ImportError: Gtk-based backends require cairo | No module named 'gi._gi_cairo'

Question

The original problem

On Ubuntu 22.04, I'm getting the following error with this MWE:

import matplotlib as mpl
from matplotlib import pyplot as plt

mpl.use("GTK4Agg")

The full traceback being

niko@niko-ubuntu:~/myproj$ python script.py 
Traceback (most recent call last):
  File "/usr/lib/python3/dist-packages/gi/__init__.py", line 176, in require_foreign
    _gi.require_foreign(namespace, symbol)
ModuleNotFoundError: No module named 'gi._gi_cairo'

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/home/niko/.local/lib/python3.10/site-packages/matplotlib/backends/_backend_gtk.py", line 23, in <module>
    gi.require_foreign("cairo")
  File "/usr/lib/python3/dist-packages/gi/__init__.py", line 178, in require_foreign
    raise ImportError(str(e))
ImportError: No module named 'gi._gi_cairo'

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

Traceback (most recent call last):
  File "/home/niko/myproj/script.py", line 4, in <module>
    mpl.use("GTK4Agg")
  File "/home/niko/.local/lib/python3.10/site-packages/matplotlib/__init__.py", line 1233, in use
    plt.switch_backend(name)
  File "/home/niko/.local/lib/python3.10/site-packages/matplotlib/pyplot.py", line 271, in switch_backend
    backend_mod = importlib.import_module(
  File "/usr/lib/python3.10/importlib/__init__.py", line 126, in import_module
    return _bootstrap._gcd_import(name[level:], package, level)
  File "<frozen importlib._bootstrap>", line 1050, in _gcd_import
  File "<frozen importlib._bootstrap>", line 1027, in _find_and_load
  File "<frozen importlib._bootstrap>", line 1006, in _find_and_load_unlocked
  File "<frozen importlib._bootstrap>", line 688, in _load_unlocked
  File "<frozen importlib._bootstrap_external>", line 883, in exec_module
  File "<frozen importlib._bootstrap>", line 241, in _call_with_frames_removed
  File "/home/niko/.local/lib/python3.10/site-packages/matplotlib/backends/backend_gtk4agg.py", line 4, in <module>
    from . import backend_agg, backend_gtk4
  File "/home/niko/.local/lib/python3.10/site-packages/matplotlib/backends/backend_gtk4.py", line 26, in <module>
    from . import _backend_gtk
  File "/home/niko/.local/lib/python3.10/site-packages/matplotlib/backends/_backend_gtk.py", line 25, in <module>
    raise ImportError("Gtk-based backends require cairo") from e
ImportError: Gtk-based backends require cairo

Checking cairo

By just reading to last line of the traceback, the first thing to do would be to try to install pycairo, which I tried, and got

niko@niko-ubuntu:~/myproj$ python -m pip install pycairo
Defaulting to user installation because normal site-packages is not writeable
Requirement already satisfied: pycairo in /usr/lib/python3/dist-packages (1.20.1)

I can also verify that cairo is importable:

niko@niko-ubuntu:~/myproj$ ipython
>>> import cairo
>>> cairo.__file__
'/usr/lib/python3/dist-packages/cairo/__init__.py'

No module named 'gi._gi_cairo'

Then the next thing to do would be to read the full traceback, which I did, and realized that I should try to require_foreign("cairo"), which I did:

>>> import gi
>>> gi.require_foreign("cairo")
---------------------------------------------------------------------------
ModuleNotFoundError                       Traceback (most recent call last)
File /usr/lib/python3/dist-packages/gi/__init__.py:176, in require_foreign(namespace, symbol)
    175 try:
--> 176     _gi.require_foreign(namespace, symbol)
    177 except Exception as e:

ModuleNotFoundError: No module named 'gi._gi_cairo'

During handling of the above exception, another exception occurred:

ImportError                               Traceback (most recent call last)
Cell In[4], line 1
----> 1 gi.require_foreign("cairo")

File /usr/lib/python3/dist-packages/gi/__init__.py:178, in require_foreign(namespace, symbol)
    176     _gi.require_foreign(namespace, symbol)
    177 except Exception as e:
--> 178     raise ImportError(str(e))
    179 importlib.import_module('gi.repository', namespace)

ImportError: No module named 'gi._gi_cairo'

Matplotlib docs & google

Googling for that exact phrase did not result to any search results, hence this question (to help future googles): What should I install in order to make matplotlib work with GTK4Agg backend? The matplotlib documentation says:

requires PyGObject and pycairo

Question

Since I can import both gi and pycairo, one could assume they're both installed. Something is missing, as gi._gi_cairo is not there. Why is that so, and how to fix this?

Answer 1

Apparently, it is possible to install PyGObject, pycairo and cairo without installing the cairo bindings for the GObject library. These are included in the python3-gi-cairo package, and should be installed when PyGObject is installed, as mentioned in PyGObject documentation.

Therefore, the fix is to install the cairo bindings with

sudo apt install python3-gi-cairo