Change Locator MAXTICKS checking to emitting a log at WARNING level.

anntzer · anntzer · commit 06bdf1ed7ef6 · 2019-02-25T07:49:13.000+01:00
I chose not to create a new helper method and instead change the
behavior of raise_if_exceeds, so that third-party locators also benefit
from the change.
diff --git a/doc/api/next_api_changes/2019-02-25-AL.rst b/doc/api/next_api_changes/2019-02-25-AL.rst
@@ -0,0 +1,6 @@
+API changes
+```````````
+
+When more than `Locator.MAXTICKS` ticks are generated, the behavior of
+`Locator.raise_if_exceeds` changed from raising a RuntimeError to emitting a
+log at WARNING level.
diff --git a/lib/matplotlib/tests/test_dates.py b/lib/matplotlib/tests/test_dates.py
@@ -143,12 +143,13 @@ def test_date_axvline():
     fig.autofmt_xdate()
 
 
-def test_too_many_date_ticks():
+def test_too_many_date_ticks(caplog):
     # Attempt to test SF 2715172, see
     # https://sourceforge.net/tracker/?func=detail&aid=2715172&group_id=80706&atid=560720
     # setting equal datetimes triggers and expander call in
     # transforms.nonsingular which results in too many ticks in the
-    # DayLocator.  This should trigger a Locator.MAXTICKS RuntimeError
+    # DayLocator.  This should emit a log at WARNING level.
+    caplog.set_level("WARNING")
     t0 = datetime.datetime(2000, 1, 20)
     tf = datetime.datetime(2000, 1, 20)
     fig = plt.figure()
@@ -160,8 +161,13 @@ def test_too_many_date_ticks():
             'Attempting to set identical left == right' in str(rec[0].message)
     ax.plot([], [])
     ax.xaxis.set_major_locator(mdates.DayLocator())
-    with pytest.raises(RuntimeError):
-        fig.savefig('junk.png')
+    fig.canvas.draw()
+    # The warning is emitted multiple times because the major locator is also
+    # called both when placing the minor ticks (for overstriking detection) and
+    # during tick label positioning.
+    assert caplog.records and all(
+        record.name == "matplotlib.ticker" and record.levelname == "WARNING"
+        for record in caplog.records)
 
 
 @image_comparison(baseline_images=['RRuleLocator_bounds'], extensions=['png'])
diff --git a/lib/matplotlib/ticker.py b/lib/matplotlib/ticker.py
@@ -1455,12 +1455,21 @@ def __call__(self):
         raise NotImplementedError('Derived must override')
 
     def raise_if_exceeds(self, locs):
-        """raise a RuntimeError if Locator attempts to create more than
-           MAXTICKS locs"""
+        """
+        Log at WARNING level if *locs* is longer than `Locator.MAXTICKS`.
+
+        This is intended to be called immediately before returning *locs* from
+        ``__call__`` to inform users in case their Locator returns a huge
+        number of ticks, causing Matplotlib to run out of memory.
+
+        The "strange" name of this method dates back to when it would raise an
+        exception instead of emitting a log.
+        """
         if len(locs) >= self.MAXTICKS:
-            raise RuntimeError("Locator attempting to generate {} ticks from "
-                               "{} to {}: exceeds Locator.MAXTICKS".format(
-                                   len(locs), locs[0], locs[-1]))
+            _log.warning(
+                "Locator attempting to generate %s ticks ([%s, ..., %s]), "
+                "which exceeds Locator.MAXTICKS (%s).",
+                len(locs), locs[0], locs[-1], self.MAXTICKS)
         return locs
 
     def nonsingular(self, v0, v1):
