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解决报错: qt.qpa.plugin Could not find the Qt platform plugin "xcb" in ""

问题

使用plotly绘图生成图片的时候遇到如下报错：

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qt.qpa.plugin: Could not find the Qt platform plugin "xcb" in ""
This application failed to start because no Qt platform plugin could be initialized. Reinstalling the application may fix this problem.

解决过程

尝试一

参考这篇Askubuntu文章

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指定环境变量`QT_QPA_PLATFORM_PLUGIN_PATH`。
```bash
export QT_QPA_PLATFORM_PLUGIN_PATH=/usr/lib/x86_64-linux-gnu/qt5/plugins/platforms

报错变为：

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qt.core.plugin.loader: In /usr/lib/x86_64-linux-gnu/qt5/plugins/platforms/libqeglfs.so:
  Plugin uses incompatible Qt library (5.15.0) [release]
qt.core.plugin.loader: In /usr/lib/x86_64-linux-gnu/qt5/plugins/platforms/libqlinuxfb.so:
  Plugin uses incompatible Qt library (5.15.0) [release]
qt.core.plugin.loader: In /usr/lib/x86_64-linux-gnu/qt5/plugins/platforms/libqminimal.so:
  Plugin uses incompatible Qt library (5.15.0) [release]
qt.core.plugin.loader: In /usr/lib/x86_64-linux-gnu/qt5/plugins/platforms/libqminimalegl.so:
  Plugin uses incompatible Qt library (5.15.0) [release]
qt.core.plugin.loader: In /usr/lib/x86_64-linux-gnu/qt5/plugins/platforms/libqoffscreen.so:
  Plugin uses incompatible Qt library (5.15.0) [release]
qt.core.plugin.loader: In /usr/lib/x86_64-linux-gnu/qt5/plugins/platforms/libqvnc.so:
  Plugin uses incompatible Qt library (5.15.0) [release]
qt.core.plugin.loader: In /usr/lib/x86_64-linux-gnu/qt5/plugins/platforms/libqxcb.so:
  Plugin uses incompatible Qt library (5.15.0) [release]
qt.qpa.plugin: Could not find the Qt platform plugin "xcb" in "/usr/lib/x86_64-linux-gnu/qt5/plugins/platforms"
This application failed to start because no Qt platform plugin could be initialized. Reinstalling the application may fix this problem.

尝试二

考虑python程序用了PySide6包和QT6相关，考虑是不是因为这个版本不匹配，于是安装了Qt6并将环境变量指向Qt6的位置：

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sudo apt install qt6-base-dev qt6-base-dev-tools
export QT_QPA_PLATFORM_PLUGIN_PATH=/usr/lib/x86_64-linux-gnu/qt6/plugins/platforms

报错变为：

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qt.qpa.plugin: Could not load the Qt platform plugin "xcb" in "/usr/lib/x86_64-linux-gnu/qt6/plugins/platforms" even though it was found.
This application failed to start because no Qt platform plugin could be initialized. Reinstalling the application may fix this problem.

Available platform plugins are: eglfs, linuxfb, minimal, minimalegl, offscreen, vkkhrdisplay, vnc, xcb.

再考虑是不是python包版本不匹配，于是强制升级了PySide6和PyQt6。
报错变为：

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  File "/anaconda3/lib/python3.9/site-packages/matplotlib/_api/deprecation.py", line 454, in wrapper
    return func(*args, **kwargs)
  File "/anaconda3/lib/python3.9/site-packages/matplotlib/pyplot.py", line 771, in figure
    manager = new_figure_manager(
  File "/anaconda3/lib/python3.9/site-packages/matplotlib/pyplot.py", line 346, in new_figure_manager
    _warn_if_gui_out_of_main_thread()
  File "/anaconda3/lib/python3.9/site-packages/matplotlib/pyplot.py", line 336, in _warn_if_gui_out_of_main_thread
    if (_get_required_interactive_framework(_get_backend_mod()) and
  File "/anaconda3/lib/python3.9/site-packages/matplotlib/pyplot.py", line 206, in _get_backend_mod
