What are Python Variable Annotations and How Do They Work?

What are variable annotations?

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.

How does primes: List[int] = [] assign a type?

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.

What changes does it bring?

Variable annotations bring a number of changes to Python, including:

• Syntax: The new : syntax for variable annotations
• Attribute: The __annotations__ attribute for classes and modules
• Type checking: Variable annotations can be used by type checking tools to ensure code is type-safe

Will I be forced to use it?

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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