From 3c84dc30a3d73dca992c064d61aae50ad4049dab Mon Sep 17 00:00:00 2001
From: Jody Klymak <jklymak@gmail.com>
Date: Thu, 9 Jul 2020 09:52:02 -0700
Subject: [PATCH] FIX: new date rcParams weren't being evaluated

---
 lib/matplotlib/dates.py            |  2 ++
 lib/matplotlib/rcsetup.py          | 18 +++++++++++-------
 lib/matplotlib/tests/test_dates.py |  2 ++
 3 files changed, 15 insertions(+), 7 deletions(-)

diff --git a/lib/matplotlib/dates.py b/lib/matplotlib/dates.py
index df916d44b60c..131eac48093c 100644
--- a/lib/matplotlib/dates.py
+++ b/lib/matplotlib/dates.py
@@ -1926,6 +1926,8 @@ class _rcParam_helper:
     @classmethod
     def set_converter(cls, s):
         """Called by validator for rcParams date.converter"""
+        if s not in ['concise', 'auto']:
+            raise ValueError('Converter must be one of "concise" or "auto"')
         cls.conv_st = s
         cls.register_converters()
 
diff --git a/lib/matplotlib/rcsetup.py b/lib/matplotlib/rcsetup.py
index 7abde998778a..cb474ea4e4d2 100644
--- a/lib/matplotlib/rcsetup.py
+++ b/lib/matplotlib/rcsetup.py
@@ -176,20 +176,24 @@ def validate_bool_maybe_none(b):
 
 
 def _validate_date_converter(s):
+    if s is None:
+        return
     s = validate_string(s)
-    mdates = sys.modules.get("matplotlib.dates")
-    if mdates:
-        mdates._rcParam_helper.set_converter(s)
+    if s not in ['auto', 'concise']:
+        cbook._warn_external(f'date.converter string must be "auto" '
+                             f'or "concise", not "{s}".  Check your '
+                             'matplotlibrc')
+        return
+    import matplotlib.dates as mdates
+    mdates._rcParam_helper.set_converter(s)
 
 
 def _validate_date_int_mult(s):
     if s is None:
         return
     s = validate_bool(s)
-    # only do this if dates is already imported...
-    mdates = sys.modules.get("matplotlib.dates")
-    if mdates:
-        mdates._rcParam_helper.set_int_mult(s)
+    import matplotlib.dates as mdates
+    mdates._rcParam_helper.set_int_mult(s)
 
 
 def _validate_tex_preamble(s):
diff --git a/lib/matplotlib/tests/test_dates.py b/lib/matplotlib/tests/test_dates.py
index 70fa15a17d0f..47634befc013 100644
--- a/lib/matplotlib/tests/test_dates.py
+++ b/lib/matplotlib/tests/test_dates.py
@@ -977,6 +977,8 @@ def test_change_converter():
     fig.canvas.draw()
     assert ax.get_xticklabels()[0].get_text() == 'Jan 01 2020'
     assert ax.get_xticklabels()[1].get_text() == 'Jan 15 2020'
+    with pytest.warns(UserWarning) as rec:
+        plt.rcParams['date.converter'] = 'boo'
 
 
 def test_change_interval_multiples():