Field validation rules in FastAPI - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the purpose of field validation rules in FastAPI?

Field validation rules ensure that the data received in API requests meets expected formats and constraints before processing. This helps prevent errors and improves data quality.

beginner

How do you declare a required string field with a minimum length of 3 in FastAPI using Pydantic?

Use Pydantic's Field with min_length parameter: <br><pre>from pydantic import BaseModel, Field

class Item(BaseModel):
    name: str = Field(..., min_length=3)</pre>

beginner

What does the ellipsis (...) mean when used as a default value in a Pydantic Field?

The ellipsis (...) means the field is required. FastAPI will return an error if this field is missing in the request.

intermediate

How can you validate that an integer field is between 1 and 10 in FastAPI?

Use Field with ge (greater or equal) and le (less or equal) parameters: <br><pre>from pydantic import BaseModel, Field

class Item(BaseModel):
    quantity: int = Field(..., ge=1, le=10)</pre>

advanced

How do you add a custom error message for a field validation in FastAPI?

You can add a custom error message by using Pydantic validators or by customizing the Field with constraints and error messages inside a validator function.

In FastAPI, what does Field(..., min_length=5) mean for a string field?

A The field is required and must have at least 5 characters

B The field is optional and must have at least 5 characters

C The field is required and must have exactly 5 characters

D The field is optional and must have exactly 5 characters

The ellipsis (...) means the field is required, and min_length=5 means it must have at least 5 characters.

Which Pydantic Field parameters enforce a number to be between 10 and 20 inclusive?

A ge=10, le=20

B min=10, max=20

C gt=10, lt=20

D min_length=10, max_length=20

ge means greater or equal, le means less or equal, so ge=10 and le=20 enforce the number between 10 and 20 inclusive.

What happens if a required field is missing in a FastAPI request?

A FastAPI sets the field to None

B FastAPI ignores the missing field

C FastAPI returns a validation error response

D FastAPI crashes

FastAPI returns a validation error response indicating the required field is missing.

How do you specify a default value for a field in FastAPI?

A Use Field(...)

B Assign the value directly in the model, e.g., field: str = 'default'

C Use Field(default=None)

D You cannot specify default values

Assigning a value directly in the model sets a default value for that field.

Which of these is NOT a valid Pydantic Field constraint?

A min_length

B max_length

C ge

D max_value

max_value is not a valid Pydantic Field constraint; use le (less or equal) instead.

Explain how to use FastAPI and Pydantic to validate that a string field is required and has a minimum length.

Think about how you tell FastAPI a field must be there and how long it should be.

Describe how to validate numeric fields in FastAPI to ensure they fall within a specific range.

Consider how to say a number must be between two values.