How to use type hints in python 3.6?

Question

I noticed Python 3.5 and Python 3.6 added a lot of features about static type checking, so I tried with the following code (in python 3.6, stable version).

from typing import List


a: List[str] = []
a.append('a')
a.append(1)
print(a)

What surprised me was that, Python didn't give me an error or warning, although 1 was appended to a list which should only contain strings. Pycharm detected the type error and gave me a warning about it, but it was not obvious and it was not shown in the output console, I was afraid sometimes I might miss it. I would like the following effects:

1. If it's obvious that I used the wrong type just as shown above, throw out a warning or error.
2. If the compiler couldn't reliably check if the type I used was right or wrong, ignore it.

Is that possible? Maybe mypy could do it, but I'd prefer to use Python-3.6-style type checking (like a: List[str]) instead of the comment-style (like # type List[str]) used in mypy. And I'm curious if there's a switch in native python 3.6 to achieve the two points I said above.

Answer 1

Type hints are entirely meant to be ignored by the Python runtime, and are checked only by 3rd party tools like mypy and Pycharm's integrated checker. There are also a variety of lesser known 3rd party tools that do typechecking at either compile time or runtime using type annotations, but most people use mypy or Pycharm's integrated checker AFAIK.

In fact, I actually doubt that typechecking will ever be integrated into Python proper in the foreseable future -- see the 'non-goals' section of PEP 484 (which introduced type annotations) and PEP 526 (which introduced variable annotations), as well as Guido's comments here.

I'd personally be happy with type checking being more strongly integrated with Python, but it doesn't seem the Python community at large is ready or willing for such a change.

The latest version of mypy should understand both the Python 3.6 variable annotation syntax and the comment-style syntax. In fact, variable annotations were basically Guido's idea in the first place (Guido is currently a part of the mypy team) -- basically, support for type annotations in mypy and in Python was developed pretty much simultaneously.