New in Python: Syntax for Variable Annotations

Python 3.6 added another interesting new feature that is known as Syntax for variable annotations. This new feature is outlined in PEP 526. The basic premise of this PEP is to take the idea of Type Hinting (PEP 484) to its next logical step, which is basically adding option type definitions to Python variables, including class variables and instance variables. Please note that adding these annotations or definitions does not suddenly make Python a statically typed language. The interpreter still doesn’t care what type the variable is. However, a Python IDE or other utility like pylint could have an annotation checker added to them that could highlight when you use a variable that you have annotated as one type and then used incorrectly by changing its type mid-function.

Let’s look at a simple example so we can see how this works:

# annotate.py
name: str = 'Mike'

What we have here is a Python file that we have named annotate.py. In it, we have created a variable, name, and annotated it to indicate it is a string. This is done by adding a colon after the variable name and then specifying what type it should be. You don’t have to assign anything to the variable if you don’t want to. The following is just as valid:

# annotate.py
name: str

When you annotate a variable, it will get added to the module’s or class’s __annotations__ attribute. Let’s try importing the first version of our annotate module and access that attribute:

>>> import annotate
>>> annotate.__annotations__
{'name': <class 'str'>}
>>> annotate.name
'Mike'

As you can see, the __annotations__ attribute returns a Python dict with the variable name as the key and the type as the value. Let’s add a couple of other annotations to our module and see how the __annotations__ attribute updates.

# annotate2.py
name: str = 'Mike'

ages: list = [12, 20, 32]

class Car:
    variable: dict

In this code, we add an annotated list variable and a class with an annotated class variable. Now let’s import our new version of the annotate module and check out its __annotations__ attribute:

>>> import annotate
>>> annotate.__annotations__
{'name': <class 'str'>, 'ages': <class 'list'>}
>>> annotate.Car.__annotations__
{'variable': <class 'dict'>}
>>> car = annotate.Car()
>>> car.__annotations__
{'variable': <class 'dict'>}

This time around, we see that the annotations dictionary contains two items. You will note that the module level __annotations__ attribute does not contain the annotated class variable. To get access to that, we need to access the Car class directly or create a Car instance and access the attribute that way.

Wrapping Up

I found this new feature to be really interesting. While I love Python’s dynamic nature, I can also see the value in knowing what type a variable should be after working with C++ for the past couple of years. Of course, due to Python’s excellent introspection support, figuring out an object’s type is trivial. But this new feature could make static checkers better and also make your code more obvious, especially when you have to go back and update a piece of software you haven’t worked with in a few months or years.

Additional Reading

• PEP 526 – Syntax for variable annotations
• PEP 484 – Type Hinting
• Python 3.6: What’s New