Multiple query parameters in FastAPI - Step-by-Step Execution

Concept Flow - Multiple query parameters

Client sends HTTP GET request

↓

FastAPI receives request

↓

Extract query parameters

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Pass parameters to endpoint function

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Function processes parameters

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Return response with parameters info

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Client receives response

The client sends a GET request with multiple query parameters. FastAPI extracts these parameters and passes them to the endpoint function, which processes and returns a response.

Execution Sample

FastAPI

from fastapi import FastAPI
from typing import List

app = FastAPI()

@app.get("/items/")
async def read_items(q: List[str] = []):
    return {"q": q}

This FastAPI endpoint accepts multiple query parameters named 'q' and returns them as a list.

Execution Table

Step	Request URL	Extracted Query Params	Function Parameter 'q'	Response
1	/items/?q=apple&q=banana&q=cherry	q=['apple', 'banana', 'cherry']	['apple', 'banana', 'cherry']	{"q": ["apple", "banana", "cherry"]}
2	/items/?q=dog	q=['dog']	['dog']	{"q": ["dog"]}
3	/items/	q=[] (no query)	[]	{"q": []}
4	/items/?q=one&q=two&q=three&q=four	q=['one', 'two', 'three', 'four']	['one', 'two', 'three', 'four']	{"q": ["one", "two", "three", "four"]}
5	Request ends	No more requests	No more calls	No more responses

💡 No more requests to process; server waits for next client call.

Variable Tracker

Variable	Start	After 1	After 2	After 3	After 4	Final
q	[]	['apple', 'banana', 'cherry']	['dog']	[]	['one', 'two', 'three', 'four']	[]

Key Moments - 2 Insights

Why is 'q' a list even if only one query parameter is sent?

Because the endpoint defines 'q' as List[str], FastAPI always collects all 'q' parameters into a list, even if there is only one or none, as shown in execution_table rows 1 and 2.

What happens if no 'q' parameter is provided in the URL?

The default value [] is used, so 'q' is an empty list. This is shown in execution_table row 3 where no query parameters are sent.

Visual Quiz - 3 Questions

Look at the execution_table row 1. What is the value of 'q' inside the function?

A apple, banana, cherry

B ['apple']

C ['apple', 'banana', 'cherry']

D None

💡 Hint

Check the 'Function Parameter q' column in row 1 of the execution_table.

At which step does the function receive an empty list for 'q'?

A Step 3

B Step 2

C Step 4

D Step 1

💡 Hint

Look for the row where 'Extracted Query Params' shows no 'q' values.

If the endpoint parameter 'q' was changed to a single string instead of a list, what would happen when multiple 'q' parameters are sent?

A FastAPI would combine all 'q' values into one string.

B FastAPI would take only the last 'q' value.

C FastAPI would raise an error.

D FastAPI would ignore all 'q' parameters.

💡 Hint

Think about how FastAPI handles single vs multiple query parameters for string vs list types.

Concept Snapshot

Multiple query parameters in FastAPI:
- Use List[type] to accept multiple values for the same query key.
- FastAPI collects all values into a list.
- If no parameters sent, default list is empty.
- Example: q: List[str] = []
- URL: /items/?q=one&q=two&q=three
- Response returns list of all q values.

Full Transcript

This example shows how FastAPI handles multiple query parameters with the same name. The endpoint defines a parameter 'q' as a list of strings. When the client sends a GET request with multiple 'q' parameters, FastAPI collects them all into a list and passes it to the function. The function then returns this list in the response. If no 'q' parameters are sent, the default empty list is used. This behavior is consistent and predictable, making it easy to handle multiple query inputs.

Practice

1. What is the main purpose of using multiple query parameters in a FastAPI endpoint?

easy

A. To accept several inputs from the URL to customize the response

B. To define multiple routes in one function

C. To handle only POST request data

D. To create database models automatically

Solution

1. Step 1: Understand query parameters in FastAPI

   Query parameters allow passing data in the URL after a question mark, like ?name=John.

2. Step 2: Purpose of multiple query parameters

   Using multiple query parameters lets the API accept many inputs to customize what it returns.

3. Final Answer:

   To accept several inputs from the URL to customize the response -> Option A

4. Quick Check:

   Multiple query parameters = multiple inputs [OK]

Hint: Multiple query parameters let you send many inputs in URL [OK]

Common Mistakes:

• Confusing query parameters with request body
• Thinking multiple query parameters define routes
• Assuming query parameters only work with POST

2. Which of the following is the correct way to declare two query parameters name (string) and age (integer) in a FastAPI path operation function?

easy

A. def read_user(name: str = Query(...), age: int = Query(...)):

B. def read_user(name: str, age: int = None):

C. def read_user(name: int, age: str):

D. def read_user(name, age):

Solution

1. Step 1: Understand FastAPI query parameter declaration

   Using Query(...) marks parameters as required query parameters with type hints.

