问题

我正在尝试使FastAPI中的查询参数成为必需的，但给它一个默认值，但我在他们的用户指南中找不到任何信息。

在OpenAPI中，可以像这样做：

```python
parameters:
      - name: "some_name"
        in: "query"
        description: "a description"
        required: true
        type: "string"
        default: "First"
        enum:
        - "First"
        - "Second"
        - "Third"

如您所见，required 被分配为 true，我有一个默认值 ("first")。
使用 FastAPI，我有这个：

class ModelNames(str, Enum):
  first = "first"
  second = "second"
  third = "third"

@app.post("/path")
async def this_function(
    modelInstance = Query(
      default=Required, # 我希望以某种方式将 "first" 和 Required 分配给 default
      description="a description"
    )
  ):
  return None

这是您的代码部分的翻译。

<details>
<summary>英文:</summary>

I am trying to make a query parameter required but give it a default value on FastAPI, but i am not finding anything on their user guide.

On openapi, it would be something like this:

parameters:
- name: "some_name"
in: "query"
description: "a description"
required: true
type: "string"
default: "First"
enum:
- "First"
- "Second"
- "Third"

as you can see required is assigned to true and i have a default value ("first")
With fastapi i have this:

class ModelNames(str, Enum):
first = "first"
second = "second"
third = "third"

@app.post("/path")
async def this_function(
modelInstance = Query(
default=Required, # i would like to somehow assign "first" and Required to default
description="a description"
)
):
return None

I've tried directly assigning "first" but it makes it optional.

</details>


# 答案1
**得分**: 4

# Required parameter can't get default value by design

无法。

如果你想为自定义示例值的副作用定义默认值，请使用 example 参数。

https://fastapi.tiangolo.com/tutorial/schema-extra-example/#example-and-examples-in-openapi

<details>
<summary>英文:</summary>

# Required parameter can&#39;t get default value by design

You can&#39;t.

If you want to define a default value for the side effect of a custom example value, use the example parameter instead.

https://fastapi.tiangolo.com/tutorial/schema-extra-example/#example-and-examples-in-openapi

</details>