Boolean query parameters in FastAPI - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a boolean query parameter in FastAPI?

A boolean query parameter is a way to pass true or false values in the URL query string, which FastAPI automatically converts to Python's bool type.

beginner

How does FastAPI interpret the query parameter ?active=true when declared as active: bool?

FastAPI converts the string "true" (case-insensitive) to the boolean value True. Similarly, "false" becomes False.

intermediate

What happens if a boolean query parameter is not provided in the URL?

If the boolean query parameter is optional (e.g., active: bool = None), FastAPI sets it to None. If it is required without a default, FastAPI returns an error.

beginner

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

You assign a default value in the function signature, like active: bool = False. This means if the parameter is missing, active will be False.

beginner

Why is it important to use boolean query parameters carefully in URLs?

Because URLs are typed by users or generated by clients, using clear true/false values helps avoid confusion and ensures the API behaves as expected.

In FastAPI, what value does the query parameter ?flag=false convert to if declared as flag: bool?

A True

B False

C "false" (string)

D None

FastAPI converts the string "false" (case-insensitive) to the boolean value False.

What happens if a required boolean query parameter is missing in the request URL?

A FastAPI raises a validation error

B FastAPI sets it to False automatically

C FastAPI sets it to None

D FastAPI ignores the parameter

If a required boolean query parameter is missing, FastAPI raises a validation error because it expects a value.

How do you make a boolean query parameter optional in FastAPI?

A Use <code>flag: bool</code> without default

B Use <code>flag: Optional[str]</code>

C Use <code>flag: bool = None</code>

D Use <code>flag: int = 0</code>

Setting flag: bool = None makes the parameter optional and allows it to be missing.

Which of these is a valid boolean query parameter value FastAPI recognizes as True?

A "true"

B "on"

C "1"

D "yes"

FastAPI recognizes "true" (case-insensitive) as True. It also recognizes other strings like "yes", "1", or "on" (case-insensitive).

How can you provide a default value of True for a boolean query parameter named enabled?

A <code>enabled: bool</code>

B <code>enabled: bool = false</code>

C <code>enabled: Optional[bool]</code>

D <code>enabled: bool = True</code>

Assigning enabled: bool = True sets the default value to True if the parameter is missing.

Explain how FastAPI handles boolean query parameters and how to make them optional or required.

Think about function parameter defaults and type conversion.

Describe best practices for using boolean query parameters in URLs to ensure clear API behavior.

Consider how users and clients read URLs.