Prevent patch_attribute from patching instance attributes at classes

[fornellas]

This:

import testslide

class A:
  def __inti__(self):
    self.attr = 1


class TestA(testslide.TestCase):
  def test_attr(self):
    self.patch_attribute(A, "attr", "mock")

yields:

AttributeError: type object 'A' has no attribute 'attr'

However, patching instance attributes at classes should not be allowed. mock_callable already prevents that.

Let's make patch_attribute refuse to patch instance attributes at classes as well.

Probably a good idea to reuse logic from mock_callable to define if "is this good to patch?".

[ldfsilva]

Hey @fornellas, I got a question while refactoring the fix for this one, are classes attributes allowed to be patched at class level or should we enforce the same behavior as in for instance attributes?

For example, should the following work? (it works now)

class A:
    ATTR: str = "ATTR"


class TestA(testslide.TestCase):
  
    def test_class_attribute_work(self):
        self.patch_attribute(A, "ATTR", "mock")
        self.assertEqual(A.ATTR, "mock")

[fornellas]

What you described LGTM as:

• Attribute exists at the class, patching it there is no different from patching an attribute at a module or instance.
  • If it were to be patched at the instance, would only affect that instance, nothing more.
• The issue here is when the attribute does not exist at the class, but only at the instance. Allowing to patch it at the class would mean not respecting the class interface (which does not have the attribute).

I crossed some wire when I mentioned mock_callable blocking patching instance methods at the class, which is a different issue rooted at this RSPec decision to not support it, as it is kind of a code smell, sorry about the confusion.