Generics/templates in python?

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The other answers are totally fine:

  • One does not need a special syntax to support generics in Python
  • Python uses duck typing as pointed out by AndrĂ©.

However, if you still want a typed variant, there is a built-in solution since Python 3.5.

A full list of available type annotations is available in the Python documentation.

Generic classes:

from typing import TypeVar, Generic, List

T = TypeVar('T')

class Stack(Generic[T]):
    def __init__(self) -> None:
        # Create an empty list with items of type T
        self.items: List[T] = []

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

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

    def empty(self) -> bool:
        return not self.items

# Construct an empty Stack[int] instance
stack = Stack[int]()
stack.push(2)
stack.pop()
stack.push('x')        # Type error

Generic functions:

from typing import TypeVar, Sequence

T = TypeVar('T')      # Declare type variable

def first(seq: Sequence[T]) -> T:
    return seq[0]

def last(seq: Sequence[T]) -> T:
    return seq[-1]


n = first([1, 2, 3])  # n has type int.

Static type checking:

You must use a static type checker such as mypy or Pyre (developed by Meta/FB) to analyze your source code.

Install mypy:

python3 -m pip install mypy

Analyze your source code, for example a certain file:

mypy foo.py

or directory:

mypy some_directory

mypy will detect and print type errors. A concrete output for the Stack example provided above:

foo.py:23: error: Argument 1 to "push" of "Stack" has incompatible type "str"; expected "int"

References: mypy documentation about generics and running mypy

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