Explaining Variable Annotations as Extended Functional Type Hints in Python

2 min readOct 14, 2022

The aim of this page📝 is to resolve the “wtf” I experienced with looking into this code (does not look like Good Ol Python I’ve been used to)

tf: Terraform = tf_context.version[version]

TYPE HINTS, CONTINUED

Once upon a time, the PEP 484 introduced type hints, a.k.a. type annotations.
While its main focus was function annotations, it also introduced the notion of type comments to annotate variables
Then, PEP 526 came along and finalized that by adding type hints/annotations for variables
Variable Type Hints (Type annotations) can be added with : <type> to an assignment statement or a single expression indicating the desired type of the annotation target to a third-party type checker or just a reader

my_var: int
my_var = 5  # Passes type check.
other_var: int  = 'a'  # Flagged as error by type checker,
                       # but OK at runtime.

Although a variable annotation is parsed as part of an annotated assignment, the assignment statement is optional
The PEP states that “ the target of the annotation can be any valid single assignment target, at least syntactically (it is up to the type checker what to do with this)”,
Note that the type doesn’t need to exist to be annotated.

d = {}
    d['a']: int = 0  # Annotates d['a'] with int.
    d['b']: int      # Annotates d['b'] with int.

Normally, the annotation expression should evaluate to a Python type
After all the main use of annotations is type hinting but it is not enforced.
The annotation can be any valid Python expression, regardless of the type or value of the result
As you can see, at this time type hints are very permissive and rarely mechanically useful, unless you have a static type checker such as mypy
Or you want to signalize to a reader that the binding has a specific custom type which is exactly what was the motive behind the initial WTF of

tf: Terraform = tf_context.version[version]