String validation (min, max, regex) in FastAPI - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What happens when a FastAPI endpoint receives a string shorter than the minimum length?

Consider this FastAPI model using Pydantic for validation:

from pydantic import BaseModel, constr

class User(BaseModel):
    username: constr(min_length=5, max_length=10)

@app.post('/user')
async def create_user(user: User):
    return {'username': user.username}

If a client sends {"username": "abc"}, what will FastAPI do?

FastAPI

from fastapi import FastAPI
from pydantic import BaseModel, constr

app = FastAPI()

class User(BaseModel):
    username: constr(min_length=5, max_length=10)

@app.post('/user')
async def create_user(user: User):
    return {'username': user.username}

A Accept the input and return the username as is

B Return a 400 Bad Request error with a generic message

C Ignore the username field and return an empty string

D Return a 422 Unprocessable Entity error because the string is too short

💡 Hint

Think about how Pydantic validates string length constraints.

FastAPI uses Pydantic to validate input data. If the string is shorter than the min_length, Pydantic raises a validation error. FastAPI returns a 422 error with details.

📝 Syntax

intermediate

Which Pydantic field declaration correctly enforces a regex pattern for a string?

Choose the correct way to declare a Pydantic model field that only accepts strings matching the pattern ^[a-z]{3}\d{2}$ (three lowercase letters followed by two digits).

A code: username: constr(regex='^[a-z]{3}\d{2}$')

B code: username: str(regex='^[a-z]{3}\d{2}$')

C code: username: constr(pattern='^[a-z]{3}\d{2}$')

D code: username: str(pattern='^[a-z]{3}\d{2}$')

💡 Hint

Remember that constr is used for constrained strings in Pydantic.

The constr type accepts a regex parameter to enforce patterns. Using str with regex or pattern is invalid.

❓ state_output

advanced

What is the response when a string exceeds the max_length in a FastAPI request?

Given this Pydantic model:

class Item(BaseModel):
    code: constr(min_length=2, max_length=4)

@app.post('/items')
async def create_item(item: Item):
    return {'code': item.code}

If the client sends {"code": "ABCDE"}, what will be the HTTP status code and response?

FastAPI

from fastapi import FastAPI
from pydantic import BaseModel, constr

app = FastAPI()

class Item(BaseModel):
    code: constr(min_length=2, max_length=4)

@app.post('/items')
async def create_item(item: Item):
    return {'code': item.code}

A 400 Bad Request with a generic error message

B 200 OK with the code returned as "ABCDE"

C 422 Unprocessable Entity with validation error about max_length

D 500 Internal Server Error due to validation failure

💡 Hint

Consider how Pydantic handles max_length violations.

Pydantic raises a validation error if the string exceeds max_length. FastAPI returns a 422 error with details about the violation.

🔧 Debug

advanced

Why does this FastAPI endpoint accept invalid strings despite constraints?

Examine this code snippet:

from fastapi import FastAPI
from pydantic import BaseModel

app = FastAPI()

class Data(BaseModel):
    code: str

@app.post('/data')
async def post_data(data: Data):
    return {'code': data.code}

The developer wants to enforce min_length=3 and a regex pattern ^[A-Z]+$ on code, but the endpoint accepts any string. Why?

FastAPI

from fastapi import FastAPI
from pydantic import BaseModel

app = FastAPI()

class Data(BaseModel):
    code: str

@app.post('/data')
async def post_data(data: Data):
    return {'code': data.code}

A The field uses plain str type without constraints, so no validation is applied

B FastAPI does not support string validation on POST requests

C The regex pattern must be applied in the endpoint function, not the model

D The code field is missing a default value, so validation is skipped

💡 Hint

Think about how Pydantic enforces validation rules.

Using str alone does not enforce any constraints. To validate length or regex, use constr with parameters.

🧠 Conceptual

expert

How does FastAPI handle regex validation errors internally when using Pydantic's constr?

When a FastAPI endpoint receives a string that fails the regex validation defined by constr(regex=...), what is the internal process FastAPI follows before returning a response?

A FastAPI ignores the regex failure and returns the string as is

B Pydantic raises a ValidationError, FastAPI catches it and returns a 422 response with error details

C FastAPI raises a 500 Internal Server Error due to unhandled exception

D The regex validation is performed by FastAPI middleware, which returns a 400 error on failure

💡 Hint

Recall how FastAPI integrates with Pydantic for validation.

Pydantic performs validation and raises ValidationError on failure. FastAPI catches this and returns a 422 response with detailed error messages.