Is it possible to modify a method without altering its functionality?

Question

Is it possible to modify a method without altering its functionality?

I am currently attempting to verify that a pandas method is being called with specific values.

However, I have encountered an issue where applying a @patch decorator results in the patched method throwing a ValueError within pandas, even though the original method does not. My goal is simply to confirm that Stock.calc_sma is indeed invoking the underlying pandas.rolling_mean function.

My understanding is that the @patch decorator adds certain "magic" methods to the object being patched, allowing me to verify if the function was called. That said, why does the behavior of the pandas.rolling_mean function differ when it is patched versus when it is not?

app/models.py

import pandas as pd
class Stock:  
    def calc_sma(self, num_days)
        if self.data.shape[0] > num_days:
                column_title = 'sma' + str(num_days)
                self.data[column_title] = pd.rolling_mean(self.data['Adj Close'], num_days)

app/tests/TestStockModel.py

def setUp(self):
    self.stock = MagicMock(Stock)
    self.stock.ticker = "AAPL"
    self.stock.data = DataFrame(aapl_test_data.data)

@patch('app.models.pd.rolling_mean')
def test_calc_sma(self, patched_rolling_mean):
    Stock.calc_sma(self.stock, 3)
    assert(isinstance(self.stock.data['sma3'], Series))
    patched_rolling_mean.assert_any_call()

ERROR: test_calc_sma (TestStockModel.TestStockModel)

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

python pandas mocking patch

Answer 1

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

Typically, patch replaces objects with generic mock objects. The example demonstrates that calling the mock object returns another mock, displaying a length of 0 even if the original object wouldn't have a length attribute. Moreover, its attributes are also mock objects.

To replicate actual behavior, additional arguments are needed:

>>> with patch('os.getcwd', return_value='/a/wonderful/place'):
...     os.getcwd()
...
'/a/wonderful/place'

Alternatively, to "pass through":

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

A comparable example is available at https://docs.python.org/3.5/library/unittest.mock-examples.html

Answer 2

>>> 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

Typically, patch replaces objects with generic mock objects. The example demonstrates that calling the mock object returns another mock, displaying a length of 0 even if the original object wouldn't have a length attribute. Moreover, its attributes are also mock objects.

To replicate actual behavior, additional arguments are needed:

>>> with patch('os.getcwd', return_value='/a/wonderful/place'):
...     os.getcwd()
...
'/a/wonderful/place'

Alternatively, to "pass through":

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

A comparable example is available at https://docs.python.org/3.5/library/unittest.mock-examples.html

Is it possible to modify a method without altering its functionality?

ERROR: test_calc_sma (TestStockModel.TestStockModel)

Answer №1

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