Default values in FastAPI - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the purpose of default values in FastAPI path or query parameters?

Default values allow parameters to be optional by providing a fallback value if the client does not send that parameter. This makes the API more flexible and user-friendly.

beginner

How do you set a default value for a query parameter in FastAPI?

You assign a value to the parameter in the function definition, for example: def read_items(q: str = "default"). If the client omits q, FastAPI uses "default".

intermediate

Can default values be used with path parameters in FastAPI?

No, path parameters are required by design and cannot have default values. Only query parameters and request body fields can have defaults.

beginner

What happens if a client sends a value for a parameter that has a default value in FastAPI?

FastAPI uses the client-provided value instead of the default. The default is only used when the client omits the parameter.

intermediate

How can you make a query parameter optional without a default value in FastAPI?

You can use Optional from typing and set the default to None, like q: Optional[str] = None. This means the parameter can be missing or null.

In FastAPI, what does setting a default value for a query parameter do?

A Makes the parameter optional with a fallback value

B Makes the parameter required

C Removes the parameter from the API

D Changes the parameter type

Setting a default value makes the parameter optional and uses the default if the client does not provide it.

Can you set a default value for a path parameter in FastAPI?

A Yes, always

B No, path parameters are always required

C Only if you use a special decorator

D Only for integer path parameters

Path parameters are required and cannot have default values in FastAPI.

What is the correct way to make a query parameter optional without a default value in FastAPI?

A Use a default empty string

B Set the parameter type to int

C Remove the parameter from the function

D Set the parameter type to Optional and default to None

Using Optional[type] with default None makes the parameter optional without a fixed default.

If a client sends a value for a parameter that has a default in FastAPI, what happens?

A An error is raised

B The default value is used

C The client value is used

D The parameter is ignored

FastAPI uses the client-provided value, overriding the default.

Which of these is a valid default value for a FastAPI query parameter?

A "hello"

B A required path parameter

C A missing parameter

D A function without a return

Default values can be strings, numbers, or other valid types, like "hello".

Explain how default values work for query parameters in FastAPI and why they are useful.

Think about how you might want to let users skip some inputs.

Describe the difference between path parameters and query parameters regarding default values in FastAPI.

Consider how URLs are structured and what parts must always be present.