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.

Practice

1. What does the constr function in FastAPI's Pydantic models help you do?

easy

A. Validate string fields with rules like minimum length, maximum length, and regex patterns

B. Create integer fields with range validation

C. Automatically generate API documentation

D. Handle database connections

Solution

1. Step 1: Understand constr purpose

   constr is a Pydantic type used to validate string fields with constraints like length and regex.

2. Step 2: Compare options

   Only Validate string fields with rules like minimum length, maximum length, and regex patterns correctly describes constr usage for string validation.

3. Final Answer:

   Validate string fields with rules like minimum length, maximum length, and regex patterns -> Option A

4. Quick Check:

   constr = string validation [OK]

Hint: Remember constr is for string validation rules [OK]

Common Mistakes:

• Confusing constr with integer validation
• Thinking constr manages database or docs
• Assuming constr auto-generates API docs

2. Which of the following is the correct way to define a string field in a Pydantic model with minimum length 3 and maximum length 10?

easy

A. name: str(min_length=3, max_length=10)

B. name: constr(min=3, max=10)

C. name: constr(min_length=3, max_length=10)

D. name: str(min=3, max=10)

Solution

1. Step 1: Recall correct syntax for constr

   The correct parameters are min_length and max_length inside constr().

2. Step 2: Check options

   Only name: constr(min_length=3, max_length=10) uses constr(min_length=3, max_length=10) correctly. Options A, B, and D use wrong parameter names or types.

3. Final Answer:

   name: constr(min_length=3, max_length=10) -> Option C

4. Quick Check:

   Use min_length and max_length in constr [OK]

Hint: Use min_length and max_length, not min or max [OK]

Common Mistakes:

• Using min and max instead of min_length and max_length
• Trying to add length constraints directly to str type
• Mixing parameter names

3. Given this Pydantic model snippet:

from pydantic import BaseModel, constr

class User(BaseModel):
    username: constr(min_length=4, max_length=8, regex=r'^[a-zA-Z]+$')

What happens if you try to create User(username='abc123')?

medium

A. Validation error because username is too long

B. User instance created successfully

C. Validation error because username is too short

D. Validation error because username contains digits

Solution

1. Step 1: Analyze regex pattern

   The regex r'^[a-zA-Z]+$' allows only letters, no digits.

2. Step 2: Check input username

   Input 'abc123' contains digits '123', violating the regex.

3. Final Answer:

   Validation error because username contains digits -> Option D

4. Quick Check:

   Regex restricts digits, so error occurs [OK]

Hint: Regex restricts characters; digits cause validation error [OK]

Common Mistakes:

• Ignoring regex and only checking length
• Assuming digits are allowed by regex
• Confusing length errors with pattern errors

4. Identify the error in this FastAPI Pydantic model field:

from pydantic import BaseModel, constr

class Item(BaseModel):
    code: constr(min_length=5, max=10)

medium

A. code field must be an int, not constr

B. max should be max_length to specify maximum string length

C. constr cannot be used for string validation

D. min_length should be min to specify minimum length

Solution

1. Step 1: Check parameter names in constr

   The correct parameter for maximum length is max_length, not max.

2. Step 2: Identify error in code

   Using max=10 causes a syntax or validation error because it's invalid.

3. Final Answer:

   max should be max_length to specify maximum string length -> Option B

4. Quick Check:

   Use max_length, not max, in constr [OK]

Hint: Use max_length, not max, for string max length [OK]

Common Mistakes:

• Using max instead of max_length
• Confusing min_length with min
• Thinking constr is for int fields

5. You want to create a FastAPI model field password that must be between 8 and 16 characters and include at least one digit. Which is the correct way to define it?

hard

A. password: constr(min_length=8, max_length=16, regex=r'^(?=.*\\d).+$')

B. password: constr(min_length=8, max_length=16, regex=r'^[a-zA-Z]+$')

C. password: constr(min=8, max=16, regex=r'^(?=.*\\d).+$')

D. password: str(min_length=8, max_length=16, regex=r'^(?=.*\\d).+$')

Solution

1. Step 1: Check length parameters

   Use min_length=8 and max_length=16 in constr for length limits.

2. Step 2: Verify regex for digit inclusion

   The regex r'^(?=.*\\d).+$' ensures at least one digit is present.

3. Step 3: Validate option correctness

   password: constr(min_length=8, max_length=16, regex=r'^(?=.*\\d).+$') correctly combines length and regex. password: constr(min_length=8, max_length=16, regex=r'^[a-zA-Z]+$')'s regex disallows digits. password: constr(min=8, max=16, regex=r'^(?=.*\\d).+$') uses invalid min and max. password: str(min_length=8, max_length=16, regex=r'^(?=.*\\d).+$') wrongly applies regex to str type.

4. Final Answer:

   password: constr(min_length=8, max_length=16, regex=r'^(?=.*\\d).+$') -> Option A

5. Quick Check:

   Use constr with min_length, max_length, and digit regex [OK]

Hint: Use constr with min_length, max_length, and digit regex [OK]

Common Mistakes:

• Using min and max instead of min_length and max_length
• Using regex that excludes digits
• Trying to add regex to plain str type