Move svg basename detection down to RendererSVG.

This means one fewer argument to track down the call stack (avoiding
passing it from print_svg to RendererSVG).  Wrapping the file object
in a TextIOWrapper (as done in print_svg) forwards the `.name` to the
wrapper, so we're fine there.

Author: anntzer

lib/matplotlib/backends/backend_svg.py

@@ -280,8 +280,13 @@ def __init__(self, width, height, svgwriter, basename=None, image_dpi=72,
         self.writer = XMLWriter(svgwriter)
         self.image_dpi = image_dpi  # actual dpi at which we rasterize stuff
 
-        self._groupd = {}
+        if basename is None:
+            basename = getattr(svgwriter, "name", "")
+            if not isinstance(basename, str):
+                basename = ""
         self.basename = basename
+
+        self._groupd = {}
         self._image_counter = itertools.count()
         self._clipd = {}
         self._markers = {}
@@ -1311,9 +1316,6 @@ def print_svg(self, filename, *args, **kwargs):
             __ DC_
         """
         with cbook.open_file_cm(filename, "w", encoding="utf-8") as fh:
-            filename = getattr(fh, 'name', '')
-            if not isinstance(filename, str):
-                filename = ''
             if not cbook.file_requires_unicode(fh):
                 fh = codecs.getwriter('utf-8')(fh)
             self._print_svg(filename, fh, **kwargs)
@@ -1336,7 +1338,7 @@ def _print_svg(self, filename, fh, *, dpi=None, bbox_inches_restore=None,
 
         renderer = MixedModeRenderer(
             self.figure, width, height, dpi,
-            RendererSVG(w, h, fh, filename, dpi, metadata=metadata),
+            RendererSVG(w, h, fh, image_dpi=dpi, metadata=metadata),
             bbox_inches_restore=bbox_inches_restore)
 
         self.figure.draw(renderer)