Merge pull request matplotlib#2142 from punchagan/fix-qt-import

Try importing PySide, if we can't find sip.

Author: dmcdougall

doc/faq/usage_faq.rst

@@ -284,7 +284,20 @@ macosx         Cocoa rendering in OSX windows
 .. _TkInter: http://wiki.python.org/moin/TkInter
 .. _PyQt4: http://www.riverbankcomputing.co.uk/software/pyqt/intro
 
+How do I select PyQt4 or PySide?
+========================================
 
+You can choose either PyQt4 or PySide when using the `qt4` backend by setting
+the appropriate value for `backend.qt4` in your :file:`matplotlibrc` file. The
+default value is `PyQt4`.
+
+The setting in your :file:`matplotlibrc` file can be overridden by setting the
+`QT_API` environment variable to either `pyqt` or `pyside` to use `PyQt4` or
+`PySide`, respectively.
+
+Since the default value for the bindings to be used is `PyQt4`,
+:mod:`matplotlib` first tries to import it, if the import fails, it tries to
+import `PySide`.
 
 .. _interactive-mode:
 

lib/matplotlib/backends/qt4_compat.py

@@ -31,9 +31,19 @@
 # of file dialog.
 _getSaveFileName = None
 
+# Flag to check if sip could be imported
+_sip_imported = False
+
 # Now perform the imports.
 if QT_API in (QT_API_PYQT, QT_API_PYQTv2):
-    import sip
+    try:
+        import sip
+        _sip_imported = True
+    except ImportError:
+        # Try using PySide
+        QT_API = QT_API_PYSIDE
+
+if _sip_imported:
     if QT_API == QT_API_PYQTv2:
         if QT_API_ENV == 'pyqt':
             cond = ("Found 'QT_API=pyqt' environment variable. "
@@ -76,7 +86,7 @@
         # call to getapi() can fail in older versions of sip
         _getSaveFileName = QtGui.QFileDialog.getSaveFileName
 
-else:  # can only be pyside
+else: # try importing pyside
     from PySide import QtCore, QtGui, __version__, __version_info__
     if __version_info__ < (1, 0, 3):
         raise ImportError(