Example data in schema in FastAPI - Step-by-Step Execution

Concept Flow - Example data in schema

Define Pydantic Schema

↓

Add example data in Config

↓

Use schema in FastAPI endpoint

↓

Auto-generate docs show example

↓

Client sees example data in docs

This flow shows how to add example data to a Pydantic schema and how FastAPI uses it to show examples in API docs.

Execution Sample

FastAPI

from pydantic import BaseModel

class User(BaseModel):
    name: str
    age: int

    class Config:
        schema_extra = {"example": {"name": "Alice", "age": 30}}

Defines a User schema with example data that FastAPI will show in API docs.

Execution Table

Step	Action	Evaluation	Result
1	Define User schema with fields name and age	Class created	User schema ready
2	Add example data in Config.schema_extra	Example attached	{"name": "Alice", "age": 30} available
3	Use User schema in FastAPI endpoint	Endpoint accepts User	Example shown in docs
4	Client opens docs	Docs render example	Example data visible to client
5	Client sends request with example data	Request accepted	Server processes User data
6	End of flow	No more steps	Execution stops

💡 All steps completed, example data is used in docs and requests

Variable Tracker

Variable	Start	After Step 2	After Step 3	Final
User.schema_extra	None	{"example": {"name": "Alice", "age": 30}}	{"example": {"name": "Alice", "age": 30}}	{"example": {"name": "Alice", "age": 30}}
FastAPI endpoint	Not defined	Not defined	Uses User schema	Uses User schema with example

Key Moments - 2 Insights

Why does the example data appear in the API docs?

Because the example is set inside the schema's Config.schema_extra, FastAPI reads it and shows it automatically in the docs (see execution_table step 4).

Does the example data affect the actual data validation?

No, example data is only for documentation and testing help. Validation uses the schema fields, not the example (see execution_table step 5).

Visual Quiz - 3 Questions

Look at the execution table, at which step is the example data attached to the schema?

A Step 3

B Step 1

C Step 2

D Step 4

💡 Hint

Check the 'Action' column for when example data is added to Config.schema_extra.

According to the variable tracker, what is the value of User.schema_extra after step 3?

A {"example": {"name": "Alice", "age": 30}}

B None

C Empty dictionary

D Undefined

💡 Hint

Look at the 'After Step 3' column for User.schema_extra in variable_tracker.

If we remove the example from Config, what will happen in the docs?

A Docs will break and not load

B Docs will show no example data

C Docs will show the last example data anyway

D Docs will show an error message

💡 Hint

Refer to the concept_flow where example data is added to show in docs.

Concept Snapshot

Define a Pydantic schema with fields
Add example data inside Config.schema_extra
Use schema in FastAPI endpoint
FastAPI auto-shows example in docs
Example helps users understand API input
Example does not affect validation

Full Transcript

This visual execution shows how to add example data to a Pydantic schema in FastAPI. First, you define a schema class with fields like name and age. Then, inside the Config class, you add schema_extra with an example dictionary. When you use this schema in a FastAPI endpoint, the example data automatically appears in the generated API documentation. This helps clients see sample input data. The example data is only for docs and does not affect how FastAPI validates incoming requests. The execution table traces each step from defining the schema to the client seeing the example in docs. The variable tracker shows how the example data is stored in the schema. Key moments clarify common confusions about example usage and validation. The quiz tests understanding of when and how example data is attached and used.

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