Suggest unexisting default filename (Implement matplotlib#3608)

Author: Umair Idris

lib/matplotlib/backend_bases.py

@@ -2253,9 +2253,20 @@ def get_default_filename(self):
         Return a string, which includes extension, suitable for use as
         a default filename.
         """
-        default_filename = self.get_window_title() or 'image'
-        default_filename = default_filename.lower().replace(' ', '_')
-        return default_filename + '.' + self.get_default_filetype()
+        default_basename = self.get_window_title() or 'image'
+        default_basename = default_basename.lower().replace(' ', '_')
+        default_filetype = self.get_default_filetype()
+        default_filename = default_basename + '.' + default_filetype
+
+        dir_path = os.path.expanduser(rcParams.get('savefig.directory', ''))
+
+        i = 1
+        while os.path.isfile(os.path.join(dir_path, default_filename)): # file aleady exists
+            # attach numerical value to filename
+            default_filename = '{}-{}.{}'.format(default_basename, i, default_filetype)
+            i += 1
+
+        return default_filename
 
     def switch_backends(self, FigureCanvasClass):
         """