FIX: ensure that qt5agg and qt5cairo backends actually use qt5

tacaswell · tacaswell · commit 0c77b9aa8cd9 · 2021-12-18T21:06:52.000-05:00
Because the code in qt_compat tries qt6 bindings first, backend_qt supports both Qt5 and Qt6, and the qt5 named backends are shims to the generic Qt backend, if you imported matplotlib.backends.backend_qt5agg, matplotlib.backends.backend_qt5cairo, or matplotlib.backends.backend_qt5, and 1. had PyQt6 or pyside6 installed 2. had not previously imported a Qt5 binding Then you will end up with a backend that (by name) claims to be Qt5, but will be using Qt6 bindings. If you then subsequently import a Qt6 binding and try to embed the canvas it will fail (due to being Qt6 objects not Qt5 objects!). closes #21998
diff --git a/lib/matplotlib/backends/__init__.py b/lib/matplotlib/backends/__init__.py
@@ -1,2 +1,3 @@
 # NOTE: plt.switch_backend() (called at import time) will add a "backend"
 # attribute here for backcompat.
+_QT_MODE = None
diff --git a/lib/matplotlib/backends/backend_qt5.py b/lib/matplotlib/backends/backend_qt5.py
@@ -1,3 +1,8 @@
+from .. import backends
+
+backends._QT_MODE = 5
+
+
 from .backend_qt import (
     backend_version, SPECIAL_KEYS,
     # Public API
diff --git a/lib/matplotlib/backends/backend_qt5agg.py b/lib/matplotlib/backends/backend_qt5agg.py
@@ -1,8 +1,10 @@
 """
 Render to qt from agg
 """
+from .. import backends
 
-from .backend_qtagg import (
+backends._QT_MODE = 5
+from .backend_qtagg import (    # noqa
     _BackendQTAgg, FigureCanvasQTAgg, FigureManagerQT, NavigationToolbar2QT,
     backend_version,  FigureCanvasAgg,  FigureCanvasQT
 )
diff --git a/lib/matplotlib/backends/backend_qt5cairo.py b/lib/matplotlib/backends/backend_qt5cairo.py
@@ -1,5 +1,9 @@
+from .. import backends
+
+backends._QT_MODE = 5
 from .backend_qtcairo import (
-    _BackendQTCairo, FigureCanvasQTCairo, FigureCanvasCairo, FigureCanvasQT)
+    _BackendQTCairo, FigureCanvasQTCairo, FigureCanvasCairo, FigureCanvasQT
+)
 
 
 @_BackendQTCairo.export
diff --git a/lib/matplotlib/backends/qt_compat.py b/lib/matplotlib/backends/qt_compat.py
@@ -25,6 +25,7 @@
 import matplotlib as mpl
 from matplotlib import _api
 
+from . import _QT_MODE
 
 QT_API_PYQT6 = "PyQt6"
 QT_API_PYSIDE6 = "PySide6"
@@ -117,12 +118,20 @@ def _isdeleted(obj):
 if QT_API in [QT_API_PYQT6, QT_API_PYQT5, QT_API_PYSIDE6, QT_API_PYSIDE2]:
     _setup_pyqt5plus()
 elif QT_API is None:  # See above re: dict.__getitem__.
-    _candidates = [
-        (_setup_pyqt5plus, QT_API_PYQT6),
-        (_setup_pyqt5plus, QT_API_PYSIDE6),
-        (_setup_pyqt5plus, QT_API_PYQT5),
-        (_setup_pyqt5plus, QT_API_PYSIDE2),
-    ]
+    if _QT_MODE is None:
+        _candidates = [
+            (_setup_pyqt5plus, QT_API_PYQT6),
+            (_setup_pyqt5plus, QT_API_PYSIDE6),
+            (_setup_pyqt5plus, QT_API_PYQT5),
+            (_setup_pyqt5plus, QT_API_PYSIDE2),
+        ]
+    elif _QT_MODE == 5:
+        _candidates = [
+            (_setup_pyqt5plus, QT_API_PYQT5),
+            (_setup_pyqt5plus, QT_API_PYSIDE2),
+        ]
+    else:
+        raise ValueError("Should never hit here")
     for _setup, QT_API in _candidates:
         try:
             _setup()
diff --git a/lib/matplotlib/tests/test_backends_interactive.py b/lib/matplotlib/tests/test_backends_interactive.py
@@ -17,6 +17,13 @@
 from matplotlib import _c_internal_utils
 
