An introduction to Python Type Hints - Deepstash
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Type Annotations

Type Annotations

PEP 484 introduced type hints (typing module), a.k.a type annotations. While its main focus was function annotations, it also introduced the notion of type comments to annotate variables.

Ex: primes = [] #type: List[int]

Although type comments work well enough, there are some downsides:

  • text editors highlight comments differently
  • one can’t annotate uninitialized variables
  • mixing type comments with normal ones is confusing

The solution: Variable Annotations


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Variable Annotations

Variable Annotations

PEP 526 introduced the syntax for Variable Annotations:

var: annotation = value

Variable Annotations don’t affect the Python principle of being a dynamically typed language.

Type annotations can’t create errors, they are checked only by third party type checkers like mypy.

See image example.


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Class and Instance Variable Annotations

Class and Instance Variable Annotations

Type annotations can also be used to annotate class and instance variables in class bodies and methods.

In particular, the value-less notation: a: int allows one to annotate instance variables that should be initialized in __init__ or __new__. See the syntax in the picture.

As one knows, defining class member variables is not mandatory but it’s useful for providing autocomplete functionality while coding.

The differences between class and instance variables are discussed in the following idea


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Class Variables vs. Instance Variables

Class Variables vs. Instance Variables

When one defines a member variable in the class body definition, the interpreter considers it a class variable.

class Dog:

name: str = ‘Doggo’ # class variable

In the __init__ constructor, one does have access at the ‘name’ class variable using ‘self’. But as self represents the instance of the class, in the constructor scope, ‘’ becomes an instance variable.

By deleting the ‘name’ attribute from a Dog instance, one can access the ‘name’ class variable once again.


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