Query parameter validation in FastAPI - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What is the output when accessing the endpoint with ?q=fastapi123?

Consider this FastAPI endpoint that validates a query parameter q to be a string with a minimum length of 5 and maximum length of 10:

from fastapi import FastAPI, Query

app = FastAPI()

@app.get("/items/")
async def read_items(q: str = Query(..., min_length=5, max_length=10)):
    return {"q": q}

What will be the response when a client requests /items/?q=fastapi123?

FastAPI

from fastapi import FastAPI, Query

app = FastAPI()

@app.get("/items/")
async def read_items(q: str = Query(..., min_length=5, max_length=10)):
    return {"q": q}

A 422 Unprocessable Entity error due to length exceeding max_length

B 500 Internal Server Error

C {"q": "fastapi"}

D {"q": "fastapi123"}

💡 Hint

Check the length of the string fastapi123 and compare it with the min and max length constraints.

The string fastapi123 has 10 characters, which is within the allowed range of 5 to 10 characters. So the endpoint returns the query parameter as JSON.

📝 Syntax

intermediate

Which option correctly validates an optional integer query parameter with a default of 10 and a range 1 to 100?

Choose the correct FastAPI query parameter declaration for an optional integer count with default 10, minimum 1, and maximum 100.

A count: int = Query(default=10, min_length=1, max_length=100)

B count: int = Query(10, ge=1, le=100)

C count: int = Query(10, min=1, max=100)

D count: Optional[int] = Query(default=10, min=1, max=100)

💡 Hint

Remember that ge and le are used for numeric bounds, and min_length and max_length are for strings.

Option B correctly uses ge and le for numeric bounds and sets default to 10. Option B uses wrong parameter names and Optional without import. Options A and D misuse string length parameters.

❓ state_output

advanced

What is the response when calling /search/?tags=python&tags=fastapi&tags=api?

Given this FastAPI endpoint:

from fastapi import FastAPI, Query
from typing import List

app = FastAPI()

@app.get("/search/")
async def search(tags: List[str] = Query([])):
    return {"tags": tags}

What will be the JSON response when the client calls /search/?tags=python&tags=fastapi&tags=api?

FastAPI

from fastapi import FastAPI, Query
from typing import List

app = FastAPI()

@app.get("/search/")
async def search(tags: List[str] = Query([])):
    return {"tags": tags}

A {"tags": ["python", "fastapi", "api"]}

B {"tags": "python,fastapi,api"}

C 422 Unprocessable Entity error

D {"tags": ["python"]}

💡 Hint

Think about how FastAPI handles multiple query parameters with the same name when using a List type.

FastAPI collects all query parameters with the same name into a list when the type is List[str]. So the response contains all three tags as a list.

🔧 Debug

advanced

Why does this FastAPI endpoint raise a validation error when called with ?age=twenty?

Examine this FastAPI endpoint:

from fastapi import FastAPI, Query

app = FastAPI()

@app.get("/users/")
async def get_user(age: int = Query(..., ge=0, le=120)):
    return {"age": age}

What is the reason for a 422 validation error when the client calls /users/?age=twenty?

FastAPI

from fastapi import FastAPI, Query

app = FastAPI()

@app.get("/users/")
async def get_user(age: int = Query(..., ge=0, le=120)):
    return {"age": age}

A The string 'twenty' cannot be converted to an integer, causing a validation error.

B The age value is below the minimum allowed (0).

C The age value exceeds the maximum allowed (120).

D The Query parameter is missing a default value.

💡 Hint

Consider the data type expected and the actual input value.

The query parameter expects an integer. The string 'twenty' cannot be converted to int, so FastAPI returns a 422 validation error before checking range constraints.

🧠 Conceptual

expert

Which option best describes how FastAPI validates query parameters with Pydantic constraints?

FastAPI uses Pydantic models and field constraints to validate query parameters. Which statement correctly explains this process?

A FastAPI validates query parameters only after the endpoint function runs, raising errors inside the function.

B FastAPI only checks the type of query parameters but ignores any additional constraints like min_length or ge.

C FastAPI converts query parameters to the declared type, then applies Pydantic validators and constraints, returning errors if validation fails.

D FastAPI requires manual validation code inside the endpoint to enforce constraints on query parameters.

💡 Hint

Think about when and how FastAPI applies validation rules declared with Query and Pydantic.

FastAPI uses Pydantic to parse and validate query parameters before calling the endpoint. It converts types and applies constraints like min_length, ge, le, etc. If validation fails, it returns a 422 error automatically.

