Fix %matplotlib

args.gui may not be a str

IPython/core/magics/pylab.py

@@ -96,7 +96,7 b' class PylabMagics(Magics):'
             backends_list = list(backends.keys())
             print("Available matplotlib backends: %s" % backends_list)
         else:
-            gui, backend = self.shell.enable_matplotlib(args.gui.lower())
+            gui, backend = self.shell.enable_matplotlib(args.gui.lower() if isinstance(args.gui, str) else args.gui)
             self._show_matplotlib_backend(args.gui, backend)
 
     @skip_doctest