brain: Add __class_getitem__ to subprocess.Popen starting from Python 3.9

[dbaty]

Type of Changes

	Type
✓	🐛 Bug fix

Related Issue

Closes pylint-dev/pylint#4034

Description

This fix is necessary for pylint to detect that subprocess.Popen is subscriptable, starting from Python 3.9 (see pylint-dev/pylint#4034).

pylint relies on the presence of __class_getitem__() to determine if a class is subcriptable. Starting from Python 3.9, subprocess.Popen implements this method (and is thus subscriptable):

$ python3.9
>>> import subprocess
>>> subprocess.Popen.__class_getitem__
<bound method GenericAlias of <class 'subprocess.Popen'>>

[hippo91]

Thanks a lot @dbaty!
It's all good for me.
Just left a comment dealing with textwrap.

[hippo91]

Shouldn't we use textwrap.dedent here too?

[dbaty]

@hippo91 : I don't think we can. dedent() removes all common leading whitespaces, which means that calling this:

textwrap.dedent(
    """
    def __class_getitem__(cls, item):
        pass
    """
)

... returns this string:

def __class_getitem__(cls, item):
    pass

... which looks like a function, whereas we want the code to be indented to denote a method. Of course we could use textwrap.indent() too, like this:

textwrap.indent(
    textwrap.dedent(
        """
        def __class_getitem__(cls, item):
            pass
        """
    ),
    ' ' * 4,
)

but I am not sure it's more readable. :)

I thus prefer the current version. What do you think?

[hippo91]

@dbaty you are totally right!
Thanks for this explanation and this PR!