Matplotlib Qt backend does not work on Wayland

[jbuchermn]

Describe the bug

Cannot start the Qt backend on matplotlib on Wayland. The following errors are thrown:

qt.qpa.plugin: Could not find the Qt platform plugin "wayland" in ""
qt.qpa.plugin: Could not find the Qt platform plugin "xcb" in ""

Probably qtwayland (and also something like qtxcb?) is missing as a dependency. See https://github.com/NixOS/nixpkgs/blob/master/pkgs/applications/video/vlc/default.nix which had the same issue.

Steps To Reproduce

Steps to reproduce the behavior:

1. Install matplotlib using matplotlib = super.matplotlib.override { enableQt = true; };
2. Try simple plot script python3 -c "import matplotlib.pyplot as plt; plt.figure()"

Expected behavior

Plotting works on Qt natively on Wayland

Screenshots

Additional context

Notify maintainers

@lovek323 @veprbl

Metadata

Please run nix-shell -p nix-info --run "nix-info -m" and paste the result.

[user@system:~]$ nix-shell -p nix-info --run "nix-info -m"
 - system: `"x86_64-linux"`
 - host os: `Linux 5.10.95, NixOS, 22.05 (Quokka)`
 - multi-user?: `yes`
 - sandbox: `yes`
 - version: `nix-env (Nix) 2.6.0`
 - nixpkgs: `/nix/var/nix/profiles/per-user/root/channels/nixos`

[rnhmjoj]

I think this is just #80147, unrelated to wayland.

[jbuchermn]

That‘s a 2 year old issue - are you sure?

[rnhmjoj]

It has never been fixed, so yes, sadly.

[veprbl]

Is this a bug with pyqt5 though? I suppose a reasonable solution would be to patch it to set the variables similarly to how it's done in the bash wrapper.

[rnhmjoj]

No, the problem is there is currently no way to propagate variables from python packages to the interpreter wrapper.

A quick solution was #83321, but it was scrapped by @FRidh who later proposed #102949, now stalled.

Basically everything is waiting for a unified makeWrapper interface, which is never coming for some reason.

[veprbl]

I was thinking about implementing https://github.com/NixOS/nixpkgs/blob/master/pkgs/development/libraries/qt-5/hooks/wrap-qt-apps-hook.sh in python and putting it in the pyqt5 module. This would act only for use of pyqt5 as a library in a development environment. The applications would need to be wrapped to "freeze" the paths.

[jbuchermn]

Okay, thanks for the additional infos. I understand it's not wayland related but more general.

[rnhmjoj]

If you want to use the Qt backend, for now, the only way is this workaround: #80147 (comment)

[veprbl]

Could we plug the QT_PLUGIN_PATH setting into qtbase as a setup-hook?

[jbuchermn]

If you want to use the Qt backend, for now, the only way is this workaround: #80147 (comment)

Thanks a lot, this works and I realize that this issue is a duplicate of #80147. For anyone stumbling upon this: Additionally to the comment one needs to add qt5.qtwayland as a buildInput.

Do you want me to close this issue, or do you want to continue the conversation here?

[FRidh]

Let's use #80147.