delete
            - options
            - head

site_name: FastAPI
site_description: FastAPI framework, high performance, easy to learn, fast to code, ready for production
site_url: https://fastapi.tiangolo.com/
theme:
  name: material
  custom_dir: ../en/overrides
  palette:
  - media: '(prefers-color-scheme: light)'
    scheme: default
    primary: teal
    accent: amber
    toggle:
      icon: material/lightbulb
      name: Switch to dark mode