Path parameter types and validation in FastAPI - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to declare a path parameter as an integer in FastAPI.

FastAPI

from fastapi import FastAPI
app = FastAPI()

@app.get('/items/{item_id}')
async def read_item(item_id: [1]):
    return {"item_id": item_id}

Drag options to blanks, or click blank then click option'

Astr

Bbool

Cfloat

Dint

Attempts:

3 left

💡 Hint

Common Mistakes

Using str instead of int for numeric path parameters.

Forgetting to specify a type, which defaults to str.

The path parameter item_id should be typed as int to ensure FastAPI treats it as an integer.

2fill in blank

medium

Complete the code to validate that the path parameter item_id is greater than zero.

FastAPI

from fastapi import FastAPI, Path
app = FastAPI()

@app.get('/items/{item_id}')
async def read_item(item_id: int = Path(..., gt=[1])):
    return {"item_id": item_id}

Drag options to blanks, or click blank then click option'

A1

B-1

C0

DNone

Attempts:

3 left

💡 Hint

Common Mistakes

Using ge=0 which allows zero, not strictly greater.

Setting gt to a negative number.

The gt=0 argument ensures item_id must be greater than zero.

3fill in blank

hard

Fix the error in the code by specifying the correct type for the path parameter user_id.

FastAPI

from fastapi import FastAPI
app = FastAPI()

@app.get('/users/{user_id}')
async def get_user(user_id: [1]):
    return {"user_id": user_id}

Drag options to blanks, or click blank then click option'

Afloat

Bint

Cstr

Dbool

Attempts:

3 left

💡 Hint

Common Mistakes

Using str when numeric IDs are expected.

Using float which is not typical for IDs.

The user_id should be an int to match expected numeric user IDs.

4fill in blank

hard

Fill both blanks to declare a path parameter item_id as an integer and validate it to be between 1 and 100.

FastAPI

from fastapi import FastAPI, Path
app = FastAPI()

@app.get('/items/{item_id}')
async def read_item(item_id: [1] = Path(..., ge=[2], le=100)):
    return {"item_id": item_id}

Drag options to blanks, or click blank then click option'

Aint

Bstr

C1

D0

Attempts:

3 left

💡 Hint

Common Mistakes

Using str instead of int.

Setting ge to 0 instead of 1.

Use int for the type and ge=1 to ensure the value is at least 1.

5fill in blank

hard

Fill all three blanks to declare a path parameter user_id as an integer, validate it to be positive, and add a description.

FastAPI

from fastapi import FastAPI, Path
app = FastAPI()

@app.get('/users/{user_id}')
async def get_user(user_id: [1] = Path(..., gt=[2], description=[3])):
    return {"user_id": user_id}

Drag options to blanks, or click blank then click option'

Astr

B0

C"The ID of the user"

Dint

Attempts:

3 left

💡 Hint

Common Mistakes

Using str instead of int.

Omitting the description or using wrong quotes.

Use int type, gt=0 to ensure positive values, and add a descriptive string for documentation.

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