Field types and constraints in FastAPI - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the purpose of field types in FastAPI models?

Field types define the kind of data a field can hold, like strings, integers, or dates. They help FastAPI validate and convert incoming data automatically.

beginner

How do constraints improve data validation in FastAPI?

Constraints limit the values a field can accept, such as minimum or maximum length for strings, or value ranges for numbers. This ensures data meets specific rules before processing.

intermediate

Which FastAPI/Pydantic field type would you use for a required string with a minimum length of 3?

You would use str with a constraint like Field(..., min_length=3) to require a string with at least 3 characters.

beginner

What does the ellipsis ... mean when used in a FastAPI field declaration?

The ellipsis ... means the field is required and must be provided by the user. It tells FastAPI not to use a default value.

intermediate

How can you restrict a numeric field to accept only values between 1 and 10 in FastAPI?

Use Field(..., ge=1, le=10) where ge means 'greater or equal' and le means 'less or equal'. This limits the number to the range 1 to 10.

In FastAPI, which of these makes a field required?

A Using <code>Optional[str]</code>

B Using <code>Field(...)</code>

C Setting <code>default=None</code>

D Leaving the field out of the model

Using Field(...) tells FastAPI the field must be provided. default=None makes it optional.

Which constraint limits a string's minimum length in FastAPI?

A max_length

B le

C ge

D min_length

min_length sets the minimum number of characters allowed in a string.

How do you restrict a number to be at least 5 in FastAPI?

A Field(..., ge=5)

B Field(..., min=5)

C Field(..., max=5)

D Field(..., le=5)

ge=5 means 'greater than or equal to 5', which restricts the number to 5 or more.

What type would you use for a field that holds a date in FastAPI?

A str

B int

C datetime.date

D bool

Use datetime.date to represent dates properly in FastAPI models.

Which of these is NOT a valid constraint in FastAPI's Field?

A contains

B min_length

C max_length

D le

contains is not a valid constraint. Use min_length, max_length, ge, and le instead.

Explain how to use field types and constraints in FastAPI to validate user input.

Think about how you tell FastAPI what data to expect and what rules it must follow.

Describe the difference between optional and required fields in FastAPI models.

Consider how FastAPI knows if a field must be given or can be skipped.

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