Body - Fields

Body - Fields

🎯 What You'll Learn

This lesson follows the official FastAPI "Body - Fields" tutorial exactly. You'll learn how to use Pydantic's Field() function to add validation and metadata to model attributes, making your APIs more robust and well-documented.

By the end of this lesson, you'll understand how to:

- Import and use Field from pydantic for model attributes

- Add validation constraints to model fields (gt, max_length, etc.)

- Include metadata like titles and descriptions for better documentation

- Understand the relationship between Field, Query, Path, and Body

📚 Import Field

First, you need to import Field from pydantic:

[Code Example]

⚠️ Important: Notice that Field is imported directly from pydantic, not from fastapi like Query, Path, and Body.

🔧 Declare Model Attributes with Field

You can use Field with model attributes to add validation and metadata:

[Code Example]

Field Parameters

Field works the same way as Query, Path, and Body - it accepts the same validation parameters:

- Validation constraints: gt, ge, lt, le, min_length, max_length, etc.

- Metadata: title, description, example, etc.

- Default values: default, default_factory

🎯 Complete Example

Following the official FastAPI tutorial:

[Code Example]

Request Body Structure

With Body(embed=True), the request expects:

[Code Example]

🌟 Key Concepts

Field vs Function Parameters

Notice the similarity between model attributes with Field and function parameters:

Model attribute with Field:

[Code Example]

Function parameter with Path:

[Code Example]

Both follow the same pattern: parameter: type = ValidationFunction(constraints)

Validation Benefits

Field validation provides:

- Automatic validation: Invalid data returns HTTP 422 with detailed error messages

- Type conversion: Automatic conversion between compatible types

- Documentation: Metadata appears in OpenAPI schema and interactive docs

- IDE support: Better autocomplete and type checking

Technical Details

- Query, Path, Body create objects of subclasses of Param class

- Param is a subclass of Pydantic's FieldInfo class

- Field returns an instance of FieldInfo directly

- All use the same underlying validation system

💡 Best Practices

Validation Constraints

[Code Example]

Descriptive Metadata

[Code Example]

Optional Fields with Defaults

[Code Example]

🔗 Add Extra Information

You can include additional metadata in Field that will appear in the generated JSON Schema:

[Code Example]

⚠️ Warning: Extra keys passed to Field will be present in the OpenAPI schema. Some OpenAPI tools may not work with non-standard keys.

🎯 Validation Examples

Valid Request

[Code Example]

✅ Result: Success - all validations pass

Invalid Requests

Negative price:

[Code Example]

❌ Result: HTTP 422 - "ensure this value is greater than 0"

Description too long:

[Code Example]

❌ Result: HTTP 422 - "ensure this value has at most 300 characters"

🔗 What's Next?

In the next lesson, you'll learn about Body - Nested Models, where you'll discover how to create complex nested Pydantic models with lists, sets, and deeply nested structures.

📖 Additional Resources

- FastAPI Official Tutorial - Body - Fields

- Pydantic Field Documentation

- Pydantic Validators

- OpenAPI Schema Documentation