Path parameters in FastAPI - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What is the output when accessing a path with a parameter?

Consider this FastAPI code snippet. What will be the response when a client requests /items/42?

FastAPI

from fastapi import FastAPI

app = FastAPI()

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

A {"item_id": "42"}

B {"item_id": 42}

C 404 Not Found error

D 500 Internal Server Error

💡 Hint

Remember that FastAPI converts path parameters to the declared type.

FastAPI converts the path parameter 'item_id' to an integer because the function argument is typed as int. So the response JSON contains the integer 42.

📝 Syntax

intermediate

Which option correctly defines a path parameter in FastAPI?

Which of the following FastAPI route definitions correctly declares a path parameter named user_id of type str?

A

@app.get("/users/{user_id}")
async def get_user(user_id: str):
    return {"user_id": user_id}

B

@app.get("/users/user_id")
async def get_user(user_id: str):
    return {"user_id": user_id}

C

@app.get("/users/{user_id:int}")
async def get_user(user_id: str):
    return {"user_id": user_id}

D

@app.get("/users/{user_id}")
async def get_user():
    return {"user_id": user_id}

💡 Hint

Path parameters must be included in the route string with curly braces and matched in the function parameters.

Option A correctly uses curly braces to declare the path parameter and matches it in the function argument with the correct type. Option A treats user_id as a literal path segment. Option A uses invalid syntax for type hinting in the path. Option A misses the parameter in the function signature.

🔧 Debug

advanced

Why does this FastAPI path parameter cause a 422 error?

Given this code, why does a request to /products/abc return a 422 Unprocessable Entity error?

FastAPI

from fastapi import FastAPI

app = FastAPI()

@app.get("/products/{product_id}")
async def get_product(product_id: int):
    return {"product_id": product_id}

A Because the return statement is missing a JSON response model.

B Because the route path is missing a trailing slash.

C Because the function is missing an async keyword.

D Because 'abc' cannot be converted to int as required by the path parameter type.

💡 Hint

Check the type of the path parameter and the value passed in the URL.

FastAPI tries to convert the path parameter 'product_id' to int. The string 'abc' cannot be converted to int, so FastAPI returns a 422 error indicating the input is invalid.

❓ state_output

advanced

What is the value of user_id after this request?

Given this FastAPI route, what will be the value of user_id inside the function when a client requests /users/007?

FastAPI

from fastapi import FastAPI

app = FastAPI()

@app.get("/users/{user_id}")
async def read_user(user_id: str):
    return {"user_id": user_id}

A "7" (string, leading zeros removed)

B 7 (integer, leading zeros removed)

C "007" (string with leading zeros preserved)

D Error because of leading zeros in path parameter

💡 Hint

Check the declared type of the path parameter and how FastAPI treats strings.

The path parameter is typed as str, so FastAPI passes the exact string from the URL, including leading zeros.

🧠 Conceptual

expert

Which statement about FastAPI path parameters is true?

Select the correct statement about how FastAPI handles path parameters.

A FastAPI automatically converts path parameters to the declared type and returns a 422 error if conversion fails.

B FastAPI treats all path parameters as strings and never performs type conversion.

C FastAPI requires explicit type conversion inside the route function for path parameters.

D FastAPI only supports integer path parameters and rejects others.

💡 Hint

Think about how FastAPI validates input before calling your function.

FastAPI uses the type hints to convert and validate path parameters automatically. If conversion fails, it returns a 422 error before your function runs.