2. Step 2: Check parameter types and usage

   Parameters must have correct types and use Query(...) to be recognized as query parameters.

3. Final Answer:

   def read_user(name: str = Query(...), age: int = Query(...)): -> Option A

4. Quick Check:

   Use Query(...) with correct types for query parameters [OK]

Hint: Use Query(...) with type hints for required query parameters [OK]

Common Mistakes:

• Swapping types of parameters
• Not using Query for required parameters
• Omitting type hints

3. Given this FastAPI endpoint code:

from fastapi import FastAPI, Query
from typing import List

app = FastAPI()

@app.get('/items/')
async def read_items(q: List[str] = Query([])):
    return {"q": q}

What will be the output if the URL is /items/?q=apple&q=banana?

medium

A. {"q": ["apple"]}

B. {"q": "apple,banana"}

C. {"q": ["apple", "banana"]}

D. {"q": []}

Solution

1. Step 1: Understand List query parameters in FastAPI

   Using List[str] with Query([]) allows multiple values for the same query key to be collected into a list.

2. Step 2: Analyze the URL query parameters

   The URL has q=apple and q=banana, so FastAPI collects both into a list.

3. Final Answer:

   {"q": ["apple", "banana"]} -> Option C

4. Quick Check:

   Multiple query keys = list of values [OK]

Hint: Multiple same keys in URL become list in FastAPI [OK]

Common Mistakes:

• Expecting a single string with commas
• Getting only the first value
• Getting an empty list

4. What is wrong with this FastAPI endpoint code that tries to accept two query parameters name and age?:

from fastapi import FastAPI
app = FastAPI()

@app.get('/user/')
def get_user(name: str, age: int = None):
    return {"name": name, "age": age}

medium

A. The route path should not end with a slash

B. The function must be async

C. Missing Query() for both parameters

D. The parameter age should use Optional[int] for optional query

Solution

1. Step 1: Check optional query parameter typing

   In FastAPI, optional parameters should use Optional[type] or type | None to mark them optional properly.

2. Step 2: Analyze age: int = None

   Just setting default to None is not enough; the type hint should be Optional[int] or int | None for FastAPI to treat it as optional.

3. Final Answer:

   The parameter age should use Optional[int] for optional query -> Option D

4. Quick Check:

   Optional query params need Optional[type] or type | None [OK]

Hint: Use Optional[int] or int | None for optional query params [OK]

Common Mistakes:

• Assuming default None alone makes param optional
• Forgetting to import Optional
• Thinking async is required for all endpoints

5. You want to create a FastAPI endpoint that accepts multiple optional query parameters: tags (list of strings), limit (integer, default 10), and search (optional string). Which of the following function signatures correctly implements this?

hard

A. def get_items(tags: List[str] = Query([]), limit: int = 10, search: str = None):

B. def get_items(tags: List[str] = Query([]), limit: int = Query(10), search: Optional[str] = Query(None)):

C. def get_items(tags: List[str] = [], limit: int = 10, search: str = ''):

D. def get_items(tags: List[str], limit: int = Query(10), search: Optional[str] = Query(None)):

Solution

1. Step 1: Use Query() for query parameters with defaults

   FastAPI requires using Query() to specify default values and metadata for query parameters.

2. Step 2: Correctly type optional parameters

   Use Optional[str] or str | None with Query(None) for optional string parameters.

3. Step 3: Check all parameters in def get_items(tags: List[str] = Query([]), limit: int = Query(10), search: Optional[str] = Query(None)):

   def get_items(tags: List[str] = Query([]), limit: int = Query(10), search: Optional[str] = Query(None)): uses Query([]) for list default, Query(10) for limit default, and Optional[str] = Query(None) for optional search, which is correct.

4. Final Answer:

   def get_items(tags: List[str] = Query([]), limit: int = Query(10), search: Optional[str] = Query(None)): -> Option B

5. Quick Check:

   Use Query() with correct types and defaults for multiple optional queries [OK]

Hint: Use Query() with defaults and Optional for optional query params [OK]

Common Mistakes:

• Using mutable default [] without Query()
• Not marking optional string with Optional
• Omitting Query() for default values