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Type Hints & Annotations

Generics

TypeVar and Generic — building reusable, type-safe containers and functions that work with any type while maintaining static checking.

Interview: Advanced typing — shows ability to design reusable, type-safe abstractions.

Last Updated: June 12, 2026 • 8 min read

Generics allow you to write classes and functions that work with any type while maintaining type safety. Using TypeVar and Generic, you can create reusable containers (Stack, Queue, Cache) where the element type is tracked by the type checker.

TypeVar

  • T = TypeVar('T') — unconstrained, accepts any type
  • T = TypeVar('T', int, str) — constrained to int or str only
  • T = TypeVar('T', bound=Number) — must be a subclass of Number
  • Same TypeVar used in params and return type links them together

Generic Classes

  • Inherit from Generic[T] to make a class parameterized by type
  • Stack[int] creates a stack that only accepts ints
  • Python 3.12+ syntax: class Stack[T]: (no TypeVar needed)

Interview Insight

Be able to implement a generic Stack or Cache class. Know how TypeVar links input and output types — if input is T, output is also T, not Any.

Use Cases

Reusable containers — Stack, Queue, Cache, LinkedList with type safety

Utility functions — first, last, identity that work with any type

Repository pattern — generic data access layer

Middleware — generic request/response handlers

Builder patterns — type-safe fluent API builders

Common Mistakes

Using Any instead of TypeVar — loses type safety benefits

Not linking TypeVar in params and return type — checker can't infer output

Forgetting to inherit from Generic[T] in generic classes

Using mutable default arguments in generic class __init__

Not using bound or constraints when the type should be restricted