[3.7] bpo-40126: Fix reverting multiple patches in unittest.mock. (GH-19351) (GH-19484)

Patcher's __exit__() is now never called if its __enter__() is failed.
Returning true from __exit__() silences now the exception.
(cherry picked from commit 4b222c9)

Co-authored-by: Serhiy Storchaka <[email protected]>

Author: serhiy-storchaka

Lib/unittest/mock.py

@@ -30,6 +30,7 @@
 import pprint
 import sys
 import builtins
+import contextlib
 from types import ModuleType, MethodType
 from functools import wraps, partial
 
@@ -1243,33 +1244,16 @@ def decorate_callable(self, func):
         @wraps(func)
         def patched(*args, **keywargs):
             extra_args = []
-            entered_patchers = []
-
-            exc_info = tuple()
-            try:
+            with contextlib.ExitStack() as exit_stack:
                 for patching in patched.patchings:
-                    arg = patching.__enter__()
-                    entered_patchers.append(patching)
+                    arg = exit_stack.enter_context(patching)
                     if patching.attribute_name is not None:
                         keywargs.update(arg)
                     elif patching.new is DEFAULT:
                         extra_args.append(arg)
 
                 args += tuple(extra_args)
                 return func(*args, **keywargs)
-            except:
-                if (patching not in entered_patchers and
-                    _is_started(patching)):
-                    # the patcher may have been started, but an exception
-                    # raised whilst entering one of its additional_patchers
-                    entered_patchers.append(patching)
-                # Pass the exception to __exit__
-                exc_info = sys.exc_info()
-                # re-raise the exception
-                raise
-            finally:
-                for patching in reversed(entered_patchers):
-                    patching.__exit__(*exc_info)
 
         patched.patchings = [self]
         return patched
@@ -1411,19 +1395,23 @@ def __enter__(self):
 
         self.temp_original = original
         self.is_local = local
-        setattr(self.target, self.attribute, new_attr)
-        if self.attribute_name is not None:
-            extra_args = {}
-            if self.new is DEFAULT:
-                extra_args[self.attribute_name] =  new
-            for patching in self.additional_patchers:
-                arg = patching.__enter__()
-                if patching.new is DEFAULT:
-                    extra_args.update(arg)
-            return extra_args
-
-        return new
-
+        self._exit_stack = contextlib.ExitStack()
+        try:
+            setattr(self.target, self.attribute, new_attr)
+            if self.attribute_name is not None:
+                extra_args = {}
+                if self.new is DEFAULT:
+                    extra_args[self.attribute_name] =  new
+                for patching in self.additional_patchers:
+                    arg = self._exit_stack.enter_context(patching)
+                    if patching.new is DEFAULT:
+                        extra_args.update(arg)
+                return extra_args
+
+            return new
+        except:
+            if not self.__exit__(*sys.exc_info()):
+                raise
 
     def __exit__(self, *exc_info):
         """Undo the patch."""
@@ -1444,9 +1432,9 @@ def __exit__(self, *exc_info):
         del self.temp_original
         del self.is_local
         del self.target
-        for patcher in reversed(self.additional_patchers):
-            if _is_started(patcher):
-                patcher.__exit__(*exc_info)
+        exit_stack = self._exit_stack
+        del self._exit_stack
+        return exit_stack.__exit__(*exc_info)
 
 
     def start(self):
@@ -1464,7 +1452,7 @@ def stop(self):
             # If the patch hasn't been started this will fail
             pass
 
-        return self.__exit__()
+        return self.__exit__(None, None, None)
 
 
 

Misc/NEWS.d/next/Library/2020-04-04-00-47-40.bpo-40126.Y-bTNP.rst

@@ -0,0 +1,3 @@
+Fixed reverting multiple patches in unittest.mock. Patcher's ``__exit__()``
+is now never called if its ``__enter__()`` is failed. Returning true from
+``__exit__()`` silences now the exception.