How Do Variable Annotations Enhance Python's Type System?

Variable Annotations: Enhancing Python's Type System

Variable annotations were introduced in Python 3.6, following the implementation of type hints with PEP 484. While type hints merely hinted at the expected type of a variable, annotations take the concept further by allowing you to directly specify a variable's type.

Syntax and Features

The new annotation syntax allows for both standalone annotations and annotations during assignments:

# Standalone annotation
number: int

# Annotation during assignment
primes: List[int] = []

The annotated_assignment_stmt syntax covers this new syntax, introducing the ":" character as a delimiter.

In addition, Python 3.6 introduces the annotations attribute for modules and classes. This attribute contains the type annotations for the defined variables.

Accessing Annotations

To access annotations, you can use the get_type_hints function from the typing module. For example:

>>> from typing import get_type_hints
>>> primes: List[int] = []
>>> captain: str
>>> get_type_hints(__main__)
{'primes': typing.List[int]}

Example

Consider the example provided in the question:

primes: List[int] = []

This annotation indicates that the primes variable is a list of integers. Any assignment to this variable must adhere to this type.

Class Variables and StarShip

The Stats variable in the StarShip class is an instance variable, not a class variable. ClassVar is a special type that denotes class variables. Currently, Starship.stats is an instance variable with the type Dict[str, int].

Usage

Variable annotations are optional and primarily intended for use by type-checking tools. They provide an easy and structured way to specify type metadata, enhancing code readability and facilitating the development of custom type-related tools and libraries.

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