Practice

1. What is the main purpose of using Query() in FastAPI for query parameters?

easy

A. To validate and enforce rules on query parameter values

B. To create new API endpoints automatically

C. To handle database connections

D. To format the response data

Solution

1. Step 1: Understand the role of Query()

   Query() is used in FastAPI to add validation rules to query parameters, such as minimum length or value ranges.

2. Step 2: Identify what Query() does not do

   It does not create endpoints, handle databases, or format responses; it only validates input parameters.

3. Final Answer:

   To validate and enforce rules on query parameter values -> Option A

4. Quick Check:

   Query() validates parameters = B [OK]

Hint: Query() is for input rules, not endpoints or data handling [OK]

Common Mistakes:

• Thinking Query() creates API routes
• Confusing Query() with response formatting
• Assuming Query() manages database connections

2. Which of the following is the correct syntax to require a query parameter q with a minimum length of 3 in FastAPI?

easy

A. Query(q: str, min_length=3)

B. q: Query = str(min_length=3)

C. q = Query(str, min_length=3)

D. q: str = Query(min_length=3)

Solution

1. Step 1: Recall FastAPI query parameter syntax

   In FastAPI, you declare a query parameter with its type and assign it to Query() with validation rules, e.g., q: str = Query(min_length=3).

2. Step 2: Check each option's syntax correctness

   q: str = Query(min_length=3) matches the correct pattern. Options B, C, and D misuse type annotations or function calls.

3. Final Answer:

   q: str = Query(min_length=3) -> Option D

4. Quick Check:

   Correct syntax = A [OK]

Hint: Type then = Query(...) with rules inside [OK]

Common Mistakes:

• Swapping type and Query() order
• Calling Query() incorrectly as a function
• Missing type annotation before Query()

3. Given this FastAPI endpoint code, what will be the response if the query parameter age is set to 15?

from fastapi import FastAPI, Query
app = FastAPI()

@app.get("/items/")
async def read_items(age: int = Query(..., ge=18)):
    return {"age": age}

medium

A. {"age": 15}

B. 422 Unprocessable Entity error

C. 500 Internal Server Error

D. {"age": null}

Solution

1. Step 1: Understand the validation rule

   The query parameter age requires a value greater or equal to 18 (ge=18).

2. Step 2: Check the input value against the rule

   Input age=15 is less than 18, so FastAPI will reject it with a 422 validation error.

3. Final Answer:

   422 Unprocessable Entity error -> Option B

4. Quick Check:

   age < 18 triggers 422 error = C [OK]

Hint: ge=18 means age must be 18 or more [OK]

Common Mistakes:

• Assuming invalid input returns null or default
• Expecting server error instead of validation error
• Ignoring validation and returning input as is

4. Identify the error in this FastAPI query parameter validation code:

from fastapi import FastAPI, Query
app = FastAPI()

@app.get("/search/")
async def search_items(q: str = Query(min_length=5)):
    return {"query": q}

medium

A. There is no error; the code correctly requires the query parameter with min_length validation

B. min_length should be max_length

C. Query() cannot be used with strings

D. Function must be synchronous, not async

Solution

1. Step 1: Check if query parameter is required

   Query(min_length=5) implicitly sets default=Ellipsis, making q required with string length validation.

2. Step 2: Understand FastAPI requirement for required params

   Explicit ... is not needed; the code correctly validates required query parameters. No error.

3. Final Answer:

   There is no error; the code correctly requires the query parameter with min_length validation -> Option A

4. Quick Check:

   Required param needs Query(...) = A [OK]

Hint: Query(min_length=5) makes param required implicitly [OK]

Common Mistakes:

• Thinking Query(min_length=5) makes param optional
• Confusing min_length with max_length
• Thinking async is invalid for FastAPI endpoints

5. You want to create a FastAPI endpoint that accepts a query parameter tags which must be a list of strings, each at least 2 characters long. Which is the correct way to validate this?

hard

A. tags: list[str] = Query(..., min_length=2)

B. tags: list[str] = Query(..., min_items=1, min_length=2)

C. tags: list[str] = Query(..., min_items=1), with custom validation for string length

D. tags: str = Query(..., min_length=2)

Solution

1. Step 1: Understand FastAPI's Query validation limits

   FastAPI's Query() supports min_items for lists but min_length applies to strings, not list elements.

2. Step 2: Validate list and individual string lengths

   To ensure each string in the list is at least 2 characters, you must add custom validation logic beyond Query().

3. Final Answer:

   tags: list[str] = Query(..., min_items=1), with custom validation for string length -> Option C

4. Quick Check:

   List length via Query, element length via custom code = D [OK]

Hint: Query() validates list size; use extra code for element rules [OK]

Common Mistakes:

• Using min_length on list instead of strings inside
• Not validating individual string lengths
• Declaring tags as a single string instead of list