Type Hinting for Homogeneous Collections

Python 3's type annotations provide a convenient way to specify the expected type of a function's arguments. However, it is not immediately clear whether these annotations can be applied to collections to enforce homogeneity within their elements.

Initial Inability of Function Annotations

As of August 2014, Python's function annotations did not support type hinting for items within collections. This meant that pseudo-code such as the following example was invalid:

def my_func(l: list):
    pass

Instead, formatted docstrings were the recommended method for type hinting within collections:

def my_func(l):
    """
    :type l: list[int]
    """
    pass

Introduction of Type Hinting for Collections

With the advent of PEP 484, Python 3.5 introduced full support for type annotations, including the ability to specify types within collections. The new typing module allowed for explicit declaration of collection types:

from typing import List

def do_something(l: List[str]):
    for s in l:
        s  # str

This improvement enabled IDEs such as PyCharm to provide accurate auto-completion and type checking for collections.

Conclusion

While Python 3 initially lacked support for type hinting within collections, the introduction of PEP 484 and the typing module have made it a breeze to specify and enforce homogeneity in collections. This enhancement has greatly improved type safety and the development experience for Python programmers.