Field types and constraints in FastAPI - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to declare a string field with a maximum length of 50 characters.

FastAPI

from pydantic import BaseModel, Field

class User(BaseModel):
    name: str = Field(max_length=[1])

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

A50

B100

C0

D10

Attempts:

3 left

💡 Hint

Common Mistakes

Using min_length instead of max_length

Setting length to 0 which means no characters allowed

The max_length parameter limits the string length to 50 characters.

2fill in blank

medium

Complete the code to declare an integer field that must be greater than or equal to 18.

FastAPI

from pydantic import BaseModel, Field

class User(BaseModel):
    age: int = Field(ge=[1])

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

A1

B0

C21

D18

Attempts:

3 left

💡 Hint

Common Mistakes

Using gt instead of ge

Setting the value too low like 0

The ge parameter means "greater than or equal to". Setting it to 18 enforces the minimum age.

3fill in blank

hard

Fix the error in the code by completing the field declaration to require a string with a minimum length of 3.

FastAPI

from pydantic import BaseModel, Field

class Product(BaseModel):
    code: str = Field(min_length=[1])

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

A1

B0

C3

D5

Attempts:

3 left

💡 Hint

Common Mistakes

Using max_length instead of min_length

Setting the value too low like 0 or 1

The min_length parameter sets the minimum number of characters required. Here, 3 ensures the string is at least 3 characters long.

4fill in blank

hard

Fill both blanks to declare a float field that must be between 0.0 and 100.0 inclusive.

FastAPI

from pydantic import BaseModel, Field

class Score(BaseModel):
    value: float = Field(ge=[1], le=[2])

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

A0.0

B100.0

C50.0

D1.0

Attempts:

3 left

💡 Hint

Common Mistakes

Mixing up ge and le

Using integer values instead of floats

The ge parameter sets the minimum value, and le sets the maximum value. Here, the score must be between 0.0 and 100.0.

5fill in blank

hard

Fill both blanks to create a string field that is required, has a minimum length of 5, and a maximum length of 20.

FastAPI

from pydantic import BaseModel, Field

class Account(BaseModel):
    username: str = Field(..., min_length=[1], max_length=[2])

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

Arequired=True

B5

C20

Dnullable=False

Attempts:

3 left

💡 Hint

Common Mistakes

Using invalid arguments like required=True or nullable=False

Setting the length values too low like 0

The min_length and max_length set the string length limits. The ... means the field is required.

Practice

1. In FastAPI, what is the main purpose of using Field() in a Pydantic model?

easy

A. To define API routes

B. To create database tables automatically

C. To add validation rules and metadata to model fields

D. To handle HTTP requests directly

Solution

1. Step 1: Understand the role of Pydantic models in FastAPI

   Pydantic models define the data structure and validation for request and response data.

2. Step 2: Identify what Field() adds

   Field() allows adding constraints like min/max length, default values, and descriptions to model fields.

3. Final Answer:

   To add validation rules and metadata to model fields -> Option C

4. Quick Check:

   Field() = validation and metadata [OK]

Hint: Field() is for validation and extra info on model fields [OK]

Common Mistakes:

• Thinking Field() creates routes or handles requests
• Confusing Field() with database table creation
• Assuming Field() is for HTTP methods

2. Which of the following is the correct way to require a string field with a minimum length of 3 in a Pydantic model using Field()?

easy

A. name: str = Field(required=True, length=3)

B. name: str = Field(min=3)

C. name: str = Field(length=3)

D. name: str = Field(min_length=3)

Solution

1. Step 1: Recall the correct parameter for minimum length in Field()

   The correct parameter to set minimum string length is min_length.

2. Step 2: Check each option for correct syntax

   name: str = Field(min_length=3) uses min_length=3, which is correct. Others use invalid or incorrect parameters.

3. Final Answer:

   name: str = Field(min_length=3) -> Option D

4. Quick Check:

   Minimum string length uses min_length [OK]

Hint: Use min_length for string length constraints in Field() [OK]

Common Mistakes:

• Using min instead of min_length for strings
• Using length instead of min_length
• Adding required=True which is not a Field() parameter

3. Given this Pydantic model snippet:

from pydantic import BaseModel, Field

class Item(BaseModel):
    price: float = Field(gt=0)
    name: str = Field(min_length=2, max_length=10)

What happens if you create Item(price=-5, name='A')?

medium

A. Validation error because price is not greater than 0 and name is too short

B. Item is created successfully with given values

C. Validation error only for price, name is accepted

D. Validation error only for name, price is accepted

Solution

1. Step 1: Check the constraints on price

   Price must be greater than 0 (gt=0). Given price is -5, which fails this.

2. Step 2: Check the constraints on name

   Name must have length between 2 and 10. Given name is 'A' (length 1), which fails min_length=2.

3. Final Answer:

   Validation error because price is not greater than 0 and name is too short -> Option A

4. Quick Check:

   Both price and name fail constraints [OK]

Hint: Check all Field constraints; both price and name fail here [OK]

Common Mistakes:

• Assuming only one field causes error
• Ignoring min_length constraint on strings
• Thinking negative price passes gt=0

4. What is wrong with this FastAPI Pydantic model field definition?

from pydantic import BaseModel, Field

class User(BaseModel):
    age: int = Field(min_length=18)

medium

A. Field cannot be used with integers

B. min_length is invalid for integer fields

C. Missing default value for age

D. Age should be a string, not int

Solution

1. Step 1: Understand Field constraints for int type

   Integer fields use gt or ge for minimum value, not min_length.

2. Step 2: Identify the error in the code

   Using min_length on an int field causes a validation error because it applies only to strings or collections.

3. Final Answer:

   min_length is invalid for integer fields -> Option B

4. Quick Check:

   min_length applies to strings, not ints [OK]

Hint: Use gt/ge for numbers, min_length for strings only [OK]

Common Mistakes:

• Using string constraints on numeric fields
• Thinking Field() can't be used with int
• Confusing default value requirement

5. You want to create a FastAPI model for a product with a tags field that must be a list of strings, each string at least 2 characters long. Which is the best way to enforce this using Pydantic Field()?

hard

A. from typing import List
from pydantic import BaseModel, Field, conlist, constr

class Product(BaseModel):
tags: conlist(constr(min_length=2))

B. from typing import List
from pydantic import BaseModel, Field

class Product(BaseModel):
tags: List[str] = Field(min_length=2)

C. from typing import List
from pydantic import BaseModel, Field

class Product(BaseModel):
tags: List[str] = Field(min_items=2)

D. from typing import List
from pydantic import BaseModel, Field

class Product(BaseModel):
tags: List[str] = Field(min_length=2, max_length=10)

Solution

1. Step 1: Understand the requirement for list of strings with string length constraints

   We need to enforce each string in the list to have at least 2 characters.

2. Step 2: Identify how to apply constraints on list items

   Pydantic's constr can constrain strings, and conlist can constrain lists of constrained types.

3. Step 3: Evaluate options

   from typing import List from pydantic import BaseModel, Field, conlist, constr class Product(BaseModel): tags: conlist(constr(min_length=2)) uses conlist(constr(min_length=2)) which correctly enforces each string's min length inside the list. Others either apply min_length to the list itself or incorrectly.

4. Final Answer:

   Use conlist with constr to enforce string length inside list -> Option A

5. Quick Check:

   conlist(constr(min_length=2)) = correct nested constraints [OK]

Hint: Use conlist with constr for list of strings with length limits [OK]

Common Mistakes:

• Applying min_length to the list instead of strings inside
• Not using constr for string constraints
• Confusing min_items with min_length