Define the location attribute on base class ProcessEnv

[jdufresne]

Fixes errors when running mypy on a project's noxfile.py:

noxfile.py:42: error: "ProcessEnv" has no attribute "location"

Discovered while adding types pip's noxfile.py:
pypa/pip#9411

[stsewd]

I'm not that familiar with mypy, but location is a per instance attribute, not sure if there is a way to declare it like that.

[jdufresne]

@stsewd The syntanx used here is a type declaration. It does not not assign the attribute to the class (or even the instance). A true assignment is still required for that. Observe:

class MyClass:
    a: str

obj = MyClass()
obj.a

$ python blah.py
Traceback (most recent call last):
  File "/home/jon/blah.py", line 5, in <module>
    obj.a
AttributeError: 'MyClass' object has no attribute 'a'

The syntax is use for type declaration only so it makes mypy happy. Please let me know if I can provide additional detail.

[theacodes]

Looks good to me. Thanks, @jdufresne!

[cjolowicz]

FTR this annotation is not strictly correct: PassthroughEnv does not have a location attribute, only VirtualEnv and CondaEnv do. And for these, mypy should be able to work it out from the type annotations on __init__.

The problem is that typical access is via session.virtualenv.location, where session.virtualenv is annotated as ProcessEnv. That's also the use case in pip's noxfile, AFAICS from the linked PR.

A simple fix would be the following:

class ProcessEnv:
    location: Optional[str] = None

This would result in a runtime change though, adding a class attribute on ProcessEnv. I can't see a simple way to solve this purely on the typing level. 🤔

Another possibility, maybe preferable, would be to keep the type annotation from this PR, but make it raise on PassthroughEnv:

class PassthroughEnv(ProcessEnv):
    @property
    def location(self) -> str:
        raise RuntimeError("PassthroughEnv has no location")

This would also have the advantage that client code does not need to check against None to pass mypy.