This isn't so much about the variable scope in list comprehensions as about the way classes work. In Python 3 (but not in Python 2!), list comprehensions don't affect the scope around them:
>>> [i for i in range(10)]
[0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
>>> i
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
NameError: name 'i' is not defined
>>> i = 0
>>> [i for i in range(10)]
[0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
>>> i
0
However, when you do that in a class, it won't look for b among the class attributes the way it would in the local scope of a module or function. To do what you are trying to do, use the @property decorator:
>>> class a:
... s = 'python'
... b = 'py'
... @property
... def c(self):
... return [x for x in self.s if x in self.b]
...
>>> A = a()
>>> A.c
['p', 'y']
Also, remember that strings are iterable too (they're just lists of their component characters), so no need to explicitly make b a list.
