修补方法而不更改方法的工作方式？

Question

我试图测试一个pandas方法是否被调用了一些值。在

然而，仅仅通过应用@patch decorator，patched方法就会在pandas中抛出一个ValueError，而实际的方法没有。我只是想测试Stock.calc_sma正在调用底层的pandas.rolling_mean函数。在

我假设@patch修饰符基本上给我正在修补的东西添加了一些“魔法”方法，允许我检查函数是否被调用。如果是这样的话，为什么pandas.rolling_mean函数在修补和未修补时的行为不一样？在

应用程序/模型.py在

import pandas as pd
class Stock:  # i've excluded a bunch of class methods, including the one that sets self.data, which is a DataFrame of stock prices.
    def calc_sma(self, num_days)
        if self.data.shape[0] > num_days:  # Stock.data holds a DataFrame of stock prices
                column_title = 'sma' + str(num_days)
                self.data[column_title] = pd.rolling_mean(self.data['Adj Close'], num_days)

应用程序/测试/TestStockModel.py在

^{pr2}$

错误：测试计算sma(TestStockModel.TestStockModel)在

Traceback (most recent call last):
  File "/Users/grant/Code/python/chartflux/env/lib/python2.7/site-packages/mock.py", line 1201, in patched
    return func(*args, **keywargs)
  File "/Users/grant/Code/python/chartflux/app/tests/TestStockModel.py", line 26, in test_calc_sma
    Stock.calc_sma(self.stock, 3)
  File "/Users/grant/Code/python/chartflux/app/models.py", line 27, in calc_sma
    self.data[column_title] = pd.rolling_mean(self.data['Adj Close'], num_days)
  File "/Users/grant/Code/python/chartflux/env/lib/python2.7/site-packages/pandas/core/frame.py", line 1887, in __setitem__
    self._set_item(key, value)
  File "/Users/grant/Code/python/chartflux/env/lib/python2.7/site-packages/pandas/core/frame.py", line 1967, in _set_item
    value = self._sanitize_column(key, value)
  File "/Users/grant/Code/python/chartflux/env/lib/python2.7/site-packages/pandas/core/frame.py", line 2017, in _sanitize_column
    raise ValueError('Length of values does not match length of '
ValueError: Length of values does not match length of index

Answer 1

>>> import os
>>> os.getcwd()
'/'
>>> from unittest.mock import patch
>>> with patch('os.getcwd'):
...     print(os.getcwd)
...     print(os.getcwd())
...     print(len(os.getcwd()))
...
<MagicMock name='getcwd' id='4472112296'>
<MagicMock name='getcwd()' id='4472136928'>
0

默认情况下，patch用真正通用的模拟对象替换对象。如您所见，调用mock只会返回另一个mock。它的len为0，即使被替换的对象没有len。它的属性也是通用的mock。在

因此，模拟行为需要额外的参数，比如：

^{pr2}$

或“通过”：

>>> _cwd = os.getcwd
>>> with patch('os.getcwd') as p:
...     p.side_effect = lambda: _cwd()
...     print(os.getcwd())
...
/

在https://docs.python.org/3.5/library/unittest.mock-examples.html中有一个类似的例子