Don't fallback to agg in tight_layout.get_renderer.

anntzer · anntzer · commit b9cea08163bb · 2020-05-26T21:01:58.000+02:00
tight_layout.get_renderer currently falls back to agg if it is unable to
get a renderer for the current canvas, but we actually have a way to
coerce the canvas to produce a renderer, via
backend_bases._get_renderer.

This renderer gets used by the tight_layout machinery to estimate text
size.  Falling back to Agg would produce bad results in the (admittedly
rare) cases where text size estimation yields very different results
depending on the renderer -- the easiest way to trigger this is to use
a "thin" font, e.g. Tex Gyre Chorus, while using the pdf backend with
the "pdf.use14corefonts" rcParam set to True, which effectively forces
Helvetica. e.g.
```
from pylab import *
matplotlib.use("pdf")
rcdefaults()
rcParams.update({"pdf.use14corefonts": True, "font.family": "TeX Gyre Chorus"})
plot()
margins(0)
tight_layout(0)
savefig("/tmp/test.pdf")
```
would crop the tick labels before this PR.

(The real point is not to fix an obscure edge case, but actually to have
fewer places that fall back to Agg as "favorited" backend.)
diff --git a/lib/matplotlib/backend_bases.py b/lib/matplotlib/backend_bases.py
@@ -1501,7 +1501,7 @@ def __init__(self, name, canvas, key, x=0, y=0, guiEvent=None):
         self.key = key
 
 
-def _get_renderer(figure, print_method, *, draw_disabled=False):
+def _get_renderer(figure, print_method=None, *, draw_disabled=False):
     """
     Get the renderer that would be used to save a `~.Figure`, and cache it on
     the figure.
@@ -1520,8 +1520,11 @@ class Done(Exception):
     def _draw(renderer): raise Done(renderer)
 
     with cbook._setattr_cm(figure, draw=_draw):
+        if print_method is None:
+            fmt = figure.canvas.get_default_filetype()
+            print_method = getattr(figure.canvas, f"print_{fmt}")
         try:
-            print_method(io.BytesIO())
+            print_method(io.BytesIO(), dpi=figure.dpi)
         except Done as exc:
             renderer, = figure._cachedRenderer, = exc.args
 
@@ -2089,7 +2092,7 @@ def print_figure(
                     renderer = _get_renderer(
                         self.figure,
                         functools.partial(
-                            print_method, dpi=dpi, orientation=orientation),
+                            print_method, orientation=orientation),
                         draw_disabled=True)
                     self.figure.draw(renderer)
                     bbox_inches = self.figure.get_tightbbox(
diff --git a/lib/matplotlib/tests/test_tightlayout.py b/lib/matplotlib/tests/test_tightlayout.py
@@ -310,3 +310,18 @@ def test_suptitle():
     t = ax.set_title("bar")
     fig.canvas.draw()
     assert st.get_window_extent().y0 > t.get_window_extent().y1
+
+
+@pytest.mark.backend("pdf")
+def test_non_agg_renderer(monkeypatch, recwarn):
+    unpatched_init = mpl.backend_bases.RendererBase.__init__
+
+    def __init__(self, *args, **kwargs):
+        # Check that we don't instantiate any other renderer than a pdf
+        # renderer to perform pdf tight layout.
+        assert isinstance(self, mpl.backends.backend_pdf.RendererPdf)
+        unpatched_init(self, *args, **kwargs)
+
+    monkeypatch.setattr(mpl.backend_bases.RendererBase, "__init__", __init__)
+    fig, ax = plt.subplots()
+    fig.tight_layout()
diff --git a/lib/matplotlib/tight_layout.py b/lib/matplotlib/tight_layout.py
@@ -166,19 +166,14 @@ def auto_adjust_subplotpars(
 
 def get_renderer(fig):
     if fig._cachedRenderer:
-        renderer = fig._cachedRenderer
+        return fig._cachedRenderer
     else:
         canvas = fig.canvas
-
         if canvas and hasattr(canvas, "get_renderer"):
-            renderer = canvas.get_renderer()
-        else:  # Some noninteractive backends have no renderer until draw time.
-            cbook._warn_external("tight_layout: falling back to Agg renderer")
-            from matplotlib.backends.backend_agg import FigureCanvasAgg
-            canvas = FigureCanvasAgg(fig)
-            renderer = canvas.get_renderer()
-
-    return renderer
+            return canvas.get_renderer()
+        else:
+            from . import backend_bases
+            return backend_bases._get_renderer(fig, draw_disabled=True)
 
 
 def get_subplotspec_list(axes_list, grid_spec=None):
