Chapter 32: Custom Web Components and Framework Extensions

ASP.NET Core developers often build custom model binders, tag helpers, middleware, and filters. Python gives you several smaller extension points.

Useful extension targets:

• Pydantic custom types for reusable validation.
• FastAPI dependencies for request-specific services.
• Starlette middleware for cross-cutting behavior.
• Jinja2 filters and macros for presentation helpers.
• Exception handlers for domain-to-HTTP mapping.
• Custom response classes for specialized output.

Example custom Pydantic type usage:

from pydantic import BaseModel, Field


class SlugCommand(BaseModel):
    slug: str = Field(pattern=r"^[a-z0-9]+(?:-[a-z0-9]+)*$")

Extension rule: start boring. Most teams need fewer framework extensions than they think. Reach for custom components only when repeated code becomes harder to understand than the abstraction.