String validation (min, max, regex) in FastAPI - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the purpose of string validation in FastAPI?

String validation ensures that input strings meet specific rules like minimum length, maximum length, or matching a pattern. This helps keep data clean and safe.

beginner

How do you set a minimum and maximum length for a string in FastAPI using Pydantic?

Use the Field function with min_length and max_length parameters inside your Pydantic model. For example: <br>name: str = Field(..., min_length=3, max_length=50)

intermediate

How can you validate a string against a regular expression in FastAPI?

Use <code>constr</code> from Pydantic with the <code>regex</code> parameter. For example: <br><code>from pydantic import constr<br>email: constr(regex=r'^[\w\.-]+@[\w\.-]+\.\w+$')</code>

beginner

What happens if a string input does not meet the validation rules in FastAPI?

FastAPI automatically returns a clear error response with details about which validation rule failed. This helps users fix their input.

intermediate

Write a Pydantic model field that requires a username string between 4 and 20 characters and only letters or numbers.

Use constr with min_length=4, max_length=20, and a regex for letters and numbers: <br>username: constr(min_length=4, max_length=20, regex=r'^[a-zA-Z0-9]+$')

Which Pydantic function is used to add min and max length constraints to a string field in FastAPI?

A BaseModel

B constr

C validator

D Field

The Field function allows setting min_length and max_length for string fields.

How do you enforce a string to match a pattern using regex in FastAPI's Pydantic models?

A Use <code>constr(regex=...)</code>

B Use <code>BaseModel(regex=...)</code>

C Use <code>Field(regex=...)</code>

D Use <code>validator(regex=...)</code>

The constr type allows regex pattern matching for strings.

What error response does FastAPI return if string validation fails?

A 500 Internal Server Error

B 422 Unprocessable Entity with details

C 200 OK with warning

D 404 Not Found

FastAPI returns a 422 status with details about validation errors.

Which import is needed to use constr in FastAPI models?

A from pydantic import constr

B from fastapi import Field

C from typing import constr

D from fastapi import constr

constr is imported from Pydantic.

What does min_length=5 do in a string field validation?

A Sets the string length to exactly 5

B Allows strings shorter than 5

C Requires string length to be at least 5 characters

D Limits string length to 5 characters maximum

min_length=5 means the string must be 5 or more characters long.

Explain how to validate a string field in FastAPI to have a minimum length, maximum length, and match a regex pattern.

Think about combining Field and constr from Pydantic.

Describe what happens when a user sends a string that does not meet validation rules in a FastAPI app.

Focus on FastAPI's automatic error handling for validation.