Path parameters in FastAPI - Step-by-Step Execution

Concept Flow - Path parameters

Client sends request with URL

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FastAPI matches URL path

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Extract path parameter value

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Pass parameter to function

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Function processes parameter

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Return response with parameter data

FastAPI takes the part of the URL defined as a path parameter, passes it to your function, and uses it to create a response.

Execution Sample

FastAPI

from fastapi import FastAPI
app = FastAPI()

@app.get("/items/{item_id}")
async def read_item(item_id: int):
    return {"item_id": item_id}

This code defines a path parameter 'item_id' that FastAPI extracts from the URL and sends to the function.

Execution Table

Step	Request URL	Path Parameter Extracted	Function Parameter	Response
1	/items/5	item_id = 5	item_id = 5	{"item_id": 5}
2	/items/42	item_id = 42	item_id = 42	{"item_id": 42}
3	/items/abc	item_id = 'abc'	Error: invalid int	422 Unprocessable Entity

💡 Execution stops when the request URL does not match the path pattern or parameter type is invalid.

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 3
item_id	None	5	42	Error (invalid int)

Key Moments - 2 Insights

Why does the request to /items/abc cause an error?

Because the path parameter 'item_id' is declared as int, FastAPI tries to convert 'abc' to int and fails, causing a 422 error as shown in execution_table step 3.

How does FastAPI know which part of the URL is the path parameter?

FastAPI uses the curly braces {} in the route path to identify path parameters, as shown in the code sample and execution_table where /items/{item_id} extracts the value after /items/.

Visual Quiz - 3 Questions

Look at the execution_table, what is the response when the request URL is /items/5?

A {"item_id": 5}

B {"item_id": "5"}

C 422 Unprocessable Entity

D 404 Not Found

💡 Hint

Check execution_table row 1 under Response column.

At which step does FastAPI return an error due to invalid path parameter type?

A Step 2

B Step 3

C Step 1

D No error occurs

💡 Hint

Look at execution_table row 3 showing 'Error: invalid int' and 422 response.

If the path parameter type is changed from int to str, how would the response for /items/abc change?

A It would return 404 Not Found

B It would still return 422 error

C It would return {"item_id": "abc"} without error

D It would crash the server

💡 Hint

Refer to variable_tracker and key_moments about type conversion and errors.

Concept Snapshot

Path parameters in FastAPI are parts of the URL path wrapped in {}.
They are extracted from the URL and passed as arguments to your function.
You must declare the parameter type (e.g., int, str) for validation.
If the URL part does not match the type, FastAPI returns a 422 error.
Use path parameters to create dynamic URLs easily.

Full Transcript

In FastAPI, path parameters let you capture parts of the URL and use them in your code. When a client sends a request like /items/5, FastAPI sees the {item_id} in the route and extracts '5' as the item_id parameter. It converts it to the declared type, here int, and passes it to the function. The function then returns a response using that value. If the client sends a value that cannot convert to int, like 'abc', FastAPI returns a 422 error. This process helps create dynamic and flexible web APIs.