ENH: Roll own util.patch() for monkey-patching objects

Author: kernc

backtesting/_util.py

@@ -1,6 +1,7 @@
 from __future__ import annotations
 
 import warnings
+from contextlib import contextmanager
 from numbers import Number
 from typing import Dict, List, Optional, Sequence, Union, cast
 
@@ -15,6 +16,20 @@ def try_(lazy_func, default=None, exception=Exception):
         return default
 
 
+@contextmanager
+def patch(obj, attr, newvalue):
+    had_attr = hasattr(obj, attr)
+    orig_value = getattr(obj, attr, None)
+    setattr(obj, attr, newvalue)
+    try:
+        yield
+    finally:
+        if had_attr:
+            setattr(obj, attr, orig_value)
+        else:
+            delattr(obj, attr)
+
+
 def _as_str(value) -> str:
     if isinstance(value, (Number, str)):
         return str(value)

backtesting/test/_test.py

@@ -1,4 +1,5 @@
 import inspect
+import multiprocessing
 import os
 import sys
 import time
@@ -10,15 +11,14 @@
 from runpy import run_path
 from tempfile import NamedTemporaryFile, gettempdir
 from unittest import TestCase
-from unittest.mock import patch
 
 import numpy as np
 import pandas as pd
 from pandas.testing import assert_frame_equal
 
 from backtesting import Backtest, Strategy
 from backtesting._stats import compute_drawdown_duration_peaks
-from backtesting._util import _Array, _as_str, _Indicator, try_
+from backtesting._util import _Array, _as_str, _Indicator, patch, try_
 from backtesting.lib import (
     FractionalBacktest, OHLCV_AGG,
     SignalStrategy,
@@ -626,7 +626,7 @@ def test_multiprocessing_windows_spawn(self):
         kw = {'fast': [10]}
 
         stats1 = Backtest(df, SmaCross).optimize(**kw)
-        with patch('multiprocessing.get_start_method', lambda **_: 'spawn'):
+        with patch(multiprocessing, 'get_start_method', lambda **_: 'spawn'):
             with self.assertWarns(UserWarning) as cm:
                 stats2 = Backtest(df, SmaCross).optimize(**kw)
 
@@ -776,7 +776,7 @@ def init(self):
         bt.run()
         import backtesting._plotting
         with _tempfile() as f, \
-                patch.object(backtesting._plotting, '_MAX_CANDLES', 10), \
+                patch(backtesting._plotting, '_MAX_CANDLES', 10), \
                 self.assertWarns(UserWarning):
             bt.plot(filename=f, resample=True)
             # Give browser time to open before tempfile is removed
@@ -976,6 +976,15 @@ def __call__(self):
         for s in ('Open', 'High', 'Low', 'Close', 'Volume'):
             self.assertEqual(_as_str(_Array([1], name=s)), s[0])
 
+    def test_patch(self):
+        class Object:
+            pass
+        o = Object()
+        o.attr = False
+        with patch(o, 'attr', True):
+            self.assertTrue(o.attr)
+        self.assertFalse(o.attr)
+
     def test_pandas_accessors(self):
         class S(Strategy):
             def init(self):