matplotlib
diff --git a/‎lib/matplotlib/axis.py
Copy file name to clipboardExpand all lines: lib/matplotlib/axis.py
+45-8Lines changed: 45 additions & 8 deletions b/‎lib/matplotlib/axis.py
Copy file name to clipboardExpand all lines: lib/matplotlib/axis.py
+45-8Lines changed: 45 additions & 8 deletions
@@ -21,6 +21,7 @@
 import matplotlib.units as munits
 import numpy as np
 import warnings
+import itertools
 
 GRIDLINE_INTERPOLATION_STEPS = 180
 
@@ -625,6 +626,42 @@ class Ticker(object):
     formatter = None
 
 
+class LazyDefaultTickList(object):
+    """
+    A lazy evaluating placeholder for the default tick list.
+
+    On access the class replaces itself with the default 1-element list. We
+    use the lazy evaluation so that :meth:`matplotlib.axis.Axis.reset_ticks`
+    can be called multiple times without the overhead of initialzing every
+    time.
+
+    https://github.com/matplotlib/matplotlib/issues/6664
+    """
+    def __init__(self, axis, major=True):
+        self.axis = axis
+        self.major = major
+
+    def _instantiate_list(self):
+        if self.major:
+            self.axis.majorTicks = [self.axis._get_tick(major=True)]
+            self.axis._lastNumMajorTicks = 1
+            return self.axis.majorTicks
+        else:
+            self.axis.minorTicks = [self.axis._get_tick(major=False)]
+            self.axis._lastNumMinorTicks = 1
+            return self.axis.minorTicks
+
+    def __iter__(self):
+        return iter(self._instantiate_list())
+
+    def __len__(self):
+        return len(self._instantiate_list())
+
+    def __getitem__(self, key):
+        l = self._instantiate_list()
+        return l[key]
+
+
 class Axis(artist.Artist):
     """
     Public attributes
@@ -781,13 +818,12 @@ def reset_ticks(self):
         # build a few default ticks; grow as necessary later; only
         # define 1 so properties set on ticks will be copied as they
         # grow
-        del self.majorTicks[:]
-        del self.minorTicks[:]
-
-        self.majorTicks.extend([self._get_tick(major=True)])
-        self.minorTicks.extend([self._get_tick(major=False)])
-        self._lastNumMajorTicks = 1
-        self._lastNumMinorTicks = 1
+        if not isinstance(self.majorTicks, LazyDefaultTickList):
+            del self.majorTicks[:]
+            self.majorTicks = LazyDefaultTickList(self, major=True)
+        if not isinstance(self.minorTicks, LazyDefaultTickList):
+            del self.minorTicks[:]
+            self.minorTicks = LazyDefaultTickList(self, major=False)
 
     def set_tick_params(self, which='major', reset=False, **kw):
         """
@@ -872,7 +908,8 @@ def _translate_tick_kw(kw, to_init_kw=True):
 
     def set_clip_path(self, clippath, transform=None):
         artist.Artist.set_clip_path(self, clippath, transform)
-        for child in self.majorTicks + self.minorTicks:
+        for child in itertools.chain(iter(self.majorTicks),
+                                     iter(self.minorTicks)):
             child.set_clip_path(clippath, transform)
         self.stale = True