FIX: be more careful about not importing pyplot early

tacaswell · tacaswell · commit a5a1311b1ec7 · 2020-06-25T23:11:07.000-04:00
In matplotlib.use we import pyplot to use `switch_backend`, however if the user calls `mpl.use(...)` before importing pyplot then during the initial import of pyplot, before we set the selected backend we try to set the backend set via rcParams. This change only imports pyplot if it is already imported, otherwise it is safe to just set the rcParams and not go through the full `plt.switch_backend` path. closes #17763
diff --git a/lib/matplotlib/__init__.py b/lib/matplotlib/__init__.py
@@ -1140,12 +1140,15 @@ def use(backend, *, force=True):
         # Nothing to do if the requested backend is already set
         pass
     else:
-        try:
-            from matplotlib import pyplot as plt
-            plt.switch_backend(name)
-        except ImportError:
-            if force:
-                raise
+        if 'matplotlib.pyplot' in sys.modules:
+            try:
+                from matplotlib import pyplot as plt
+                plt.switch_backend(name)
+            except ImportError:
+                if force:
+                    raise
+        else:
+            rcParams['backend'] = backend
 
 
 if os.environ.get('MPLBACKEND'):
