实例与嘲讽 [英] isinstance and Mocking

问题描述

class HelloWorld(object):
    def say_it(self):
        return 'Hello I am Hello World'

def i_call_hello_world(hw_obj):
    print 'here... check type: %s' %type(HelloWorld)
    if isinstance(hw_obj, HelloWorld):
        print hw_obj.say_it()

from mock import patch, MagicMock
import unittest

class TestInstance(unittest.TestCase):
    @patch('__main__.HelloWorld', spec=HelloWorld)
    def test_mock(self,MK):
        print type(MK)
        MK.say_it.return_value = 'I am fake'
        v = i_call_hello_world(MK)
        print v

if __name__ == '__main__':
    c = HelloWorld()
    i_call_hello_world(c)
    print isinstance(c, HelloWorld)
    unittest.main()

这是回溯

here... check type: <type 'type'>
Hello I am Hello World
True
<class 'mock.MagicMock'>
here... check type: <class 'mock.MagicMock'>
E
======================================================================
ERROR: test_mock (__main__.TestInstance)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/usr/local/lib/python2.7/dist-packages/mock.py", line 1224, in patched
    return func(*args, **keywargs)
  File "t.py", line 18, in test_mock
    v = i_call_hello_world(MK)
  File "t.py", line 7, in i_call_hello_world
    if isinstance(hw_obj, HelloWorld):
TypeError: isinstance() arg 2 must be a class, type, or tuple of classes and types

----------------------------------------------------------------------
Ran 1 test in 0.002s

问题1.为什么会引发此错误?他们是<class type='MagicMock>

Q1. Why is this error thrown? They are <class type='MagicMock>

第二季度.我如何暂停模拟，以便在错误修复后第一行通过?

Q2. How do I pause the mocking so that the first line will pass if the error is fixed?

从 docs 中:

通常，对象的__class__属性将返回其类型.对于具有规格的模拟对象，__class__会返回规格类.这样，模拟对象就可以通过isinstance()测试来代替它们正在伪装的对象:

Normally the __class__ attribute of an object will return its type. For a mock object with a spec, __class__ returns the spec class instead. This allows mock objects to pass isinstance() tests for the object they are replacing / masquerading as:

mock = Mock(spec=3)
isinstance(mock, int)
True

推荐答案

不要使用isinstance，而是检查是否存在say_it方法.如果该方法存在，请调用它:

Don't use isinstance, instead check for the existence of the say_it method. If the method exists, call it:

if hasattr(hw_obj, 'say_it'):
    print hw_obj.say_it()

无论如何，这是一个更好的设计:依赖类型信息更加脆弱.

This is a better design anyway: relying on type information is much more brittle.

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