Field types and constraints in FastAPI - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the purpose of field types in FastAPI models?

Field types define the kind of data a field can hold, like strings, integers, or dates. They help FastAPI validate and convert incoming data automatically.

beginner

How do constraints improve data validation in FastAPI?

Constraints limit the values a field can accept, such as minimum or maximum length for strings, or value ranges for numbers. This ensures data meets specific rules before processing.

intermediate

Which FastAPI/Pydantic field type would you use for a required string with a minimum length of 3?

You would use str with a constraint like Field(..., min_length=3) to require a string with at least 3 characters.

beginner

What does the ellipsis ... mean when used in a FastAPI field declaration?

The ellipsis ... means the field is required and must be provided by the user. It tells FastAPI not to use a default value.

intermediate

How can you restrict a numeric field to accept only values between 1 and 10 in FastAPI?

Use Field(..., ge=1, le=10) where ge means 'greater or equal' and le means 'less or equal'. This limits the number to the range 1 to 10.

In FastAPI, which of these makes a field required?

A Using <code>Optional[str]</code>

B Using <code>Field(...)</code>

C Setting <code>default=None</code>

D Leaving the field out of the model

Using Field(...) tells FastAPI the field must be provided. default=None makes it optional.

Which constraint limits a string's minimum length in FastAPI?

A max_length

B le

C ge

D min_length

min_length sets the minimum number of characters allowed in a string.

How do you restrict a number to be at least 5 in FastAPI?

A Field(..., ge=5)

B Field(..., min=5)

C Field(..., max=5)

D Field(..., le=5)

ge=5 means 'greater than or equal to 5', which restricts the number to 5 or more.

What type would you use for a field that holds a date in FastAPI?

A str

B int

C datetime.date

D bool

Use datetime.date to represent dates properly in FastAPI models.

Which of these is NOT a valid constraint in FastAPI's Field?

A contains

B min_length

C max_length

D le

contains is not a valid constraint. Use min_length, max_length, ge, and le instead.

Explain how to use field types and constraints in FastAPI to validate user input.

Think about how you tell FastAPI what data to expect and what rules it must follow.

Describe the difference between optional and required fields in FastAPI models.

Consider how FastAPI knows if a field must be given or can be skipped.