如何模拟上下文管理器并替换其一个或多个方法

问题描述

我正在尝试模拟一个类，该类用作进行网络调用的上下文管理器。具体来说，它的Read方法通过网络返回PLC标签的值。

这是我要在main.py中测试的功能的示例：

from pylogix import PLC

def read_counter(counter_entry):
    with PLC() as comm:
        count = comm.Read(counter_entry['tag'])
        if count.Status == 'Success':
            log_count(count.Value)
            counter_entry['last_count'] == count.Value

我的测试如下：

import unittest
from unittest.mock import Mock,patch,Magicmock
from random import randint

import main

class MockComm():

    def __init__(self):
        self.tag_dict = {}

    def __enter__(self):
        print('__enter__ called')
        return self

    def __exit__(self):
        print('__exit__ called')

    def add(self,tag,value):
        print('Added ',' as ',value)
        self.tag_dict[tag] = value

    def Read(self,tag):
        print('Reading ',tag)
        if tag in self.tag_dict:
            print('returned ',self.tag_dict[tag])
            return Response(tag,Value=self.tag_dict[tag])
        else:
            print('returned Connection Failure')
            return Response(tag,Value=None,Status='Connection failure')


class Response():
    def __init__(self,Status='Success'):
        self.TagName = tag
        self.Value = Value
        self.Status = Status

class ReadPylogixCounterTestSuit(unittest.TestCase):

    def setUp(self):
        self.counter_entry = {
            'type': 'pylogix_counter','tag': 'Program:Production.DailyTotal','Part_Type_Tag': 'Stn010.PartType','Part_Type_Map': {'0': '50-4865','1': '50-5081'},# used internally to track the readings
            'nextread': 0,# timestamp of the next reading
            'lastcount': 0,# last counter value
            'lastread': 0       # timestamp of the last read
        }

    @patch('main.PLC',new_callable=MockComm)
    def test_read_tag(self,mock_PLC):

        FirsT_COUNTER_VALUE = randint(1,2500)
        mock_PLC.add(self.counter_entry['tag'],FirsT_COUNTER_VALUE)
        mock_PLC.add(self.counter_entry['Part_Type_Tag'],0)

        main.read_counter(self.counter_entry)

        assert self.counter_entry['lastcount'] == FirsT_COUNTER_VALUE

编辑：将问题中的代码更新为完整的可运行示例。
当我逐步执行以上代码时，mock_PLC是Mock_Comm的实例。当我点击with PLC() as comm行时，就会得到以下回溯：

=====================================================================
ERROR: test_read_tag (test_read_counter.ReadPylogixCounterTestSuit)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "C:\Users\cstrutto\AppData\Local\Programs\Python\python37\lib\unittest\mock.py",line 1256,in patched
    return func(*args,**keywargs)
  File "c:\programing\python-testing\test_context_manager\test_read_counter.py",line 81,in test_read_tag
    main.read_counter(self.counter_entry)
  File "c:\programing\python-testing\test_context_manager\main.py",line 4,in read_counter
    with PLC() as comm:
TypeError: 'MockComm' object is not callable

----------------------------------------------------------------------
Ran 1 test in 34.908s

Failed (errors=1)

我有必需的__enter__和__exit__方法。为什么它不能用作上下文管理器？

解决方法

我无法像上面所做的那样替换对象，但是，我确实找到了一种在模拟对象中为Read方法设置side_effect的方法。

我仍将尝试用自己的对象替换该对象，但目前，以下代码可以正常工作：

@patch('main.PLC',autospec=True)
def test_read_tag(self,mock_PLC):

    FIRST_COUNTER_VALUE = randint(1,2500)

    def mock_Read(tag):
        
        tag_dict={
            self.counter_entry['tag']: FIRST_COUNTER_VALUE,self.counter_entry['Part_Type_Tag']: 0
        }

        print('Reading ',tag)
        if tag in tag_dict:
            print('returned ',tag_dict[tag])
            return Response(tag,Value=tag_dict[tag])
        else:
            print('returned Connection Failure')
            return Response(tag,Value=None,Status='Connection failure')

    mock_client = MagicMock(spec=main.PLC)
    mock_client.Read.side_effect = mock_Read
    mock_client.__enter__.return_value = mock_client
    mock_PLC.return_value = mock_client

    main.read_counter(self.counter_entry)