Optional and nullable fields in FastAPI - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to make the 'description' field optional in a FastAPI model.

FastAPI

from typing import [1]
from pydantic import BaseModel

class Item(BaseModel):
    name: str
    description: [1][str] = None

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

ASet

BList

COptional

DDict

Attempts:

3 left

💡 Hint

Common Mistakes

Using List or Dict instead of Optional

Not importing Optional from typing

Setting default to empty string instead of None

Using Optional from typing allows the field to be optional, meaning it can be None or a string.

2fill in blank

medium

Complete the code to allow the 'price' field to be nullable (accept None) in a FastAPI model.

FastAPI

from typing import Optional
from pydantic import BaseModel

class Product(BaseModel):
    name: str
    price: Optional[[1]] = None

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

Afloat

Bstr

Cint

Dbool

Attempts:

3 left

💡 Hint

Common Mistakes

Using int instead of float for price

Not setting default to None for nullable

Using str for price

The price field should be a float or None, so we use Optional[float].

3fill in blank

hard

Fix the error in the code to correctly declare an optional nullable field 'tags' as a list of strings.

FastAPI

from typing import [1], List
from pydantic import BaseModel

class BlogPost(BaseModel):
    title: str
    tags: [1][List[str]] = None

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

ACallable

BOptional

CAny

DUnion

Attempts:

3 left

💡 Hint

Common Mistakes

Using Union without specifying None

Using Any which is too broad

Not importing Optional

To make 'tags' optional and nullable, wrap the list type with Optional.

4fill in blank

hard

Fill both blanks to define a FastAPI model with an optional nullable 'rating' field of type float and a required 'title' field.

FastAPI

from typing import [1]
from pydantic import BaseModel

class Review(BaseModel):
    title: str
    rating: [2][float] = None

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

AOptional

BList

CDict

DSet

Attempts:

3 left

💡 Hint

Common Mistakes

Using different types for import and annotation

Using List or Dict instead of Optional

Not setting default to None

Both blanks use Optional to make the 'rating' field optional and nullable.

5fill in blank

hard

Fill all three blanks to create a FastAPI model with an optional nullable 'email' field of type string, a required 'username' field, and an optional nullable 'age' field of type int.

FastAPI

from typing import [1]
from pydantic import BaseModel

class User(BaseModel):
    username: str
    email: [2][str] = None
    age: [3][int] = None

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

AOptional

BList

DDict

Attempts:

3 left

💡 Hint

Common Mistakes

Using List or Dict instead of Optional

Not setting default to None

Mixing different types for optional fields

Use Optional for all optional nullable fields. Here, email and age are optional and nullable.

Practice

1. In FastAPI, what does it mean when a field is declared as Optional[str] in a request model?

easy

A. The field is required and cannot be None.

B. The field must always be provided with a string value.

C. The field can only be an empty string but not None.

D. The field can be omitted or set to None in the request.

Solution

1. Step 1: Understand Optional type in FastAPI

   Optional[str] means the field can be missing or have a value of type str or None.

2. Step 2: Interpret the meaning for API requests

   This allows clients to omit the field or explicitly send null (None) as its value.

3. Final Answer:

   The field can be omitted or set to None in the request. -> Option D

4. Quick Check:

   Optional[str] = omitted or None [OK]

Hint: Optional means field can be missing or None [OK]

Common Mistakes:

• Thinking Optional means field is always required
• Confusing Optional with default values
• Assuming Optional forbids None

2. Which of the following is the correct way to declare a nullable string field in a FastAPI Pydantic model?

easy

A. name: Optional[str] = None

B. name: str | None = None

C. name: str = None

D. name: str?

Solution

1. Step 1: Recall Pydantic syntax for optional and nullable fields

   To allow a field to be nullable and optional, use Optional[str] with a default None.

2. Step 2: Check each option

   A is invalid because just assigning None to str without Optional is not correct. B is valid Python 3.10+ syntax but C is the recommended explicit way in FastAPI. D is invalid syntax.

3. Final Answer:

   name: Optional[str] = None -> Option A

4. Quick Check:

   Nullable string = Optional[str] = None [OK]

Hint: Use Optional[type] with = None for nullable fields [OK]

Common Mistakes:

• Using just type with = None without Optional
• Using invalid syntax like str?
• Confusing Optional with just default values

3. Given this FastAPI model:

from typing import Optional
from pydantic import BaseModel

class User(BaseModel):
    name: str
    nickname: Optional[str] = None

user = User(name='Alice')

What will be the value of user.nickname?

medium

A. '' (empty string)

B. 'Alice'

C. None

D. Raises an error

Solution

1. Step 1: Analyze the model fields and defaults

   The nickname field is Optional[str] with a default of None, so if not provided, it defaults to None.

2. Step 2: Check the instance creation

   When creating user with only name='Alice', nickname is not set, so it uses the default None.

3. Final Answer:

   None -> Option C

4. Quick Check:

   Optional field missing = None [OK]

Hint: Missing optional field defaults to None [OK]

Common Mistakes:

• Assuming nickname defaults to empty string
• Thinking nickname copies name value
• Expecting an error for missing optional field

4. What is wrong with this FastAPI model declaration?

from typing import Optional
from pydantic import BaseModel

class Item(BaseModel):
    description: Optional[str]

medium

A. Optional[str] is not allowed in Pydantic models.

B. Missing default value for Optional field causes validation error.

C. Field description must be declared as str, not Optional[str].

D. No import for Optional is present.

Solution

1. Step 1: Understand Optional fields in Pydantic

   Optional means the field can be None or missing, but Pydantic requires a default value for optional fields.

2. Step 2: Check the model declaration

   The field description is Optional[str] but has no default value, so Pydantic will expect it to be provided and raise an error if missing.

3. Final Answer:

   Missing default value for Optional field causes validation error. -> Option B

4. Quick Check:

   Optional fields need default values [OK]

Hint: Optional fields must have default values [OK]

Common Mistakes:

• Omitting default None for Optional fields
• Thinking Optional alone makes field optional
• Ignoring Pydantic validation rules

5. You want to create a FastAPI model where the field email can be omitted, but if present, it can be set to null explicitly. Which is the best way to declare this field?

hard

A. email: Optional[str] = None

B. email: str = None

C. email: str | None

D. email: Optional[str]

Solution

1. Step 1: Understand requirements for the field

   The field must be optional (can be omitted) and nullable (can be None explicitly).

2. Step 2: Evaluate each declaration

   B lacks Optional and default None, so it is not optional. C lacks default None, so field is required. D lacks default None, so field is required. A uses Optional[str] with default None, making it optional and nullable.

3. Final Answer:

   email: Optional[str] = None -> Option A

4. Quick Check:

   Optional with default None = optional and nullable [OK]

Hint: Use Optional[type] with = None for optional nullable fields [OK]

Common Mistakes:

• Forgetting to assign default None
• Using union without default value
• Assuming Optional alone makes field optional