Pydantic model basics in FastAPI - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to define a Pydantic model with a name field.

FastAPI

from pydantic import BaseModel

class User([1]):
    name: str

Drag options to blanks, or click blank then click option'

AModelBase

BBaseModel

CModel

DBase

Attempts:

3 left

💡 Hint

Common Mistakes

Using a wrong base class name like Model or Base.

Forgetting to inherit from any class.

The Pydantic model must inherit from BaseModel to work properly.

2fill in blank

medium

Complete the code to create a Pydantic model with an age field of type integer.

FastAPI

from pydantic import BaseModel

class Person(BaseModel):
    age: [1]

Drag options to blanks, or click blank then click option'

Astr

Bbool

Cfloat

Dint

Attempts:

3 left

💡 Hint

Common Mistakes

Using str for age which should be a number.

Using float which allows decimals.

The age field should be an integer, so use int as the type.

3fill in blank

hard

Fix the error in the Pydantic model by completing the missing base class.

FastAPI

from pydantic import BaseModel

class Product([1]):
    id: int
    price: float

Drag options to blanks, or click blank then click option'

AModelBase

BModel

CBaseModel

DProductBase

Attempts:

3 left

💡 Hint

Common Mistakes

Using incorrect base classes that cause runtime errors.

Forgetting to inherit from any class.

The model must inherit from BaseModel to validate fields correctly.

4fill in blank

hard

Fill both blanks to create a Pydantic model with a required string field and an optional integer field.

FastAPI

from typing import [1]
from pydantic import BaseModel

class Item(BaseModel):
    name: str
    quantity: [2][int] = None

Drag options to blanks, or click blank then click option'

AOptional

BList

CDict

DSet

Attempts:

3 left

💡 Hint

Common Mistakes

Using List or Dict which expect collections, not optional values.

Not importing Optional from typing.

Use Optional from typing to mark quantity as optional.

5fill in blank

hard

Fill all three blanks to define a Pydantic model with a string field, a float field with a default, and a boolean field.

FastAPI

from pydantic import BaseModel

class Config(BaseModel):
    env: [1]
    version: [2] = 1.0
    debug: [3] = False

Drag options to blanks, or click blank then click option'

Astr

Bfloat

Cbool

Dint

Attempts:

3 left

💡 Hint

Common Mistakes

Mixing up types like using int for version which is a float.

Forgetting to assign default values for optional fields.

The env field is a string, version is a float with default 1.0, and debug is a boolean with default False.

Practice

1. What is the main purpose of a Pydantic model in FastAPI?

easy

A. To validate and organize data automatically

B. To create HTML templates

C. To manage database connections

D. To handle user authentication

Solution

1. Step 1: Understand Pydantic model role

   Pydantic models are designed to check and organize data by validating types and structure. This matches "To validate and organize data automatically".

2. Final Answer:

   To validate and organize data automatically -> Option A

3. Quick Check:

   Pydantic = Data validation and organization [OK]

Hint: Pydantic models are for data validation, not UI or DB [OK]

Common Mistakes:

• Confusing Pydantic with template engines
• Thinking Pydantic manages databases
• Assuming Pydantic handles authentication

2. Which of the following is the correct way to define a simple Pydantic model with a required string field name?

easy

A. class User:
name = str

B. def User():
name: str

C. class User(BaseModel):
name: str

D. class User(BaseModel):
name = 'str'

Solution

1. Step 1: Recall Pydantic model syntax

   A Pydantic model must inherit from BaseModel and define fields with type annotations like class User(BaseModel): name: str. Other options miss inheritance or use assignment instead of type annotation.

2. Final Answer:

   class User(BaseModel):\n name: str -> Option C

3. Quick Check:

   BaseModel + type annotation = correct model [OK]

Hint: Use BaseModel and type annotations for fields [OK]

Common Mistakes:

• Not inheriting from BaseModel
• Using assignment instead of type annotation
• Defining model as a function

3. Given this Pydantic model and code:

from pydantic import BaseModel

class Item(BaseModel):
    id: int
    name: str

item = Item(id='10', name='Book')
print(item.id, type(item.id))

What will be the output?

medium

A. 10

B. '10'

C. Error: id must be int

D. 10

Solution

1. Step 1: Understand Pydantic type coercion

   Pydantic automatically converts compatible types like string '10' to int 10, so print(item.id, type(item.id)) outputs 10 <class 'int'>.

2. Final Answer:

   10 <class 'int'> -> Option D

3. Quick Check:

   Pydantic converts compatible types automatically [OK]

Hint: Pydantic converts strings to int if possible [OK]

Common Mistakes:

• Expecting a type error for string input
• Assuming no conversion happens
• Confusing output type with input type

4. What is wrong with this Pydantic model definition?

from pydantic import BaseModel

class Product(BaseModel):
    price: float = 'free'

medium

A. Class name should be lowercase

B. Default value 'free' is a string, but price expects a float

C. Missing import for BaseModel

D. Field price must be an integer

Solution

1. Step 1: Check field type and default value

   price: float = 'free' is invalid because the string 'free' cannot be converted to float, causing a validation error.

2. Final Answer:

   Default value 'free' is a string, but price expects a float -> Option B

3. Quick Check:

   Field type and default must match [OK]

Hint: Default values must match field types exactly [OK]

Common Mistakes:

• Using wrong default value types
• Ignoring type mismatch errors
• Thinking class names must be lowercase

5. You want to create a Pydantic model User with an optional age field that defaults to 18 if not provided. Which is the correct way?

hard

A. class User(BaseModel):
age: int = 18

B. class User(BaseModel):
age: Optional[int]

C. class User(BaseModel):
age: int = None

D. class User(BaseModel):
age: int

Solution

1. Step 1: Understand optional field with default

   To make age optional with default 18, use age: int = 18. age: Optional[int] lacks default (required but allows None), age: int = None is invalid (None not int), age: int requires a value.

2. Final Answer:

   class User(BaseModel):\n age: int = 18 -> Option A

3. Quick Check:

   Default value makes field optional [OK]

Hint: Assign default value to make field optional [OK]

Common Mistakes:

• Using Optional without default
• Assigning None to non-optional int
• Forgetting to assign default value