MNT: check PySide, PyQt4, or PyQt5 are imported

tacaswell · tacaswell · commit 7f5ae1faeba4 · 2016-03-17T13:43:46.000-04:00
If PySide, PyQt4, or PyQt5 are imported do not consult rcparams or ENV variable to decide which backend to use. This is a case where ignoring user input is sensible because the Qt bindings are mutually exclusive and attempting to import a second binding will fail. This can result in stand-alone scripts / mpl embeddings which are dependent on the rcparams to function correctly. Importing a specific Qt binding is stronger signal of user intent that either of the other two configuration options and should take precedence. closes #6164
diff --git a/lib/matplotlib/backends/qt_compat.py b/lib/matplotlib/backends/qt_compat.py
@@ -42,7 +42,23 @@
 
 QT_API = None
 
-if (QT_API_ENV is not None):
+# check if any binding is already imported, if so silently ignore the
+# rcparams/ENV settings and use what ever is already imported.
+if 'PySide' in sys.modules:
+    # user has imported PySide before importing mpl
+    QT_API = QT_API_PYSIDE
+
+if 'PyQt4' in sys.modules:
+    # user has imported PyQt4 before importing mpl
+    # this case also handles the PyQt4v2 case as once sip is imported
+    # the API versions can not be changed so do not try
+    QT_API = QT_API_PYQT
+
+if 'PyQt5' in sys.modules:
+    # the user has imported PyQt5 before importing mpl
+    QT_API = QT_API_PYQT5
+
+if (QT_API_ENV is not None) and QT_API is None:
     try:
         QT_ENV_MAJOR_VERSION = ETS[QT_API_ENV][1]
     except KeyError:
