Variable annotations are a new feature in Python 3.6 that allow you to attach type metadata to variables. This is a follow-on to PEP 484, which introduced type hints for function parameters.
Just as for function annotations, the Python interpreter does not attach any particular meaning to variable annotations. The interpreter simply stores the type information in a special attribute named __annotations__. This attribute is available for classes and modules.
The syntax for variable annotations is simple. You simply specify the type of the variable after a colon (:) character. For example, the following code defines a variable named primes that is annotated as a list of integers:
primes: List[int] = []
You can also annotate variables that are assigned a value at the same time. For example, the following code defines a variable named captain that is annotated as a string:
captain: str = "Picard"
Variable annotations are completely optional. However, they can be very useful for type checking tools and other code analysis tools. These tools can use the type annotations to ensure that your code is type-safe.
The code primes: List[int] = [] assigns the type List[int] to the variable primes. This means that primes is expected to hold a list of integers. The [] part of the code initializes primes with an empty list.
Variable annotations bring a number of changes to Python, including:
No, variable annotations are completely optional. You can continue to use Python without using annotations. However, if you are using a type checking tool, you may want to consider using variable annotations to improve the accuracy of the type checker.
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