class A(object):
    try:
        import c_ext
        method = c_ext.optimized_method
    except ImportError:
        def method(self):
            return "foo"

def optimized_method(self):
    return "fasterfoo"

>>> A().method()
exceptions.TypeError: optimized_method() takes exactly one argument (0 given)

class A(object):
    def method(self):
        try:
            import c_ext
            return c_ext.optimized_method(self)
        except ImportError:
            pass
        return "foo"

class A(object):
    def method(self):
        return "foo"

try:
    import c_ext
    import types
    A.method = types.MethodType(c_ext.optimized_method, None, A)
except ImportError:
    pass