From ba71202d484187e42ed09d9191b9e32a42ddd63d Mon Sep 17 00:00:00 2001
From: Djoume Salvetti <djoume@taket.org>
Date: Wed, 19 Nov 2025 10:03:52 -0500
Subject: [PATCH 1/2] gh-141749 fix pure python pickle.py error handling

This fix addresses error handling inconsistencies in CPython's pure Python `pickle.py` implementation to match the behavior of the C `_pickle` module. The changes make the pure Python implementation raise proper `UnpicklingError` exceptions for invalid pickle data instead of low-level `KeyError` and `IndexError` exceptions.
---
 Lib/pickle.py           | 8 +++++++-
 Lib/test/test_pickle.py | 2 +-
 2 files changed, 8 insertions(+), 2 deletions(-)

diff --git a/Lib/pickle.py b/Lib/pickle.py
index 729c215514ad24..fe984476fed346 100644
--- a/Lib/pickle.py
+++ b/Lib/pickle.py
@@ -1322,12 +1322,18 @@ def load(self):
                 if not key:
                     raise EOFError
                 assert isinstance(key, bytes_types)
-                dispatch[key[0]](self)
+                try:
+                    dispatch[key[0]](self)
+                except KeyError:
+                    raise UnpicklingError(
+                        f"invalid load key, '\\x{key[0]:02x}'.")
         except _Stop as stopinst:
             return stopinst.value
 
     # Return a list of items pushed in the stack after last MARK instruction.
     def pop_mark(self):
+        if not self.metastack:
+            raise UnpicklingError("could not find MARK")
         items = self.stack
         self.stack = self.metastack.pop()
         self.append = self.stack.append
diff --git a/Lib/test/test_pickle.py b/Lib/test/test_pickle.py
index e2384b33345a45..d1afcb5a0cc591 100644
--- a/Lib/test/test_pickle.py
+++ b/Lib/test/test_pickle.py
@@ -55,7 +55,7 @@ class PyPickleTests(AbstractPickleModuleTests, unittest.TestCase):
 class PyUnpicklerTests(AbstractUnpickleTests, unittest.TestCase):
 
     unpickler = pickle._Unpickler
-    bad_stack_errors = (IndexError,)
+    bad_stack_errors = (pickle.UnpicklingError, IndexError)
     truncated_errors = (pickle.UnpicklingError, EOFError,
                         AttributeError, ValueError,
                         struct.error, IndexError, ImportError)

From 0c0376fdf71122c5556fd0b0d302c92264ca651a Mon Sep 17 00:00:00 2001
From: "blurb-it[bot]" <43283697+blurb-it[bot]@users.noreply.github.com>
Date: Wed, 19 Nov 2025 15:23:31 +0000
Subject: [PATCH 2/2] =?UTF-8?q?=F0=9F=93=9C=F0=9F=A4=96=20Added=20by=20blu?=
 =?UTF-8?q?rb=5Fit.?=
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Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 8bit

---
 .../next/Library/2025-11-19-15-23-30.gh-issue-141749.QRBf7O.rst  | 1 +
 1 file changed, 1 insertion(+)
 create mode 100644 Misc/NEWS.d/next/Library/2025-11-19-15-23-30.gh-issue-141749.QRBf7O.rst

diff --git a/Misc/NEWS.d/next/Library/2025-11-19-15-23-30.gh-issue-141749.QRBf7O.rst b/Misc/NEWS.d/next/Library/2025-11-19-15-23-30.gh-issue-141749.QRBf7O.rst
new file mode 100644
index 00000000000000..85d38eeb682f3a
--- /dev/null
+++ b/Misc/NEWS.d/next/Library/2025-11-19-15-23-30.gh-issue-141749.QRBf7O.rst
@@ -0,0 +1 @@
+Pure python pickle.py error handling is more consistent with the c implementation, raising UnpicklingError exceptions for invalid pickle data instead of KeyError or IndexError.