Rename global var affects list comprehension

[matmarczak]

With code:

some_var = 1
compr = [some_var for some_var in range(10)]

When renaming either some_var from global or some_var from list comprehension scope both variables get renamed.

[lieryan]

Copying the investigation text from #249:

I took a look at this, it seems like there are a number of issues at play here:

1. pyscopes.py::_HoldingScopeFinder currently assumes that a single logical line of code (i.e. a python statement) can only belong only to a single variable scope, but list/generator expression and lambdas are expressions that can create their own variable scope, so this assumption is incorrect. It may take quite a bit of internal rewrite of _HoldingScopeFinder and friends to fix this part of the issue.

2. currently we don't create a variable scope for List/Generator Comprehension, this is in line with Python 2, where Comprehension leaks its loop variables to the surrounding scope, but it's incorrect for Python 3 code (Comprhensions with scope #422)

3. The loop variable of a List/Generator Comprehension currently aren't considered an assignment statement, which means that unless you have an assignment for the same name elsewhere, rope can't resolve the variable (Implement renaming of walrus operator and loop variables in comprehensions #318)

Probably the quickest way to fix this is to fix that issue 3 first. That should make rope treat comprehension with Python 2 scope semantics (i.e. leaky loop variable), so renames will work most of the time. In Python 3 though, renames will be a bit overzealous if your loop variable shadows a name in the surrounding scope. We can fix the scoping issue later on.

[lieryan]

This is now fixed in #430, which is merged in master and will be part of 0.21 release.