 
+def _yeet_to_subprocess(func):
+    func_source = '\n'.join(
+        textwrap.dedent(inspect.getsource(func)).split('\n')[1:]
+    )
+    return f"{func_source}\n{func.__name__}()"
+
+
 # Minimal smoke-testing of the backends for which the dependencies are
 # PyPI-installable on CI.  They are not available for all tested Python
 # versions so we don't fail on missing backends.
@@ -258,6 +265,7 @@ def test_interactive_thread_safety(env):
 
 def test_lazy_auto_backend_selection():
 
+    @_yeet_to_subprocess
     def _impl():
         import matplotlib
         import matplotlib.pyplot as plt
@@ -272,8 +280,69 @@ def _impl():
         assert isinstance(bk, str)
 
     proc = subprocess.run(
-        [sys.executable, "-c",
-         textwrap.dedent(inspect.getsource(_impl)) + "\n_impl()"],
+        [sys.executable, "-c", _impl],
+        env={**os.environ, "SOURCE_DATE_EPOCH": "0"},
+        timeout=_test_timeout, check=True,
+        stdout=subprocess.PIPE, universal_newlines=True)
+
+
+def test_qt5backends_uses_qt5():
+
+    qt5_bindings = [
+        dep for dep in ['PyQt5', 'pyside2']
+        if not importlib.util.find_spec(dep)
+    ]
+    qt6_bindings = [
+        dep for dep in ['PyQt6', 'pyside6']
+        if not importlib.util.find_spec(dep)
+    ]
+    if not (len(qt5_bindings) > 0 and len(qt6_bindings) > 0):
+        pytest.skip('need both QT6 and QT5 bindings')
+
+    @_yeet_to_subprocess
+    def _implagg():
+        import matplotlib.backends.backend_qt5agg
+        import sys
+        matplotlib.backends.backend_qt5agg
+
+        assert 'PyQt6' not in sys.modules
+        assert 'pyside6' not in sys.modules
+        assert 'PyQt5' in sys.modules or 'pyside2' in sys.modules
+
+    @_yeet_to_subprocess
+    def _implcairo():
+        import matplotlib.backends.backend_qt5cairo
+        import sys
+        matplotlib.backends.backend_qt5cairo
+
+        assert 'PyQt6' not in sys.modules
+        assert 'pyside6' not in sys.modules
+        assert 'PyQt5' in sys.modules or 'pyside2' in sys.modules
+
+    @_yeet_to_subprocess
+    def _implcore():
+        import matplotlib.backends.backend_qt5
+        import sys
+        matplotlib.backends.backend_qt5
+
+        assert 'PyQt6' not in sys.modules
+        assert 'pyside6' not in sys.modules
+        assert 'PyQt5' in sys.modules or 'pyside2' in sys.modules
+
+    subprocess.run(
+        [sys.executable, "-c", _implagg],
+        env={**os.environ, "SOURCE_DATE_EPOCH": "0"},
+        timeout=_test_timeout, check=True,
+        stdout=subprocess.PIPE, universal_newlines=True)
+
+    subprocess.run(
+        [sys.executable, "-c", _implcairo],
+        env={**os.environ, "SOURCE_DATE_EPOCH": "0"},
+        timeout=_test_timeout, check=True,
+        stdout=subprocess.PIPE, universal_newlines=True)
+
+    subprocess.run(
+        [sys.executable, "-c", _implcore],
         env={**os.environ, "SOURCE_DATE_EPOCH": "0"},
         timeout=_test_timeout, check=True,
         stdout=subprocess.PIPE, universal_newlines=True)

Original file line number	Diff line number	Diff line change
`@@ -1,2 +1,3 @@`
`1`	`1`	`# NOTE: plt.switch_backend() (called at import time) will add a "backend"`
`2`	`2`	`# attribute here for backcompat.`
	`3`	`+_QT_MODE = None`