Fix tight_layout() on "canvasless" figures.

This patch avoids a deprecation warning ("unexpected argument 'dpi'")
on `Figure().tight_layout()`.  To do so we stop passing `dpi` to
`print_foo`, which is actually fine because `print_figure` already sets
the figure dpi to whatever value we will want, so `print_foo` can just
read `self.figure.dpi`.  (The pgf backend already handles that
correctly.)

Author: anntzer

doc/api/next_api_changes/deprecations/19503-AL.rst

@@ -0,0 +1,6 @@
+The *dpi* parameter of ``FigureCanvas.print_foo`` printers is deprecated
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+The `~.Figure.savefig` machinery already took care of setting the figure dpi
+to the desired value, so ``print_foo`` can directly read it from there.  Not
+passing *dpi* to ``print_foo`` allows clearer detection of unused parameters
+passed to `~.Figure.savefig`.

lib/matplotlib/backend_bases.py

@@ -1573,7 +1573,7 @@ def _draw(renderer): raise Done(renderer)
             print_method = getattr(
                 figure.canvas._get_output_canvas(None, fmt), f"print_{fmt}")
         try:
-            print_method(io.BytesIO(), dpi=figure.dpi)
+            print_method(io.BytesIO())
         except Done as exc:
             renderer, = figure._cachedRenderer, = exc.args
             return renderer
@@ -2235,14 +2235,16 @@ def print_figure(
             cl_state = self.figure.get_constrained_layout()
             self.figure.set_constrained_layout(False)
             try:
-                result = print_method(
-                    filename,
-                    dpi=dpi,
-                    facecolor=facecolor,
-                    edgecolor=edgecolor,
-                    orientation=orientation,
-                    bbox_inches_restore=_bbox_inches_restore,
-                    **kwargs)
+                # _get_renderer may change the figure dpi (as vector formats
+                # force the figure dpi to 72), so we need to set it again here.
+                with cbook._setattr_cm(self.figure, dpi=dpi):
+                    result = print_method(
+                        filename,
+                        facecolor=facecolor,
+                        edgecolor=edgecolor,
+                        orientation=orientation,
+                        bbox_inches_restore=_bbox_inches_restore,
+                        **kwargs)
             finally:
                 if bbox_inches and restore_bbox:
                     restore_bbox()

lib/matplotlib/backends/backend_pdf.py

@@ -2701,10 +2701,13 @@ def get_default_filetype(self):
         return 'pdf'
 
     @_check_savefig_extra_args
+    @_api.delete_parameter("3.4", "dpi")
     def print_pdf(self, filename, *,
-                  dpi=72,  # dpi to use for images
+                  dpi=None,  # dpi to use for images
                   bbox_inches_restore=None, metadata=None):
 
+        if dpi is None:  # always use this branch after deprecation elapses.
+            dpi = self.figure.get_dpi()
         self.figure.set_dpi(72)            # there are 72 pdf points to an inch
         width, height = self.figure.get_size_inches()
         if isinstance(filename, PdfPages):

lib/matplotlib/backends/backend_ps.py

@@ -829,11 +829,14 @@ def print_ps(self, outfile, *args, **kwargs):
     def print_eps(self, outfile, *args, **kwargs):
         return self._print_ps(outfile, 'eps', *args, **kwargs)
 
+    @_api.delete_parameter("3.4", "dpi")
     def _print_ps(
             self, outfile, format, *args,
-            dpi=72, metadata=None, papertype=None, orientation='portrait',
+            dpi=None, metadata=None, papertype=None, orientation='portrait',
             **kwargs):
 
+        if dpi is None:  # always use this branch after deprecation elapses.
+            dpi = self.figure.get_dpi()
         self.figure.set_dpi(72)  # Override the dpi kwarg
 
         dsc_comments = {}

lib/matplotlib/backends/backend_svg.py

@@ -1343,9 +1343,12 @@ def print_svgz(self, filename, *args, **kwargs):
             return self.print_svg(gzipwriter)
 
     @_check_savefig_extra_args
-    def _print_svg(self, filename, fh, *, dpi=72, bbox_inches_restore=None,
+    @_api.delete_parameter("3.4", "dpi")
+    def _print_svg(self, filename, fh, *, dpi=None, bbox_inches_restore=None,
                    metadata=None):
-        self.figure.set_dpi(72.0)
+        if dpi is None:  # always use this branch after deprecation elapses.
+            dpi = self.figure.get_dpi()
+        self.figure.set_dpi(72)
         width, height = self.figure.get_size_inches()
         w, h = width * 72, height * 72
 

lib/matplotlib/backends/backend_template.py

@@ -204,9 +204,13 @@ def draw(self):
 
     def print_foo(self, filename, *args, **kwargs):
         """
-        Write out format foo.  The dpi, facecolor and edgecolor are restored
-        to their original values after this call, so you don't need to
-        save and restore them.
+        Write out format foo.
+
+        This method is normally called via `.Figure.savefig` and
+        `.FigureCanvasBase.print_figure`, which take care of setting the figure
+        facecolor, edgecolor, and dpi to the desired output values, and will
+        restore them to the original values.  Therefore, `print_foo` does not
+        need to handle these settings.
         """
         self.draw()