Path parameter types and validation in FastAPI - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a path parameter in FastAPI?

A path parameter is a variable part of the URL path that FastAPI extracts and passes to your function. It lets you capture values directly from the URL.

beginner

How do you specify the type of a path parameter in FastAPI?

You specify the type by adding a type hint in the function parameter, like item_id: int. FastAPI uses this to validate and convert the value automatically.

intermediate

What happens if a path parameter value does not match the declared type in FastAPI?

FastAPI returns a 422 Unprocessable Entity error because the value cannot be converted to the declared type, ensuring only valid data is processed.

intermediate

How can you add validation constraints like minimum or maximum values to path parameters in FastAPI?

You can use Path() from fastapi with arguments like ge (greater or equal) and le (less or equal) to set limits, e.g., item_id: int = Path(..., ge=1, le=100).

beginner

Why is it important to validate path parameters in web applications?

Validation prevents invalid or harmful data from reaching your code, which helps avoid errors, security issues, and unexpected behavior.

In FastAPI, how do you declare a path parameter named 'user_id' as an integer?

A def get_user(user_id: float):

B def get_user(user_id):

C def get_user(user_id: int):

D def get_user(user_id: str):

Using user_id: int tells FastAPI to expect an integer and validate it.

What response does FastAPI give if a path parameter fails type validation?

A 404 Not Found

B 422 Unprocessable Entity

C 200 OK with empty data

D 500 Internal Server Error

FastAPI returns 422 when the input cannot be converted to the declared type.

Which FastAPI function helps add validation constraints like minimum or maximum values to path parameters?

A Path()

B Body()

C Query()

D Depends()

Path() is used to add validation rules to path parameters.

What does Path(..., ge=1, le=10) mean for a path parameter?

A Value must be between 1 and 10 inclusive

B Value must be greater than 10

C Value must be less than 1

D Value can be any number

The parameter value must be greater or equal to 1 and less or equal to 10.

Why should you use type hints and validation for path parameters?

A To avoid writing functions

B To make URLs longer

C To slow down the app

D To improve code readability and ensure correct data

Type hints and validation help catch errors early and make code easier to understand.

Explain how FastAPI uses type hints to validate path parameters and what happens if validation fails.

Think about how FastAPI checks the URL parts against your function's expected types.

Describe how to add minimum and maximum value constraints to a path parameter in FastAPI and why this is useful.

Consider how you can tell FastAPI to accept only numbers within a certain range.

Practice

1. What is the main purpose of specifying a type for a path parameter in FastAPI?

easy

A. To disable validation for that parameter

B. To make the parameter optional in the URL

C. To automatically generate HTML forms

D. To ensure the parameter value matches the expected data type from the URL

Solution

1. Step 1: Understand path parameters in FastAPI

   Path parameters extract values from the URL and can be typed to ensure correct data types.

2. Step 2: Role of typing in path parameters

   Typing enforces that the value received matches the expected type, preventing errors and invalid data.

3. Final Answer:

   To ensure the parameter value matches the expected data type from the URL -> Option D

4. Quick Check:

   Path parameter typing = type safety [OK]

Hint: Path parameter types check URL values match expected type [OK]

Common Mistakes:

• Thinking typing makes parameters optional
• Believing typing disables validation
• Confusing path parameters with query parameters

2. Which of the following is the correct way to declare an integer path parameter with a minimum value of 1 in FastAPI?

easy

A. def read_item(item_id: int = Path(min=1)):

B. def read_item(item_id: int = Path(minimum=1)):

C. def read_item(item_id: int = Path(ge=1)):

D. def read_item(item_id: int = Path(min_value=1)):

Solution

1. Step 1: Recall FastAPI Path parameter validation syntax

   FastAPI uses Path() with keyword arguments like ge (greater or equal) for minimum values.

2. Step 2: Identify correct keyword for minimum value

   The correct keyword is ge=1, not min, minimum, or min_value.

