Backport PR #2757: check for complete pyside presence before trying to import

importing pyside partially and then falling back to pyqt causes a crash
in sip (see gh-1431)
To avoid it try to locate all modules before the import and should that
fail print a warning.

debian splits pyside into many small packages so sometimes people end up with incomplete pyside installations.

IPython/external/qt.py

@@ -1,67 +1,81 b''
 """ A Qt API selector that can be used to switch between PyQt and PySide.
 
 This uses the ETS 4.0 selection pattern of:
 PySide first, PyQt with API v2. second.
 
 Do not use this if you need PyQt with the old QString/QVariant API.
 """
 
 import os
 
 # Available APIs.
 QT_API_PYQT = 'pyqt'
 QT_API_PYSIDE = 'pyside'
 
 def prepare_pyqt4():
     # For PySide compatibility, use the new-style string API that automatically
     # converts QStrings to Unicode Python strings. Also, automatically unpack
     # QVariants to their underlying objects.
     import sip
     sip.setapi('QString', 2)
     sip.setapi('QVariant', 2)
 
 # Select Qt binding, using the QT_API environment variable if available.
 QT_API = os.environ.get('QT_API')
 if QT_API is None:
+    pyside_found = False
     try:
         import PySide
         if PySide.__version__ < '1.0.3':
             # old PySide, fallback on PyQt
             raise ImportError
+        # we can't import an incomplete pyside and pyqt4
+        # this will cause a crash in sip (#1431)
+        # check for complete presence before importing
+        import imp
+        imp.find_module("QtCore", PySide.__path__)
+        imp.find_module("QtGui", PySide.__path__)
+        imp.find_module("QtSvg", PySide.__path__)
+        pyside_found = True
         from PySide import QtCore, QtGui, QtSvg
         QT_API = QT_API_PYSIDE
     except ImportError:
         try:
             prepare_pyqt4()
             import PyQt4
             from PyQt4 import QtCore, QtGui, QtSvg
+            if pyside_found:
+                print "WARNING: PySide installation incomplete and PyQt4 " \
+                      "present.\nThis will likely crash, please install " \
+                      "PySide completely, remove PySide or PyQt4 or set " \
+                      "the QT_API environment variable to pyqt or pyside"
             if QtCore.PYQT_VERSION_STR < '4.7':
                 # PyQt 4.6 has issues with null strings returning as None
                 raise ImportError
             QT_API = QT_API_PYQT
         except ImportError:
             raise ImportError('Cannot import PySide >= 1.0.3 or PyQt4 >= 4.7')
 
 elif QT_API == QT_API_PYQT:
     # Note: This must be called *before* PyQt4 is imported.
     prepare_pyqt4()
 
 # Now peform the imports.
 if QT_API == QT_API_PYQT:
     from PyQt4 import QtCore, QtGui, QtSvg
     if QtCore.PYQT_VERSION_STR < '4.7':
         raise ImportError("IPython requires PyQt4 >= 4.7, found %s"%QtCore.PYQT_VERSION_STR)
 
     # Alias PyQt-specific functions for PySide compatibility.
     QtCore.Signal = QtCore.pyqtSignal
     QtCore.Slot = QtCore.pyqtSlot
 
 elif QT_API == QT_API_PYSIDE:
     import PySide
     if PySide.__version__ < '1.0.3':
         raise ImportError("IPython requires PySide >= 1.0.3, found %s"%PySide.__version__)
     from PySide import QtCore, QtGui, QtSvg
 
 else:
     raise RuntimeError('Invalid Qt API %r, valid values are: %r or %r' %
                        (QT_API, QT_API_PYQT, QT_API_PYSIDE))