Simplify animation writer fallback.

anntzer · anntzer · commit 4bbca8865c65 · 2020-03-31T00:26:05.000+02:00
The fallback now always returns PillowWriter, which is the first
registered writer and is always available, so we may just as well be
more explicit.

(Not all writers support all output formats, but we don't have machinery
to select writers based on the output format right now anyways.)
diff --git a/lib/matplotlib/animation.py b/lib/matplotlib/animation.py
@@ -1087,17 +1087,13 @@ def func(current_frame: int, total_frames: int) -> Any
         # If we have the name of a writer, instantiate an instance of the
         # registered class.
         if isinstance(writer, str):
-            if writers.is_available(writer):
-                writer = writers[writer](fps, **writer_kwargs)
-            else:
-                alt_writer = next(iter(writers), None)
-                if alt_writer is None:
-                    raise ValueError("Cannot save animation: no writers are "
-                                     "available. Please install ffmpeg to "
-                                     "save animations.")
-                _log.warning("MovieWriter %s unavailable; trying to use %s "
-                             "instead.", writer, alt_writer)
-                writer = alt_writer(fps, **writer_kwargs)
+            try:
+                writer_cls = writers[writer]
+            except RuntimeError:  # Raised if not available.
+                writer_cls = PillowWriter  # Always available.
+                _log.warning("MovieWriter %s unavailable; using Pillow "
+                             "instead.", writer)
+            writer = writer_cls(fps, **writer_kwargs)
         _log.info('Animation.save using %s', type(writer))
 
         if 'bbox_inches' in savefig_kwargs:
