Detect leaking `for` loops in `ClassDef` and `Module`

[sobolevn]

Right now we can have two problems:

class Some(object):
   for x in [1, 2]:
       print(x)

print(Some.x)  # oups, will show you `2`

and with modules:

# some.py
for x in [1, 2]:
    print(x)

# __main__
import some
print(some.x)  # 2

This is not users want in 99.9% of cases. There are other - functional - solutions which don't leave extra vars in the scope.

Moreover, for is already forbidden in class bodies.

[alexeev-prog]

I can work with this

[alexeev-prog]

You need to check if there is a for loop inside the top (global) level of the module and inside the classes. To do this, you can add two warnings: one for the presence of for in the class and one for the presence of for in the global scope.

[sobolevn]

There's no need in two violations, one is good enough.

But, that's not all. We also need to define, when it is ok to use such code:

1. for x in ...: ...; del x
2. for x in ...: ...; globals().pop('x')
3. What else?

for _ in ...: ... won't work, because Klass._ is still not ideal.

[alexeev-prog]

We need check actions with objects? If we see del, globals.pop, any other action without creating - its ok?

[sobolevn]

Yes, because this way - name is not leaked.

[alexeev-prog]

Please assign me