Python 3.7 vs 2.7, assignment

Question

I'm a bit of a 2.7 Python person. I came across this code, which I can assume what it does, but what does it exactly mean? Model is used, but I assume that model is a variable that is of the type of class, but then we assign a function to it. Looks very odd to me.

model: Class = a_function(f)

What does the above mean?

Answer 1

It’s a type annotation. It doesn’t actually do anything in code, but the annotation can be read by third-party applications (e.g. mypy) to perform type checking. In your case, the code declares that the type of the variable model is Class.

The rest of the expression is a regular assignment.

Answer 2

The following statement that you posted:

model: Class = a_function(f)

Means a variable declared with its type. This rule is not required in Python, but it's a way to show what is the type of the parameter or variable that is being used. This property is called a "type_annotation". It can be useful, for example, in complex functions implementations, such as the following code:

def process_some_data(client: str, properties: dict, feature: SomeClass):
   # some  code
   return something