Model inheritance in FastAPI - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

Output of inherited Pydantic model serialization

Given the following FastAPI Pydantic models using inheritance, what is the JSON output of ChildModel().dict()?

FastAPI

from pydantic import BaseModel

class ParentModel(BaseModel):
    id: int = 1
    name: str = "parent"

class ChildModel(ParentModel):
    age: int = 10

result = ChildModel().dict()

A {"id": 1, "name": "parent", "age": 10}

B {"id": 1, "age": 10}

C {"name": "parent", "age": 10}

D {"id": 1, "name": "parent"}

💡 Hint

Remember that child models inherit all fields from parent models.

The ChildModel inherits all fields from ParentModel and adds 'age'. Calling dict() returns all fields with their default values.

📝 Syntax

intermediate

Correct syntax for extending Pydantic model with extra field

Which option correctly defines a Pydantic model ExtendedModel that inherits from BaseModel and adds a new field extra: str with default "extra"?

FastAPI

from pydantic import BaseModel

class ExtendedModel(BaseModel):
    extra: str = "extra"

A

class ExtendedModel(BaseModel):
    extra = str("extra")

B

class ExtendedModel(BaseModel):
    extra str = "extra"

C

class ExtendedModel(BaseModel):
    extra: str "extra"

D

class ExtendedModel(BaseModel):
    extra: str = "extra"

💡 Hint

Check the correct syntax for type annotations in Python classes.

Option D uses correct Python type annotation syntax. Others have syntax errors or incorrect assignments.

🔧 Debug

advanced

Why does this inherited Pydantic model raise ValidationError?

Given these models, why does creating ChildModel(name=123) raise a ValidationError?

FastAPI

from pydantic import BaseModel

class ParentModel(BaseModel):
    name: str

class ChildModel(ParentModel):
    age: int = 10

child = ChildModel(name=123)

A Because ChildModel cannot inherit from ParentModel

B Because 'age' is missing and has no default value

C Because 'name' is expected to be a string but an integer was provided

D Because Pydantic models cannot have default values

💡 Hint

Check the type of the 'name' field and the value passed.

Pydantic validates types strictly. Passing an int where a str is expected causes ValidationError.

❓ state_output

advanced

Value of attribute after model inheritance and override

What is the value of child.name after running this code?

FastAPI

from pydantic import BaseModel

class ParentModel(BaseModel):
    name: str = "parent"

class ChildModel(ParentModel):
    name: str = "child"

child = ChildModel()

A Raises AttributeError

B "child"

C None

D "parent"

💡 Hint

Child class fields override parent fields with the same name.

The ChildModel overrides the 'name' field default to 'child', so child.name is 'child'.

🧠 Conceptual

expert

Effect of multiple inheritance on Pydantic model fields

Given these models, what fields does MultiChildModel have and what are their default values?

FastAPI

from pydantic import BaseModel

class ModelA(BaseModel):
    a: int = 1

class ModelB(BaseModel):
    b: int = 2

class MultiChildModel(ModelA, ModelB):
    c: int = 3

model = MultiChildModel()

A Fields: a=1, b=2, c=3

B Fields: b=2, c=3 (a is ignored)

C Fields: a=1, c=3 (b is ignored)

D Raises TypeError due to multiple inheritance

💡 Hint

Pydantic supports multiple inheritance but only from BaseModel subclasses.

MultiChildModel inherits fields from both ModelA and ModelB, plus its own field c.