There are two ways to go around this:
Either you can use A.foo(self) method explicitly as the others have suggested – use this when you want to call the method of the A class with disregard as to whether A is B‘s parent class or not:
class C(B):
def foo(self):
tmp = A.foo(self) # call A's foo and store the result to tmp
return "C"+tmp
Or, if you want to use the .foo() method of B‘s parent class regardless of whether the parent class is A or not, then use:
class C(B):
def foo(self):
tmp = super(B, self).foo() # call B's father's foo and store the result to tmp
return "C"+tmp