Optional query parameters in FastAPI - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is an optional query parameter in FastAPI?

An optional query parameter is a parameter that the client can choose to include or omit in the URL query string. In FastAPI, you make a query parameter optional by giving it a default value, often None.

beginner

How do you declare an optional query parameter in FastAPI?

You declare it by setting a default value in the function parameter, for example: def read_items(q: str | None = None):. This means the parameter q can be a string or None if not provided.

beginner

What happens if a client does not provide an optional query parameter in FastAPI?

If the client omits the optional query parameter, FastAPI uses the default value you set (like None). Your code can then check if the parameter was given or not.

beginner

Why use optional query parameters in an API?

Optional query parameters let clients customize their requests without forcing them to provide every detail. This makes your API flexible and easier to use.

beginner

Example: How to define an optional query parameter named 'search' in FastAPI?

Use this code: <pre>from fastapi import FastAPI
app = FastAPI()

@app.get("/items/")
async def read_items(search: str | None = None):
    if search:
        return {"message": f"Searching for {search}"}
    return {"message": "No search query provided"}</pre>

How do you make a query parameter optional in FastAPI?

A By giving it a default value like None

B By using @optional decorator

C By declaring it inside the path

D By importing Optional from fastapi

In FastAPI, setting a default value (like None) makes a query parameter optional.

What type hint is commonly used to declare an optional string query parameter?

A str | None

B int | None

C str

D Optional[str]

Using 'str | None' means the parameter can be a string or None, making it optional.

If a client omits an optional query parameter, what value does FastAPI assign?

A An empty string

B The default value set in the function, often None

C Zero

D Raises an error

FastAPI uses the default value you set, commonly None, when the parameter is omitted.

Which of these is NOT a benefit of optional query parameters?

A Making API requests more flexible

B Simplifying client usage

C Allowing clients to customize requests

D Forcing clients to provide all parameters

Optional parameters do the opposite of forcing clients; they allow skipping parameters.

In FastAPI, where do optional query parameters appear?

A In the URL path

B In the request body

C In the URL query string after '?'

D In HTTP headers

Query parameters appear in the URL after the '?' symbol.

Explain how to declare and use an optional query parameter in FastAPI.

Think about default values and type hints.

Why are optional query parameters useful in building APIs with FastAPI?

Consider client experience and API design.

Practice

1. What does it mean when a query parameter in FastAPI is optional?

easy

A. The parameter is required but can be empty

B. The parameter must always be included in the request

C. The user can choose to include it or not in the request

D. The parameter is ignored by FastAPI

Solution

1. Step 1: Understand optional parameters in APIs

   Optional means the user does not have to provide the parameter for the request to work.

2. Step 2: Apply to FastAPI query parameters

   FastAPI allows parameters to be optional by giving them a default value like None.

3. Final Answer:

   The user can choose to include it or not in the request -> Option C

4. Quick Check:

   Optional query parameter = user choice [OK]

Hint: Optional means user may skip it without error [OK]

Common Mistakes:

• Thinking optional means parameter is always required
• Confusing optional with ignored parameters
• Assuming optional means empty string is required

2. Which of the following is the correct way to declare an optional query parameter named q of type str in FastAPI?

easy

A. def read_items(q: Optional[str] = None):

B. def read_items(q: str = None):

C. def read_items(q: str):

D. def read_items(q: Optional[str]):

Solution

1. Step 1: Recall syntax for optional query parameters

   FastAPI uses Optional[type] with a default value None to mark parameters optional.

2. Step 2: Check each option

   def read_items(q: Optional[str] = None): uses Optional[str] and default None, which is correct syntax.

3. Final Answer:

   def read_items(q: Optional[str] = None): -> Option A

4. Quick Check:

   Optional[str] + = None = correct syntax [OK]

Hint: Use Optional[type] with = None for optional params [OK]

Common Mistakes:

• Omitting the default None value
• Using Optional without import
• Setting type to str without Optional

3. Given this FastAPI endpoint code:

from typing import Optional
from fastapi import FastAPI

app = FastAPI()

@app.get("/items/")
async def read_items(q: Optional[str] = None):
    if q:
        return {"query": q}
    return {"query": "No query"}

What will be the JSON response when the client calls /items/ without any query parameters?

medium

A. {"query": null}

B. {"query": "No query"}

C. An error occurs because q is missing

D. {"query": ""}

Solution

1. Step 1: Understand default value of q

   q is Optional[str] with default None, so if missing, q is None.

2. Step 2: Check the if condition

   If q is None (falsey), the function returns {"query": "No query"}.

3. Final Answer:

   {"query": "No query"} -> Option B

4. Quick Check:

   Missing q returns "No query" [OK]

Hint: Missing optional param uses default None [OK]

Common Mistakes:

• Expecting error when param is missing
• Assuming null is returned instead of string
• Confusing empty string with None

4. Identify the error in this FastAPI endpoint code that tries to make q optional:

from fastapi import FastAPI

app = FastAPI()

@app.get("/search/")
def search(q: str = None):
    return {"q": q}

medium

A. q should be typed as Optional[str] when default is None

B. Function must be async to use optional parameters

C. Missing import of Optional from typing

D. Default value None is not allowed for query parameters

Solution

1. Step 1: Check parameter typing and default

   q has type str but default None, which conflicts because None is not str.

2. Step 2: Correct typing for optional parameter

   Use Optional[str] from typing to allow None as a valid default.

3. Final Answer:

   q should be typed as Optional[str] when default is None -> Option A

4. Quick Check:

   Type must match default None with Optional[str] [OK]

Hint: Match type with default None using Optional[type] [OK]

Common Mistakes:

• Not importing Optional but using it
• Making function async unnecessarily
• Using None default without Optional type

5. You want to create a FastAPI endpoint that accepts an optional integer query parameter page with a default of 1, but if the user sends page=0, it should return an error. How would you declare page to achieve this behavior?

hard

A. page: Optional[int] = 1 with extra validation to reject 0

B. page: int without default and require user to send page > 0

C. page: Optional[int] = None and set page = 1 inside function if None

D. page: int = 1 and check inside function if page == 0 to raise error

Solution

1. Step 1: Understand default and validation needs

   Default 1 means page is not optional, but user can omit it to get 1.

2. Step 2: Handle invalid value 0

   Use int with default 1, then inside function check if page == 0 and raise error.

3. Final Answer:

   page: int = 1 and check inside function if page == 0 to raise error -> Option D

4. Quick Check:

   Default + validation inside function = page: int = 1 and check inside function if page == 0 to raise error [OK]

Hint: Use default param + check value inside function [OK]

Common Mistakes:

• Making page Optional[int] = 1 (invalid syntax)
• Not validating 0 value inside function
• Requiring page without default when default needed