Forbid to shadow outer scope variables with blocks

[sobolevn]

Rule request

Thesis

This example can damage your code: https://github.com/satwikkansal/wtfPython#-the-disappearing-variable-from-outer-scope

e = 7
try:
    raise Exception()
except Exception as e:
    pass

>>> print(e)
NameError: name 'e' is not defined

Reasoning

It just deletes your variables! This should not happen.

[sobolevn]

There are also different blocks that might create new variables:

• with and async with
• for and async for
• except

We need to check that they do not shadow any variables defined before.

[sobolevn]

And not just variables, but also functions. Imports are covered with flake8 (should be tested in external_plugins.py).

[sobolevn]

There are also nested functions and arguments, methods and attributes.

This task is really hard.
But, I have decided to make its scope wider in order to make it more predictable and solid.

We also have related task #401 which checks that variables defined in blocks are not used after the block itself. We check that they are not used before.

@makarchuk please check out this discussion.

[sobolevn]

Imports work from time to time. They should also be covered.

[sobolevn]

Here's the deal: we will have two separate logical checks for a single violation.
First one will check that different overlapping wps_context does not contain the same name.
Second one will check that inside a single context we do not have an overlap: methods and class attributes, variables and except names