Merge pull request matplotlib#18352 from anntzer/qt

Avoid triggering backend resolution during qt initial import.

Author: QuLogic

lib/matplotlib/backends/qt_compat.py

@@ -42,13 +42,15 @@
     QT_API = QT_API_PYSIDE
 # Otherwise, check the QT_API environment variable (from Enthought).  This can
 # only override the binding, not the backend (in other words, we check that the
-# requested backend actually matches).
-elif mpl.rcParams["backend"] in ["Qt5Agg", "Qt5Cairo"]:
+# requested backend actually matches).  Use dict.__getitem__ to avoid
+# triggering backend resolution (which can result in a partially but
+# incompletely imported backend_qt5).
+elif dict.__getitem__(mpl.rcParams, "backend") in ["Qt5Agg", "Qt5Cairo"]:
     if QT_API_ENV in ["pyqt5", "pyside2"]:
         QT_API = _ETS[QT_API_ENV]
     else:
         QT_API = None
-elif mpl.rcParams["backend"] in ["Qt4Agg", "Qt4Cairo"]:
+elif dict.__getitem__(mpl.rcParams, "backend") in ["Qt4Agg", "Qt4Cairo"]:
     if QT_API_ENV in ["pyqt4", "pyside"]:
         QT_API = _ETS[QT_API_ENV]
     else:
@@ -150,8 +152,8 @@ def is_pyqt5():
     _setup_pyqt5()
 elif QT_API in [QT_API_PYQTv2, QT_API_PYSIDE, QT_API_PYQT]:
     _setup_pyqt4()
-elif QT_API is None:
-    if mpl.rcParams["backend"] == "Qt4Agg":
+elif QT_API is None:  # See above re: dict.__getitem__.
+    if dict.__getitem__(mpl.rcParams, "backend") == "Qt4Agg":
         _candidates = [(_setup_pyqt4, QT_API_PYQTv2),
                        (_setup_pyqt4, QT_API_PYSIDE),
                        (_setup_pyqt4, QT_API_PYQT),