Example data in schema in FastAPI

Introduction

Example data helps show what kind of information your API expects or returns. It makes your API easier to understand and test.

When you want to show sample input data for an API endpoint.

When you want to document the expected format of data in your API.

When you want to help frontend developers understand what data to send.

When you want to generate automatic API docs with clear examples.

When you want to test your API with realistic sample data.

Syntax

FastAPI

from pydantic import BaseModel, Field

class Item(BaseModel):
    name: str = Field(..., example="Apple")
    price: float = Field(..., example=1.99)
    is_offer: bool = Field(default=False, example=True)

The example parameter inside Field() sets example data for each field.

This example data appears in the automatic API docs generated by FastAPI.

Examples

Example data for username and email fields helps show expected input format.

FastAPI

from pydantic import BaseModel, Field

class User(BaseModel):
    username: str = Field(..., example="johndoe")
    email: str = Field(..., example="[e-mail]")

Example data can be numbers, strings, or booleans depending on the field type.

FastAPI

from pydantic import BaseModel, Field

class Product(BaseModel):
    id: int = Field(..., example=123)
    name: str = Field(..., example="T-shirt")
    price: float = Field(..., example=19.99)

Sample Program

This FastAPI app defines an Item schema with example data. When you visit the API docs, you will see these examples. The POST endpoint returns the item you send.

FastAPI

from fastapi import FastAPI
from pydantic import BaseModel, Field

app = FastAPI()

class Item(BaseModel):
    name: str = Field(..., example="Banana")
    price: float = Field(..., example=0.99)
    is_offer: bool = Field(default=False, example=True)

@app.post("/items/")
async def create_item(item: Item):
    return item

Important Notes

Example data does not affect validation; it only helps with documentation and testing.

Use clear and realistic examples to make your API easier to use.

Summary

Example data shows sample values for API fields.

It helps users understand what data to send or expect.

FastAPI uses Field(..., example=...) to add example data.

Practice

1. What is the main purpose of adding example data in a FastAPI schema field?

easy

A. To automatically generate database tables

B. To validate the data type of the field at runtime

C. To encrypt the field data before sending

D. To show sample values for API fields to help users understand expected data

Solution

1. Step 1: Understand the role of example data in FastAPI

   Example data is used to provide sample values in API documentation, helping users know what to send or expect.

2. Step 2: Differentiate example data from validation or encryption

   Example data does not validate or encrypt; it only shows sample values in docs.

3. Final Answer:

   To show sample values for API fields to help users understand expected data -> Option D

4. Quick Check:

   Example data = sample values [OK]

Hint: Example data shows sample values in docs, not validation [OK]

Common Mistakes:

• Confusing example with validation
• Thinking example encrypts data
• Assuming example creates database schema

2. Which of the following is the correct way to add example data to a Pydantic model field in FastAPI?

easy

A. name: str = Field(example="John Doe")

B. name: str = Field(default="John Doe")

C. name: str = Field(description="John Doe")

D. name: str = Field(required=True)

Solution

1. Step 1: Recall the Field syntax for example data

   FastAPI uses Field(..., example=...) to add example data to schema fields.

2. Step 2: Identify the correct option

   name: str = Field(example="John Doe") uses Field(example="John Doe"), which is the correct syntax for example data.

3. Final Answer:

   name: str = Field(example="John Doe") -> Option A

4. Quick Check:

   Field(example=...) = correct syntax [OK]

Hint: Use Field(example=...) to add example data [OK]

Common Mistakes:

• Using default instead of example
• Confusing description with example
• Omitting Field() entirely

3. Given this Pydantic model in FastAPI:

from pydantic import BaseModel, Field

class User(BaseModel):
    username: str = Field(..., example="alice123")
    age: int = Field(..., example=30)

What example data will appear in the OpenAPI docs for the age field?

medium

A. "30"

B. 30

C. None

D. 0

Solution

1. Step 1: Check the example value assigned to the age field

   The age field has example=30, which is an integer value 30.

2. Step 2: Understand how example data is shown in docs

   FastAPI shows the example exactly as given, so the example for age will be 30 (integer).

3. Final Answer:

   30 -> Option B

4. Quick Check:

   Example for age = 30 [OK]

Hint: Example shows exact value given in Field(example=...) [OK]

Common Mistakes:

• Thinking example converts int to string
• Assuming example is None if not set
• Confusing example with default value

4. Identify the error in this FastAPI Pydantic model using example data:

from pydantic import BaseModel, Field

class Item(BaseModel):
    name: str = Field(example=123)
    price: float = Field(..., example="9.99")

medium

A. Example for name should be a string, not an integer

B. Missing default value for name field

C. Example for price should be a float, not a string

D. Field(...) is missing for name field

Solution

1. Step 1: Check example data types against field types

   The name field is str but example=123 (int) is allowed but not ideal; price is float but example="9.99" is a string, which is incorrect.

2. Step 2: Identify the type mismatch causing error

   Example for price should be a float (9.99), not a string "9.99"; this can cause schema confusion.

3. Final Answer:

   Example for price should be a float, not a string -> Option C

4. Quick Check:

   Example type must match field type [OK]

Hint: Match example type to field type exactly [OK]

Common Mistakes:

• Using string example for numeric fields
• Ignoring type mismatch warnings
• Omitting Field(...) for required fields

5. You want to create a FastAPI schema for a product with a nested model and provide example data for both levels. Which is the correct way to add example data for nested fields?

hard

A. Use example in Field() for each nested field separately

B. Add example data only to the top-level model, nested fields inherit automatically

C. Use example only in the nested model class, not in the parent

D. Example data cannot be added to nested models in FastAPI

Solution

1. Step 1: Understand example data usage in nested models

   Each field in nested models needs its own example data using Field(example=...) to show correct samples in docs.

2. Step 2: Confirm nested fields do not inherit example data

   Example data is not inherited; you must specify it explicitly for each nested field.

3. Final Answer:

   Use example in Field() for each nested field separately -> Option A

4. Quick Check:

   Example data must be set per field [OK]

Hint: Add example=... to every nested field explicitly [OK]

Common Mistakes:

• Assuming nested fields inherit example data
• Skipping example in nested models
• Thinking example data is only for top-level fields