3. Final Answer:

   def read_item(item_id: int = Path(ge=1)): -> Option C

4. Quick Check:

   Minimum value keyword = ge [OK]

Hint: Use ge= for minimum value in Path() [OK]

Common Mistakes:

• Using min or minimum instead of ge
• Using min_value which is invalid
• Omitting Path() and just typing int

3. Given this FastAPI path operation:

from fastapi import FastAPI, Path
app = FastAPI()

@app.get('/users/{user_id}')
async def get_user(user_id: int = Path(ge=1, le=10)):
    return {"user_id": user_id}

What will be the response if the client requests /users/15?

medium

A. Returns 422 Unprocessable Entity error

B. Returns {"user_id": "15"} as string

C. Returns 404 Not Found error

D. Returns {"user_id": 15}

Solution

1. Step 1: Understand validation constraints

   The path parameter user_id must be an integer between 1 and 10 inclusive.

2. Step 2: Analyze the request value

   The client requests /users/15, which is outside the allowed range (greater than 10).

3. Step 3: FastAPI validation behavior

   FastAPI automatically returns a 422 error if validation fails for path parameters.

4. Final Answer:

   Returns 422 Unprocessable Entity error -> Option A

5. Quick Check:

   Invalid path param value = 422 error [OK]

Hint: Out-of-range path params cause 422 error [OK]

Common Mistakes:

• Expecting 404 instead of 422
• Assuming FastAPI converts invalid values silently
• Thinking invalid values return as strings

4. Identify the error in this FastAPI path parameter declaration:

from fastapi import FastAPI, Path
app = FastAPI()

@app.get('/items/{item_id}')
async def read_item(item_id: str = Path(gt=0)):
    return {"item_id": item_id}

medium

A. Using gt=0 with a string type causes a validation error

B. Missing type annotation for item_id

C. Path() cannot be used with string parameters

D. The route path should not include curly braces

Solution

1. Step 1: Check parameter type and validation

   The parameter item_id is typed as str but uses gt=0 which means greater than 0, a numeric constraint.

2. Step 2: Understand validation mismatch

   Numeric validations like gt apply only to numbers, not strings, causing a validation error.

3. Final Answer:

   Using gt=0 with a string type causes a validation error -> Option A

4. Quick Check:

   Numeric validation on string param = error [OK]

Hint: Numeric validators only work with numeric types [OK]

Common Mistakes:

• Applying numeric validation to strings
• Forgetting to annotate type
• Misunderstanding route path syntax

5. You want to create a FastAPI endpoint that accepts a path parameter product_id which must be an integer between 100 and 999 inclusive. If the value is invalid, FastAPI should reject the request automatically. Which of the following is the best way to declare this parameter?

hard

A. def get_product(product_id: int = Path(min=100, max=999)):

B. def get_product(product_id: int = Path(ge=100, le=999)):

C. def get_product(product_id: int = Path(gt=100, lt=999)):

D. def get_product(product_id: int = Path(minimum=100, maximum=999)):

Solution

1. Step 1: Recall correct validation keywords in Path()

   FastAPI uses ge (greater or equal) and le (less or equal) for inclusive range validation.

2. Step 2: Analyze each option's keywords

   def get_product(product_id: int = Path(ge=100, le=999)): uses ge=100 and le=999 which matches the requirement exactly. The remaining options use invalid keywords or exclusive bounds. def get_product(product_id: int = Path(gt=100, lt=999)): uses exclusive bounds which exclude 100 and 999.

3. Final Answer:

   def get_product(product_id: int = Path(ge=100, le=999)): -> Option B

4. Quick Check:

   Inclusive range validation = ge and le [OK]

Hint: Use ge and le for inclusive min and max in Path() [OK]

Common Mistakes:

• Using min/max instead of ge/le
• Using gt/lt for inclusive bounds
• Using invalid keyword names