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.

Author: djoume

Lib/pickle.py

@@ -1349,12 +1349,19 @@ def load(self):
                 if not key:
                     raise EOFError
                 assert isinstance(key, bytes_types)
-                dispatch[key[0]](self)
+                try:
+                    func = dispatch[key[0]]
+                except KeyError:
+                    raise UnpicklingError(
+                        f"invalid load key, '\\x{key[0]:02x}'.")
+                func(self)
         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

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)

Misc/NEWS.d/next/Library/2025-11-19-15-23-30.gh-issue-141749.QRBf7O.rst

@@ -0,0 +1,3 @@
+:mod:`pickle`: align exceptions raised by the pure Python implementation of :func:`pickle.load`
+with the C implementation. Previous cases raising :exc:`KeyError` or :exc:`IndexError`
+now raise :exc:`~pickle.UnpicklingError`. Patch by Djoume Salvetti.