Don't alter a module when reducing it

CHANGES.md

@@ -4,6 +4,9 @@
 dev
 ===
 
+- Fix a side effect altering dynamic modules at pickling time.
+  ([PR #426](https://github.com/cloudpipe/cloudpickle/pull/426))
+
 - Support for pickling type annotations on Python 3.10 as per [PEP 563](
   https://www.python.org/dev/peps/pep-0563/)
   ([PR #400](https://github.com/cloudpipe/cloudpickle/pull/400))

cloudpickle/cloudpickle_fast.py

@@ -342,8 +342,13 @@ def _module_reduce(obj):
     if _is_importable(obj):
         return subimport, (obj.__name__,)
     else:
-        obj.__dict__.pop('__builtins__', None)
-        return dynamic_subimport, (obj.__name__, vars(obj))
+        # Some external libraries can populate the "__builtins__" entry of a
+        # module's `__dict__` with unpicklable objects (see #316). For that
+        # reason, we do not attempt to pickle the "__builtins__" entry, and
+        # restore a default value for it at unpickling time.
+        state = obj.__dict__.copy()
+        state.pop('__builtins__', None)
+        return dynamic_subimport, (obj.__name__, state)
 
 
 def _method_reduce(obj):

tests/cloudpickle_test.py

@@ -604,6 +604,12 @@ def __reduce__(self):
             assert hasattr(depickled_mod.__builtins__, "abs")
         assert depickled_mod.f(-1) == 1
 
+        # Additional check testing that the issue #425 is fixed: without the
+        # fix for #425, `mod.f` would not have access to `__builtins__`, and
+        # thus calling `mod.f(-1)` (which relies on the `abs` builtin) would
+        # fail.
+        assert mod.f(-1) == 1
+
     def test_load_dynamic_module_in_grandchild_process(self):
         # Make sure that when loaded, a dynamic module preserves its dynamic
         # property. Otherwise, this will lead to an ImportError if pickled in