classmethod装饰器可以在不实例化类的情况下执行函数，例如:
class TestClassMethod:
@classmethod
def print_text(self):
print "ok, cool"
TestClassMethod.print_text() # ok, cool
class TestNoClassMethod:
def print_text(self):
print "ok, cool"
TestNoClassMethod.print_text() # 报错
TestNoClassMethod().print_text() # ok, cool