String validation (min, max, regex) in FastAPI - Mini Project: Build & Apply

String validation (min, max, regex) with FastAPI

📖 Scenario: You are building a simple web API using FastAPI. The API will accept a username from users. You want to make sure the username is valid by checking its length and pattern.

🎯 Goal: Create a FastAPI app that accepts a username string with these rules: minimum length 3, maximum length 10, and only letters and numbers allowed. Use Pydantic string validation with min_length, max_length, and regex.

📋 What You'll Learn

Create a FastAPI app instance called app

Create a Pydantic model called User with a username field

Validate username with min_length=3, max_length=10, and a regex that allows only letters and numbers

Create a POST endpoint /users/ that accepts a User model and returns the username

💡 Why This Matters

🌍 Real World

Validating user input is essential in web APIs to prevent bad data and security issues. FastAPI with Pydantic makes this easy and clear.

💼 Career

Backend developers often need to validate incoming data. Knowing how to use FastAPI and Pydantic validation is a valuable skill for building reliable APIs.

1

Create FastAPI app and User model with username field

Write code to import FastAPI and BaseModel from pydantic. Create a FastAPI app instance called app. Create a Pydantic model called User with a single field username of type str.

FastAPI

from fastapi import FastAPI
from pydantic import BaseModel

# Your code here

Hint

Start by importing FastAPI and BaseModel. Then create app = FastAPI(). Define User class inheriting from BaseModel with a username field of type str.

2

Add validation rules to username field

Modify the username field in the User model to add validation: set min_length=3, max_length=10, and a regex pattern that allows only letters and numbers. Use Field(..., min_length=3, max_length=10, regex="^[a-zA-Z0-9]+$").

FastAPI

from fastapi import FastAPI
from pydantic import BaseModel, Field

app = FastAPI()

class User(BaseModel):
    # Add validation to username below
    username: str = Field(...)

Hint

Import Field from pydantic. Use Field(..., min_length=3, max_length=10, regex="^[a-zA-Z0-9]+$") to add validation to username.

3

Create POST endpoint to accept User model

Add a POST endpoint /users/ to the FastAPI app. The endpoint should accept a User model as input and return a dictionary with the key username and the value from the input model.

FastAPI

from fastapi import FastAPI
from pydantic import BaseModel, Field

app = FastAPI()

class User(BaseModel):
    username: str = Field(..., min_length=3, max_length=10, regex="^[a-zA-Z0-9]+$")

# Add POST endpoint below

Hint

Use @app.post("/users/") decorator. Define async function create_user with parameter user: User. Return dictionary with username key and user.username value.

4

Add response model to endpoint

Modify the POST endpoint /users/ to include a response_model=User parameter in the decorator. This ensures the response matches the User model.

FastAPI

from fastapi import FastAPI
from pydantic import BaseModel, Field

app = FastAPI()

class User(BaseModel):
    username: str = Field(..., min_length=3, max_length=10, regex="^[a-zA-Z0-9]+$")

# Add response_model=User to the POST decorator
@app.post("/users/")
async def create_user(user: User):
    return {"username": user.username}

Hint

Add response_model=User inside the @app.post decorator parentheses.

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