From 80d0959e4131f695fb94e814ed73b5339c98bb23 Mon Sep 17 00:00:00 2001
From: Antony Lee <anntzer.lee@gmail.com>
Date: Sat, 9 Apr 2022 23:12:33 +0200
Subject: [PATCH] Fix pickling of globally available, dynamically generated
 norm classes.

---
 lib/matplotlib/colors.py            | 23 ++++++++++++++++++++---
 lib/matplotlib/tests/test_pickle.py |  5 +++++
 2 files changed, 25 insertions(+), 3 deletions(-)

diff --git a/lib/matplotlib/colors.py b/lib/matplotlib/colors.py
index ec206b926a4d..2e519149527f 100644
--- a/lib/matplotlib/colors.py
+++ b/lib/matplotlib/colors.py
@@ -43,6 +43,7 @@
 from collections.abc import Sized, Sequence
 import copy
 import functools
+import importlib
 import inspect
 import io
 import itertools
@@ -1528,9 +1529,22 @@ def _make_norm_from_scale(scale_cls, base_norm_cls, bound_init_signature):
 
     class Norm(base_norm_cls):
         def __reduce__(self):
+            cls = type(self)
+            # If the class is toplevel-accessible, it is possible to directly
+            # pickle it "by name".  This is required to support norm classes
+            # defined at a module's toplevel, as the inner base_norm_cls is
+            # otherwise unpicklable (as it gets shadowed by the generated norm
+            # class).  If either import or attribute access fails, fall back to
+            # the general path.
+            try:
+                if cls is getattr(importlib.import_module(cls.__module__),
+                                  cls.__qualname__):
+                    return (_create_empty_object_of_class, (cls,), vars(self))
+            except (ImportError, AttributeError):
+                pass
             return (_picklable_norm_constructor,
                     (scale_cls, base_norm_cls, bound_init_signature),
-                    self.__dict__)
+                    vars(self))
 
         def __init__(self, *args, **kwargs):
             ba = bound_init_signature.bind(*args, **kwargs)
@@ -1603,11 +1617,14 @@ def autoscale_None(self, A):
     return Norm
 
 
-def _picklable_norm_constructor(*args):
-    cls = _make_norm_from_scale(*args)
+def _create_empty_object_of_class(cls):
     return cls.__new__(cls)
 
 
+def _picklable_norm_constructor(*args):
+    return _create_empty_object_of_class(_make_norm_from_scale(*args))
+
+
 @make_norm_from_scale(
     scale.FuncScale,
     init=lambda functions, vmin=None, vmax=None, clip=False: None)
diff --git a/lib/matplotlib/tests/test_pickle.py b/lib/matplotlib/tests/test_pickle.py
index 7cd23ea5c0eb..f4e35fb19b87 100644
--- a/lib/matplotlib/tests/test_pickle.py
+++ b/lib/matplotlib/tests/test_pickle.py
@@ -221,6 +221,11 @@ def test_mpl_toolkits():
     assert type(pickle.loads(pickle.dumps(ax))) == parasite_axes.HostAxes
 
 
+def test_standard_norm():
+    assert type(pickle.loads(pickle.dumps(mpl.colors.LogNorm()))) \
+        == mpl.colors.LogNorm
+
+
 def test_dynamic_norm():
     logit_norm_instance = mpl.colors.make_norm_from_scale(
         mpl.scale.LogitScale, mpl.colors.Normalize)()