Restrict protected attributes and methods access and assignment

[sobolevn]

Rule request

Thesis

Currently we allow to use this code:

bad = object()
bad._protected = 1
print(bad._protected)

No warning would be shown to the user.

What we need to do?

1. We need to forbid to use protected names (that are starting with _) in a free manner
2. These attributes and methods can not be: called, assigned, accessed from outer scope
3. But, they can be used freely with self and cls like: self._some = 1
4. This example should also be valid:

class Test(object): 
      _some = 1
      _extra_some = _some + 2

However, Test._some is not valid.

Reasoning

We should respect protected APIs. And whenever we are using one (which we should not do), we should explicitly mark these lines with noqa.