    switch_backend(dict.__getitem__(rcParams, "backend"))
  File "/anaconda3/lib/python3.9/site-packages/matplotlib/pyplot.py", line 251, in switch_backend
    switch_backend(candidate)
  File "/anaconda3/lib/python3.9/site-packages/matplotlib/pyplot.py", line 264, in switch_backend
    backend_mod = importlib.import_module(
  File "/anaconda3/lib/python3.9/importlib/__init__.py", line 127, in import_module
    return _bootstrap._gcd_import(name[level:], package, level)
  File "<frozen importlib._bootstrap>", line 1030, in _gcd_import
  File "<frozen importlib._bootstrap>", line 1007, in _find_and_load
  File "<frozen importlib._bootstrap>", line 986, in _find_and_load_unlocked
  File "<frozen importlib._bootstrap>", line 680, in _load_unlocked
  File "<frozen importlib._bootstrap_external>", line 850, in exec_module
  File "<frozen importlib._bootstrap>", line 228, in _call_with_frames_removed
  File "/anaconda3/lib/pythoadd n3.9/site-packages/matplotlib/backends/backend_qtagg.py", line 12, in <module>
    from .backend_qt import QtCore, QtGui, _BackendQT, FigureCanvasQT
  File "/anaconda3/lib/python3.9/site-packages/matplotlib/backends/backend_qt.py", line 72, in <module>
    _MODIFIER_KEYS = [
  File "/anaconda3/lib/python3.9/site-packages/matplotlib/backends/backend_qt.py", line 73, in <listcomp>
    (_to_int(getattr(_enum("QtCore.Qt.KeyboardModifier"), mod)),
TypeError: int() argument must be a string, a bytes-like object or a number, not 'KeyboardModifier'

发现涉及到matplotlib，同样升级之，错误变为：

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qt.qpa.plugin: Could not load the Qt platform plugin "xcb" in "/usr/lib/x86_64-linux-gnu/qt6/plugins/platforms" even though it was found.
This application failed to start because no Qt platform plugin could be initialized. Reinstalling the application may fix this problem.

Available platform plugins are: eglfs, offscreen, minimal, xcb, wayland-egl, linuxfb, vkkhrdisplay, wayland, vnc, minimalegl.

百思不得其解。

解决

网上找到设置export QT_DEBUG_PLUGINS=1后可以让Qt输出详细debug信息，设置后运行程序输出：

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qt.core.plugin.factoryloader: Got keys from plugin meta data QList("minimalegl")
qt.core.library: "/anaconda3/lib/python3.9/site-packages/PyQt6/Qt6/plugins/platforms/libqxcb.so" cannot load: Cannot load library /anaconda3/lib/python3.9/site-packages/PyQt6/Qt6/plugins/platforms/libqxcb.so: (libxcb-cursor.so.0: cannot open shared object file: No such file or directory)
qt.core.plugin.loader: QLibraryPrivate::loadPlugin failed on "/anaconda3/lib/python3.9/site-packages/PyQt6/Qt6/plugins/platforms/libqxcb.so" : "Cannot load library /anaconda3/lib/python3.9/site-packages/PyQt6/Qt6/plugins/platforms/libqxcb.so: (libxcb-cursor.so.0: cannot open shared object file: No such file or directory)"
qt.qpa.plugin: Could not load the Qt platform plugin "xcb" in "/usr/lib/x86_64-linux-gnu/qt6/plugins/platforms" even though it was found.
This application failed to start because no Qt platform plugin could be initialized. Reinstalling the application may fix this problem.

发现

  • 实际上被使用的是PyQt6自带的插件动态库，并没有用QT_QPA_PLATFORM_PLUGIN_PATH指定的系统库。
  • 错误原因是缺失libxcb-cursor.so.0。
    于是安装libxcb-cursor
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sudo apt install libxcb-cursor*

再次运行发现不再报错。问题解决。

删除掉QT_DEBUG_PLUGINS和QT_QPA_PLATFORM_PLUGIN_PATH环境变量（因为实际上并没有用系统的Qt6，所以QT_QPA_PLATFORM_PLUGIN_PATH删掉不影响什么）。