TypeError constructor returned NULL in wayland session

[liuyun88]

Bug report

Bug summary

When trying to run matplotlib pyplot, and setting the backend to gtk3agg or gtk3cairo, the following error is encountered in wayland session. The same code works fine in Xorg, or using Qt backend.

Gdk-Message: 12:41:05.918: Unable to load fleur from the cursor theme
Traceback (most recent call last):
  File "/usr/lib/python3.9/site-packages/matplotlib/backends/backend_gtk3.py", line 40, in <module>
    cursors.MOVE:          Gdk.Cursor.new(Gdk.CursorType.FLEUR),
TypeError: constructor returned NULL

Code for reproduction

import matplotlib.pyplot as plt
fig, ax0 = plt.subplots()
ax0.plot(1,1)
plt.show()

Actual outcome

Traceback (most recent call last):
  File "/usr/lib/python3.9/site-packages/matplotlib/backends/backend_gtk3.py", line 40, in <module>
    cursors.MOVE:          Gdk.Cursor.new(Gdk.CursorType.FLEUR),
TypeError: constructor returned NULL

The above exception was the direct cause of the following exception:

Traceback (most recent call last):
  File "/home/yl398/test.py", line 1, in <module>
    import matplotlib.pyplot as plt
  File "/usr/lib/python3.9/site-packages/matplotlib/pyplot.py", line 2336, in <module>
    switch_backend(rcParams["backend"])
  File "/usr/lib/python3.9/site-packages/matplotlib/pyplot.py", line 276, in switch_backend
    class backend_mod(matplotlib.backend_bases._Backend):
  File "/usr/lib/python3.9/site-packages/matplotlib/pyplot.py", line 277, in backend_mod
    locals().update(vars(importlib.import_module(backend_name)))
  File "/usr/lib/python3.9/importlib/__init__.py", line 127, in import_module
    return _bootstrap._gcd_import(name[level:], package, level)
  File "/usr/lib/python3.9/site-packages/matplotlib/backends/backend_gtk3agg.py", line 8, in <module>
    from . import backend_agg, backend_gtk3
  File "/usr/lib/python3.9/site-packages/matplotlib/backends/backend_gtk3.py", line 49, in <module>
    raise ImportError(exc) from exc
ImportError: constructor returned NULL

Expected outcome

No error should be encountered and the figure should show on the screen

Matplotlib version

• Operating system: Manjaro Linux 20.2.1
• Matplotlib version (import matplotlib; print(matplotlib.__version__)): 3.3.3
• Matplotlib backend (print(matplotlib.get_backend())): GTK3AGG or GTK3Cairo
• Python version: 3.9.1
• Jupyter version (if applicable):
• Other libraries:

Installed matplotlib from default manjaro/arch linux repository.

[anntzer]

Looks like the fix is

diff --git i/lib/matplotlib/backends/backend_gtk3.py w/lib/matplotlib/backends/backend_gtk3.py
index e9e78b43e..486ba9c1e 100644
--- i/lib/matplotlib/backends/backend_gtk3.py
+++ w/lib/matplotlib/backends/backend_gtk3.py
@@ -36,12 +36,13 @@ backend_version = "%s.%s.%s" % (
     Gtk.get_major_version(), Gtk.get_micro_version(), Gtk.get_minor_version())
 
 try:
+    _display = Gdk.Display.get_default()
     cursord = {
-        cursors.MOVE:          Gdk.Cursor.new(Gdk.CursorType.FLEUR),
-        cursors.HAND:          Gdk.Cursor.new(Gdk.CursorType.HAND2),
-        cursors.POINTER:       Gdk.Cursor.new(Gdk.CursorType.LEFT_PTR),
-        cursors.SELECT_REGION: Gdk.Cursor.new(Gdk.CursorType.TCROSS),
-        cursors.WAIT:          Gdk.Cursor.new(Gdk.CursorType.WATCH),
+        cursors.MOVE:          Gdk.Cursor.new_from_name(_display, "move"),
+        cursors.HAND:          Gdk.Cursor.new_from_name(_display, "pointer"),
+        cursors.POINTER:       Gdk.Cursor.new_from_name(_display, "default"),
+        cursors.SELECT_REGION: Gdk.Cursor.new_from_name(_display, "crosshair"),
+        cursors.WAIT:          Gdk.Cursor.new_from_name(_display, "wait"),
     }
 except TypeError as exc:
     # Happens when running headless.  Convert to ImportError to cooperate with

(per https://developer.gnome.org/gdk3/stable/gdk3-Cursors.html, new_from_name exists since gtk2.8, and is preferred over using GdkCursorType (for which a note says "these IDs are directly taken from the X cursor font, and many of these cursors are either not useful, or are not available on other platforms.").)

[liuyun88]

Thanks. Can confirm the fix works. The cursor changes to a different theme from the system default when hovering over the pyplot window, but that is a minor issue.

Edit: everything works fine. No longer have the different cursor theme issue.

[tacaswell]

@liuyun88 Would you be willing to open a PR with that patch?

[liuyun88]

@liuyun88 Would you be willing to open a PR with that patch?

Just opened a PR. First time doing it and welcoming all suggestions and advice.