Function Annotations for Collection Type Hinting

In Python 3, function annotations are a common approach for specifying types, particularly for homogeneous collections (e.g., lists). However, users sought a method to incorporate collection types into these annotations.

Docstring-Based Type Hinting

Initially, Python developers relied on formatted docstrings, such as reStructuredText or Sphinx, to provide collection type information. These approaches were supported by IDEs, but they were not as concise or integrated as annotations.

登場＊

A significant breakthrough came with PEP 484 (Type Hints) and Python 3.5's introduction of the typing module. This enhancement allowed developers to specify types within collections using type annotations.

For instance, a list of strings can be annotated as:

from typing import List

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

This annotation denotes that the function expects a list of strings, and type information is reflected in IDE code completion.

Constraints

Initially, specifying types within collections using annotations was not supported. However, this limitation was addressed in Python 3.5.

Conclusion

Type annotations now provide a comprehensive and convenient way to specify collection types, enhancing type checking and IDE code completion facilities for Python developers.