Make rcParams.copy() return a new RcParams instance.

... so that `rcParams.update(<previously-copied-rcparams-instance>)`
doesn't emit DeprecationWarnings even if some entries are deprecated.
(See comment in tests.)

Author: anntzer

doc/api/next_api_changes/behavior/21042-AL.rst

@@ -0,0 +1,5 @@
+rcParams.copy() returns a new RcParams instance, rather than a dict
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+This makes the copied instance still validate inputs, and additionally avoids
+emitting deprecation warnings when using a previously copied RcParams instance
+to update the global instance (even if some entries are deprecated).

lib/matplotlib/__init__.py

@@ -699,7 +699,10 @@ def find_all(self, pattern):
                         if pattern_re.search(key))
 
     def copy(self):
-        return {k: dict.__getitem__(self, k) for k in self}
+        rccopy = RcParams()
+        for k in self:  # Skip deprecations and revalidation.
+            dict.__setitem__(rccopy, k, dict.__getitem__(self, k))
+        return rccopy
 
 
 def rc_params(fail_on_error=False):
@@ -877,8 +880,8 @@ def rc_params_from_file(fname, fail_on_error=False, use_default_template=True):
 rcParams = RcParams()  # The global instance.
 dict.update(rcParams, dict.items(rcParamsDefault))
 dict.update(rcParams, _rc_params_in_file(matplotlib_fname()))
+rcParamsOrig = rcParams.copy()
 with _api.suppress_matplotlib_deprecation_warning():
-    rcParamsOrig = RcParams(rcParams.copy())
     # This also checks that all rcParams are indeed listed in the template.
     # Assigning to rcsetup.defaultParams is left only for backcompat.
     defaultParams = rcsetup.defaultParams = {

lib/matplotlib/tests/test_rcparams.py

@@ -546,3 +546,8 @@ def test_deprecation(monkeypatch):
         mpl.rcParams["svg.hashsalt"] = "foobar"
     assert mpl.rcParams["svg.hashsalt"] == "foobar"  # Doesn't warn.
     mpl.rcParams["svg.hashsalt"] = None  # Doesn't warn.
+
+    mpl.rcParams.update(mpl.rcParams.copy())  # Doesn't warn.
+    # Note that the warning suppression actually arises from the
+    # iteration over the updater rcParams being protected by
+    # suppress_matplotlib_deprecation_warning, rather than any explicit check.