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

Concept Flow - Multiple query parameters

Client sends HTTP GET request

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FastAPI receives request

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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.