gh-104745: Limit starting a patcher more than once without stopping it (#126649)

Previously, this would cause an `AttributeError` if the patch stopped more than once after this, and would also disrupt the original patched object.

---------

Co-authored-by: Peter Bierma <zintensitydev@gmail.com>
Co-authored-by: Bénédikt Tran <10796600+picnixz@users.noreply.github.com>

1 files changed, 9 insertions, 0 deletions

diff --git a/Lib/unittest/mock.py b/Lib/unittest/mock.py
index 21ca061..55cb4b1 100644
--- a/Lib/unittest/mock.py
+++ b/Lib/unittest/mock.py
@@ -1360,6 +1360,7 @@ class _patch(object):
         self.autospec = autospec
         self.kwargs = kwargs
         self.additional_patchers = []
+        self.is_started = False
 
 
     def copy(self):
@@ -1472,6 +1473,9 @@ class _patch(object):
 
     def __enter__(self):
         """Perform the patch."""
+        if self.is_started:
+            raise RuntimeError("Patch is already started")
+
         new, spec, spec_set = self.new, self.spec, self.spec_set
         autospec, kwargs = self.autospec, self.kwargs
         new_callable = self.new_callable
@@ -1603,6 +1607,7 @@ class _patch(object):
         self.temp_original = original
         self.is_local = local
         self._exit_stack = contextlib.ExitStack()
+        self.is_started = True
         try:
             setattr(self.target, self.attribute, new_attr)
             if self.attribute_name is not None:
@@ -1622,6 +1627,9 @@ class _patch(object):
 
     def __exit__(self, *exc_info):
         """Undo the patch."""
+        if not self.is_started:
+            return
+
         if self.is_local and self.temp_original is not DEFAULT:
             setattr(self.target, self.attribute, self.temp_original)
         else:
@@ -1638,6 +1646,7 @@ class _patch(object):
         del self.target
         exit_stack = self._exit_stack
         del self._exit_stack
+        self.is_started = False
         return exit_stack.__exit__(*exc_info)