Path parameter types and validation in FastAPI - Step-by-Step Execution

Concept Flow - Path parameter types and validation

Request with path parameter

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FastAPI receives request

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Extract path parameter

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Check parameter type

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Pass to endpoint

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Process request

FastAPI takes the path parameter from the URL, checks if it matches the declared type, and either passes it to the endpoint or returns an error if invalid.

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' as an integer and returns it if valid.

Execution Table

Step	Request URL	Extracted item_id	Type Check	Action	Response
1	/items/5	5	int check passes	Call endpoint with item_id=5	{'item_id': 5}
2	/items/abc	abc	int check fails	Return 422 error	422 Unprocessable Entity
3	/items/10	10	int check passes	Call endpoint with item_id=10	{'item_id': 10}

💡 Execution stops when request is processed or validation fails returning error.

Variable Tracker

Variable	Start	After Request 1	After Request 2	After Request 3
item_id	undefined	5	error (invalid)	10

Key Moments - 2 Insights

Why does FastAPI return a 422 error for /items/abc?

Because 'abc' cannot be converted to an integer as required by the path parameter type, as shown in execution_table row 2.

What happens if the path parameter matches the declared type?

FastAPI passes the parameter to the endpoint function, which processes and returns a response, as seen in execution_table rows 1 and 3.

Visual Quiz - 3 Questions

Look at the execution table, what is the response when the URL is /items/5?

A {'item_id': 5}

B 422 Unprocessable Entity

C 404 Not Found

D 500 Internal Server Error

💡 Hint

Check execution_table row 1 under Response column.

At which step does the type check fail?

A Step 3

B Step 1

C Step 2

D No failure

💡 Hint

Look at execution_table rows and see where 'int check fails' appears.

If we change item_id type to str, what happens to the response for /items/abc?

A Returns 404 error

B Returns {'item_id': 'abc'}

C Returns 422 error

D Server crashes

💡 Hint

Refer to variable_tracker and understand type validation in execution_table.

Concept Snapshot

Path parameters in FastAPI are declared with types like int or str.
FastAPI validates the parameter from the URL against the declared type.
If valid, the parameter is passed to the endpoint function.
If invalid, FastAPI returns a 422 error automatically.
This ensures only correct data types reach your code.

Full Transcript

FastAPI uses path parameters to get values from the URL. When a request comes in, FastAPI extracts the parameter and checks if it matches the declared type, such as int. If the value matches, FastAPI calls the endpoint function with that value. If it does not match, FastAPI returns a 422 error response. For example, if the path parameter is declared as int and the URL has a number like 5, it works fine. But if the URL has a string like 'abc', FastAPI rejects it with an error. This validation helps keep your API safe and predictable.