CLN/PERF: remove gc.collect from the setting_with_copy checks as not needed (#27031)

* CLN/PERF: remove gc.collect from the setting_with_copy checks as not needed

* fixup whatsnew

* add asv

Author: jreback

asv_bench/benchmarks/indexing.py

@@ -5,7 +5,7 @@
 from pandas import (Series, DataFrame, MultiIndex,
                     Int64Index, UInt64Index, Float64Index,
                     IntervalIndex, CategoricalIndex,
-                    IndexSlice, concat, date_range)
+                    IndexSlice, concat, date_range, option_context)
 
 
 class NumericSeriesIndexing:
@@ -335,4 +335,20 @@ def time_assign_with_setitem(self):
             self.df[i] = np.random.randn(self.N)
 
 
+class ChainIndexing:
+
+    params = [None, 'warn']
+    param_names = ['mode']
+
+    def setup(self, mode):
+        self.N = 1000000
+
+    def time_chained_indexing(self, mode):
+        with warnings.catch_warnings(record=True):
+            with option_context('mode.chained_assignment', mode):
+                df = DataFrame({'A': np.arange(self.N), 'B': 'foo'})
+                df2 = df[df.A > self.N // 2]
+                df2['C'] = 1.0
+
+
 from .pandas_vb_common import setup  # noqa: F401

doc/source/whatsnew/v0.25.0.rst

@@ -579,6 +579,7 @@ Performance Improvements
 - Improved performance of :attr:`IntervalIndex.is_unique` by removing conversion to ``MultiIndex`` (:issue:`24813`)
 - Restored performance of :meth:`DatetimeIndex.__iter__` by re-enabling specialized code path (:issue:`26702`)
 - Improved performance when building :class:`MultiIndex` with at least one :class:`CategoricalIndex` level (:issue:`22044`)
+- Improved performance by removing the need for a garbage collect when checking for ``SettingWithCopyWarning`` (:issue:`27031`)
 
 .. _whatsnew_0250.bug_fixes:
 

pandas/core/generic.py

@@ -3263,58 +3263,50 @@ def _check_setitem_copy(self, stacklevel=4, t='setting', force=False):
 
         """
 
-        if force or self._is_copy:
+        # return early if the check is not needed
+        if not (force or self._is_copy):
+            return
 
-            value = config.get_option('mode.chained_assignment')
-            if value is None:
-                return
-
-            # see if the copy is not actually referred; if so, then dissolve
-            # the copy weakref
-            try:
-                gc.collect(2)
-                if not gc.get_referents(self._is_copy()):
-                    self._is_copy = None
-                    return
-            except Exception:
-                pass
+        value = config.get_option('mode.chained_assignment')
+        if value is None:
+            return
 
-            # we might be a false positive
-            try:
-                if self._is_copy().shape == self.shape:
-                    self._is_copy = None
-                    return
-            except Exception:
-                pass
+        # see if the copy is not actually referred; if so, then dissolve
+        # the copy weakref
+        if self._is_copy is not None and not isinstance(self._is_copy, str):
+            r = self._is_copy()
+            if not gc.get_referents(r) or r.shape == self.shape:
+                self._is_copy = None
+                return
 
-            # a custom message
-            if isinstance(self._is_copy, str):
-                t = self._is_copy
+        # a custom message
+        if isinstance(self._is_copy, str):
+            t = self._is_copy
 
-            elif t == 'referant':
-                t = ("\n"
-                     "A value is trying to be set on a copy of a slice from a "
-                     "DataFrame\n\n"
-                     "See the caveats in the documentation: "
-                     "http://pandas.pydata.org/pandas-docs/stable/user_guide/"
-                     "indexing.html#returning-a-view-versus-a-copy"
-                     )
+        elif t == 'referant':
+            t = ("\n"
+                 "A value is trying to be set on a copy of a slice from a "
+                 "DataFrame\n\n"
+                 "See the caveats in the documentation: "
+                 "http://pandas.pydata.org/pandas-docs/stable/user_guide/"
+                 "indexing.html#returning-a-view-versus-a-copy"
+                 )
 
-            else:
-                t = ("\n"
-                     "A value is trying to be set on a copy of a slice from a "
-                     "DataFrame.\n"
-                     "Try using .loc[row_indexer,col_indexer] = value "
-                     "instead\n\nSee the caveats in the documentation: "
-                     "http://pandas.pydata.org/pandas-docs/stable/user_guide/"
-                     "indexing.html#returning-a-view-versus-a-copy"
-                     )
-
-            if value == 'raise':
-                raise com.SettingWithCopyError(t)
-            elif value == 'warn':
-                warnings.warn(t, com.SettingWithCopyWarning,
-                              stacklevel=stacklevel)
+        else:
+            t = ("\n"
+                 "A value is trying to be set on a copy of a slice from a "
+                 "DataFrame.\n"
+                 "Try using .loc[row_indexer,col_indexer] = value "
+                 "instead\n\nSee the caveats in the documentation: "
+                 "http://pandas.pydata.org/pandas-docs/stable/user_guide/"
+                 "indexing.html#returning-a-view-versus-a-copy"
+                 )
+
+        if value == 'raise':
+            raise com.SettingWithCopyError(t)
+        elif value == 'warn':
+            warnings.warn(t, com.SettingWithCopyWarning,
+                          stacklevel=stacklevel)
 
     def __delitem__(self, key):
         """