TypeVar and Generic in Python

A type parameter lets one class or function keep its caller's type.

from typing import Generic, TypeVar

T = TypeVar("T")


def first(items: list[T]) -> T | None:
    return items[0] if items else None


class Stack(Generic[T]):
    def __init__(self) -> None:
        self._items: list[T] = []

    def push(self, item: T) -> None:
        self._items.append(item)

    def pop(self) -> T:
        return self._items.pop()


numbers: Stack[int] = Stack()
numbers.push(3)
print(first([1, 2, 3]), first([]), numbers.pop())

How it works

1. A TypeVar links the argument type to the return type.
2. Generic[T] makes a class parameterized.
3. Modern syntax writes the parameter in brackets after the name.

Keywords and builtins used here

• Stack
• class
• def
• else
• first
• if
• int
• list
• pop
• print
• push
• return
• self

The run, in numbers

Lines

23

Characters to type

413

Tokens

147

Three-star pace

110 tpm

At the three-star pace of 110 tokens a minute, this run takes about 80 seconds.

Type this snippet

Step 1 of 4 in Typing in depth, step 41 of 53 in Pythonic Python.

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