@@ -98,9 +98,9 @@ deprecation warning.
 `~.Axes.errorbar` now color cycles when only errorbar color is set
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 
-Previously setting the *ecolor* would turn off automatic color cycling for the plot, leading to the 
-the lines and markers defaulting to whatever the first color in the color cycle was in the case of 
-multiple plot calls. 
+Previously setting the *ecolor* would turn off automatic color cycling for the plot, leading to the
+the lines and markers defaulting to whatever the first color in the color cycle was in the case of
+multiple plot calls.
 
 `.rcsetup.validate_color_for_prop_cycle` now always raises TypeError for bytes input
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
@@ -147,3 +147,11 @@ The parameter ``s`` to `.Axes.annotate` and  `.pyplot.annotate` is renamed to
 The old parameter name remains supported, but
 support for it will be dropped in a future Matplotlib release.
 
+`pyplot.get_cmap()` now returns a copy of the colormap
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+Previously, calling ``.pyplot.get_cmap()`` would return a pointer to
+the built-in Colormap. If you made modifications to that colormap, the
+changes would be propagated in the global state. This function now
+returns a copy of all registered colormaps to keep the built-in
+colormaps untouched.
+
@@ -15,6 +15,7 @@
   normalization.
 """
 
+import copy
 import functools
 
 import numpy as np
@@ -70,7 +71,7 @@ def _gen_cmap_d():
 
 
 cmap_d = _gen_cmap_d()
-locals().update(cmap_d)
+locals().update(copy.deepcopy(cmap_d))
 
 
 # Continue with definitions ...
@@ -95,6 +96,9 @@ def register_cmap(name=None, cmap=None, data=None, lut=None):
     and the resulting colormap is registered. Instead of this implicit
     colormap creation, create a `.LinearSegmentedColormap` and use the first
     case: ``register_cmap(cmap=LinearSegmentedColormap(name, data, lut))``.
+
+    If *name* is the same as a built-in colormap this will replace the
+    built-in Colormap of the same name.
     """
     cbook._check_isinstance((str, None), name=name)
     if name is None:
@@ -124,26 +128,27 @@ def get_cmap(name=None, lut=None):
     """
     Get a colormap instance, defaulting to rc values if *name* is None.
 
-    Colormaps added with :func:`register_cmap` take precedence over
-    built-in colormaps.
+    Colormaps added with :func:`register_cmap` with the same name as
+    built-in colormaps will replace them.
 
     Parameters
     ----------
     name : `matplotlib.colors.Colormap` or str or None, default: None
-        If a `.Colormap` instance, it will be returned.  Otherwise, the name of
-        a colormap known to Matplotlib, which will be resampled by *lut*.  The
-        default, None, means :rc:`image.cmap`.
+        If a `.Colormap` instance, a copy of it will be returned.
+        Otherwise, the name of a colormap known to Matplotlib, which will
+        be resampled by *lut*. A copy of the requested Colormap is always
+        returned. The default, None, means :rc:`image.cmap`.
     lut : int or None, default: None
         If *name* is not already a Colormap instance and *lut* is not None, the
         colormap will be resampled to have *lut* entries in the lookup table.
     """
     if name is None:
         name = mpl.rcParams['image.cmap']
     if isinstance(name, colors.Colormap):
-        return name
+        return copy.copy(name)
     cbook._check_in_list(sorted(cmap_d), name=name)
     if lut is None:
-        return cmap_d[name]
+        return copy.copy(cmap_d[name])
     else:
         return cmap_d[name]._resample(lut)
 
 
@@ -601,6 +601,18 @@ def __copy__(self):
             cmapobject._lut = np.copy(self._lut)
         return cmapobject
 
+    def __eq__(self, other):
+        if isinstance(other, Colormap):
+            # To compare lookup tables the Colormaps have to be initialized
+            if not self._isinit:
+                self._init()
+            if not other._isinit:
+                other._init()
+            if self._lut.shape != other._lut.shape:
+                return False
+            return np.all(self._lut == other._lut)
+        return False
+
     def set_bad(self, color='k', alpha=None):
         """Set the color for masked values."""
         self._rgba_bad = to_rgba(color, alpha)
 
@@ -61,7 +61,7 @@ def test_resample():
 
 
 def test_register_cmap():
-    new_cm = copy.copy(plt.cm.viridis)
+    new_cm = plt.get_cmap('viridis')
     cm.register_cmap('viridis2', new_cm)
     assert plt.get_cmap('viridis2') == new_cm
 
@@ -70,6 +70,18 @@ def test_register_cmap():
         cm.register_cmap()
 
 
+def test_colormap_builtin_immutable():
+    new_cm = plt.get_cmap('viridis')
+    new_cm.set_over('b')
+    # Make sure that this didn't mess with the original viridis cmap
+    assert new_cm != plt.get_cmap('viridis')
+
+    # Also test that pyplot access doesn't mess the original up
+    new_cm = plt.cm.viridis
+    new_cm.set_over('b')
+    assert new_cm != plt.get_cmap('viridis')
+
+
 def test_colormap_copy():
     cm = plt.cm.Reds
     cm_copy = copy.copy(cm)