Some code to understand Python type checking.

A script with today's my understanding. I do not fully utilize type checking.

I hear that mypy runs sanity check on types. I write code to set string as int property and mypy does not complain. Why doesn't it warn my mistake? What mypy offers is potential mistakes like putting a string into a property which should an integer?

mypy reports errors on reduce functions. My code returns value of int or float correctly, but mypy thinks it is wrong.

This is understandable. Real values will be known when code really executes. Software such as mypy cannot know what values are actually thrown to functions such as reduce().

I added more of type hints to helper functions that reduce() uses, but mypy raises errors anyway.

So, here is my question. What is mypy for, if it does not understand code correctly. In what ways does it help users?

When I define a class such as order, and I want its itemlines property to be a list of another class Itemline, I can write List[Itemline], which is nice. When I do this, definition of class Itemline should be come before definition of class Order. This is understandable.

Type hints and checking helps me become more organized about data structure I am trying to create.

typing package "does not enforce function and variable type annotations." Still it helps me to think ahead about data structure.

mypy seems not to be able to analyze my code correctly, although I give extensive type hints. I do not understand what mypy is for.

October 11th, 2020: Written.