[Bug]: Qt test_ipython fails on older ipython

[QuLogic]

Bug summary

I have an older conda environment with ipython 8.8.0, and this test seems to be failing. Also when building on Fedora Rawhide with ipython 8.24.0, I see the failure as well. I'm not sure why this passes (but fails with my environments) with the minimum versions in CI.

cc @ianthomas23

Code for reproduction

$ pytest lib/matplotlib/tests/test_backend_qt.py::test_ipython

Actual outcome

________ test_ipython ________

    def test_ipython():
        from matplotlib.testing import ipython_in_subprocess
>       ipython_in_subprocess("qt", "QtAgg", "qtagg")

lib/matplotlib/tests/test_backend_qt.py:381: 
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 

requested_backend_or_gui_framework = 'qt', expected_backend_old_ipython = 'QtAgg', expected_backend_new_ipython = 'qtagg'

    def ipython_in_subprocess(
        requested_backend_or_gui_framework,
        expected_backend_old_ipython,  # IPython < 8.24
        expected_backend_new_ipython,  # IPython >= 8.24
    ):
        import pytest
        IPython = pytest.importorskip("IPython")
    
        if sys.platform == "win32":
            pytest.skip("Cannot change backend running IPython in subprocess on Windows")
    
        if (IPython.version_info[:3] == (8, 24, 0) and
                requested_backend_or_gui_framework == "osx"):
            pytest.skip("Bug using macosx backend in IPython 8.24.0 fixed in 8.24.1")
    
        if IPython.version_info[:2] >= (8, 24):
            expected_backend = expected_backend_new_ipython
        else:
            # This code can be removed when Python 3.12, the latest version supported by
            # IPython < 8.24, reaches end-of-life in late 2028.
            expected_backend = expected_backend_old_ipython
    
        code = ("import matplotlib as mpl, matplotlib.pyplot as plt;"
                "fig, ax=plt.subplots(); ax.plot([1, 3, 2]); mpl.get_backend()")
        proc = subprocess_run_for_testing(
            [
                "ipython",
                "--no-simple-prompt",
                f"--matplotlib={requested_backend_or_gui_framework}",
                "-c", code,
            ],
            check=True,
            capture_output=True,
        )
    
>       assert proc.stdout.strip() == f"Out[1]: '{expected_backend}'"
E       AssertionError

lib/matplotlib/testing/__init__.py:217: AssertionError

Expected outcome

The test passes.

Additional information

If I print out the proc.stdout, then I get Out[1]: 'Qt5Agg'; the test appears to be looking for 'qtagg', instead?

Operating system

Fedora 39/Rawhide

Matplotlib Version

main (3ee509d)

Installation

git checkout

[QuLogic]

In said conda environment, I have PyQt5 and PyQt6 installed, but not PySide2 or PySide6.

[QuLogic]

It looks like this was changed in IPython 8.15:
ipython/ipython@8.14.0...8.15.0#diff-e34e08b95c23d8372b62d0224100b3ff752bda96245ab69ea12a2edf7fcb6b48L26