Simplify/improve check for pycairo in Gtk-based backends.

Gtk-based backends require pycairo (cairocffi is unsupported:
https://pygobject.readthedocs.io/en/latest/guide/cairo_integration.html).
Prior to this patch, trying to run gtk{3,4}{agg,cairo} fails *at draw
time* `TypeError: Couldn't find foreign struct converter for 'cairo.Context'`).

This patch moves the import check for cairo to the top of the common
_backend_gtk, and also simplifies backend_gtk{3,4}agg to always use
pycairo (in which case backend_cairo._to_context is a no-op) as they
cannot inter-operate with cairocffi anyways.

Author: anntzer

lib/matplotlib/backends/_backend_gtk.py

@@ -12,11 +12,17 @@
     _Backend, FigureManagerBase, NavigationToolbar2, TimerBase)
 from matplotlib.backend_tools import Cursors
 
+import gi
 # The GTK3/GTK4 backends will have already called `gi.require_version` to set
 # the desired GTK.
 from gi.repository import Gdk, Gio, GLib, Gtk
 
 
+try:
+    gi.require_foreign("cairo")
+except ImportError as e:
+    raise ImportError("Gtk-based backends require cairo") from e
+
 _log = logging.getLogger(__name__)
 
 backend_version = "%s.%s.%s" % (

lib/matplotlib/backends/backend_gtk3agg.py

@@ -1,14 +1,10 @@
 import numpy as np
 
-from .. import _api, cbook
-try:
-    from . import backend_cairo
-except ImportError as e:
-    raise ImportError('backend Gtk3Agg requires cairo') from e
+from .. import _api, cbook, transforms
 from . import backend_agg, backend_gtk3
-from .backend_cairo import cairo
 from .backend_gtk3 import Gtk, _BackendGTK3
-from matplotlib import transforms
+
+import cairo  # Presence of cairo is already checked by _backend_gtk.
 
 
 class FigureCanvasGTK3Agg(backend_gtk3.FigureCanvasGTK3,
@@ -32,8 +28,6 @@ def on_draw_event(self, widget, ctx):
         else:
             bbox_queue = self._bbox_queue
 
-        ctx = backend_cairo._to_context(ctx)
-
         for bbox in bbox_queue:
             x = int(bbox.x0)
             y = h - int(bbox.y1)

lib/matplotlib/backends/backend_gtk4agg.py

@@ -1,14 +1,11 @@
 import numpy as np
 
 from .. import _api, cbook
-try:
-    from . import backend_cairo
-except ImportError as e:
-    raise ImportError('backend Gtk4Agg requires cairo') from e
 from . import backend_agg, backend_gtk4
-from .backend_cairo import cairo
 from .backend_gtk4 import Gtk, _BackendGTK4
 
+import cairo  # Presence of cairo is already checked by _backend_gtk.
+
 
 class FigureCanvasGTK4Agg(backend_gtk4.FigureCanvasGTK4,
                           backend_agg.FigureCanvasAgg):
@@ -24,8 +21,6 @@ def on_draw_event(self, widget, ctx):
             allocation.x, allocation.y,
             allocation.width, allocation.height)
 
-        ctx = backend_cairo._to_context(ctx)
-
         buf = cbook._unmultiplied_rgba8888_to_premultiplied_argb32(
             np.asarray(self.get_renderer().buffer_rgba()))
         height, width, _ = buf.shape