Pydantic model basics in FastAPI - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a Pydantic model in FastAPI?

A Pydantic model is a Python class used to define data structures with type annotations. It validates and parses data automatically, ensuring the data matches the expected types.

beginner

How do you define a simple Pydantic model with a name (string) and age (integer)?

You create a class that inherits from BaseModel and add attributes with types, like:<br><pre>from pydantic import BaseModel

class Person(BaseModel):
    name: str
    age: int</pre>

beginner

What happens if you pass wrong data types to a Pydantic model?

Pydantic will raise a validation error explaining which fields have incorrect types. This helps catch mistakes early and keeps data safe.

intermediate

How does Pydantic help with data parsing in FastAPI?

Pydantic automatically converts input data (like JSON) into Python objects with the right types, so you can work with clean, validated data in your code.

intermediate

Can Pydantic models have default values? How?

Yes! You can assign default values to fields in the model class. For example:<br><pre>class Person(BaseModel):
    name: str
    age: int = 30  # default age</pre>

What base class must a Pydantic model inherit from?

A DataModel

B BaseModel

C ModelBase

D Model

Pydantic models always inherit from BaseModel to get validation and parsing features.

If a Pydantic model field is declared as age: int, what happens if you pass a string like '25'?

A It raises an error immediately.

B It stores the string as is.

C It converts the string '25' to integer 25 automatically.

D It ignores the field.

Pydantic tries to convert compatible types automatically, so '25' becomes integer 25.

Which of these is NOT a feature of Pydantic models?

A Data validation

B Default values for fields

C Automatic data parsing

D Database querying

Pydantic does not handle database queries; it focuses on data validation and parsing.

How do you specify an optional field in a Pydantic model?

A Use <code>Optional[type]</code> from typing and provide a default value.

B Just omit the field from the model.

C Use <code>nullable=True</code> in the field definition.

D Use <code>required=False</code> in the class.

Optional fields use typing.Optional and usually have a default value like None.

What will this code output?<br>

from pydantic import BaseModel

class User(BaseModel):
    username: str

user = User(username=123)

A User object with username as string '123'

B Error because username must be string

C User object with username as integer 123

D None

Pydantic converts the integer 123 to string '123' automatically.

Explain how Pydantic models help in validating and parsing data in FastAPI.

Think about how Pydantic checks and converts data for you.

Describe how to define default and optional fields in a Pydantic model.

Defaults and optional fields make your model flexible.