Doing type('') is effectively equivalent to str and types.StringType
so type('') == str == types.StringType will evaluate to "True"
Note that Unicode strings which only contain ASCII will fail if checking types in this way, so you may want to do something like assert type(s) in (str, unicode) or assert isinstance(obj, basestring), the latter of which was suggested in the comments by 007Brendan and is probably preferred.
isinstance() is useful if you want to ask whether an object is an instance of a class, e.g:
class MyClass: pass
print isinstance(MyClass(), MyClass) # -> True
print isinstance(MyClass, MyClass()) # -> TypeError exception
But for basic types, e.g. str, unicode, int, float, long etc asking type(var) == TYPE will work OK.