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"

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:

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

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>