Missing "from typing import List" - error in script, but not in function

I was recently surprised to see that some of our code had a linter warning on the List in something equivalent to:

class MyClass:
    def method(self):
        myVar: List[str] = []

The linter warning is because this file is missing from typing import List in the imports. So, PyCharm flags this as a linter error. Certainly the code is incorrect. What's surprising is this code runs, and has run properly for months!

Digging in, I found that this:

class MyClass:
    def method(self):
        myVar: List[str] = []

m = MyClass()
m.method()

And this:

def method(self):
    l: List[str] = []

method(None)

run properly, but this:

l: List[str] = []

Raises the expected error!

Traceback (most recent call last):
  File ".../scratches/scratch_3.py", line 5, in <module>
    l: List[str] = []
NameError: name 'List' is not defined

Why?

I am running:

Python 3.7.1 (v3.7.1:260ec2c36a, Oct 20 2018, 14:57:15) [MSC v.1915 64 bit (AMD64)] on win32
Type "help", "copyright", "credits" or "license" for more information.
>>>

But the code works on several different versions of Python on Windows and Linux (Ubuntu) builds.

Solution

Python treats annotations for global & local variables differently, as outlined in PEP 526:

Annotating a local variable will cause the interpreter to treat it as a local, even if it was never assigned to. Annotations for local variables will not be evaluated:
def f():
    x: NonexistentName  # No error.
However, if it is at a module or class level, then the type will be evaluated:
x: NonexistentName  # Error!
class X:
    var: NonexistentName  # Error!

This makes some sense, as module- and class-level annotations can be accessed through the __annotations__ dictionary, so it must evaluate the annotation and record it. On the other hand, local variable annotations can not be programmatically accessed, so there's no point in evaluating them.