demote sip API failure to warning from ImportError

IPython/external/qt_for_kernel.py

@@ -1,82 +1,87 b''
 """ Import Qt in a manner suitable for an IPython kernel.
 
 This is the import used for the `gui=qt` or `pylab=qt` initialization.
 
 Import Priority:
 
 if matplotlib has been imported and doesn't support v2 (<= 1.0.1):
     use PyQt4 @v1
 
 Next, ask ETS' QT_API env variable
 
 if QT_API not set:
     ask matplotlib via rcParams['backend.qt4']
     if it said PyQt:
         use PyQt4 @v1
     elif it said PySide:
         use PySide
 
     else: (matplotlib said nothing)
         # this is the default path - nobody told us anything
         try:
             PyQt @v1
         except:
             fallback on PySide
 else:
     use PyQt @v2 or PySide, depending on QT_API
     because ETS doesn't work with PyQt @v1.
 
 """
 
 import os
 import sys
 
+from IPython.utils.warn import warn
+
 matplotlib = sys.modules.get('matplotlib')
 if matplotlib and matplotlib.__version__ <= '1.0.1':
     # 1.0.1 doesn't support pyside or v2, so stick with PyQt @v1,
     # and ignore everything else
     from PyQt4 import QtCore, QtGui
 else:
     # ask QT_API ETS variable *first*
     QT_API = os.environ.get('QT_API', None)
     if QT_API is None:
         # QT_API not set, ask matplotlib if it was imported (e.g. `pylab=qt`)
         if matplotlib:
             mpqt = matplotlib.rcParams.get('backend.qt4', None)
         else:
             mpqt = None
         if mpqt is None:
             # matplotlib not imported or had nothing to say.
             try:
                 # default to unconfigured PyQt4
                 from PyQt4 import QtCore, QtGui
             except ImportError:
                 # fallback on PySide
                 try:
                     from PySide import QtCore, QtGui
                 except ImportError:
                     raise ImportError('Cannot import PySide or PyQt4')
         elif mpqt.lower() == 'pyqt4':
             # import PyQt4 unconfigured
             from PyQt4 import QtCore, QtGui
         elif mpqt.lower() == 'pyside':
             from PySide import QtCore, QtGui
         else:
             raise ImportError("unhandled value for backend.qt4 from matplotlib: %r"%mpqt)
     else:
         # QT_API specified, use PySide or PyQt+v2 API from external.qt
         # this means ETS is likely to be used, which requires v2
         try:
             from IPython.external.qt import QtCore, QtGui
         except ValueError as e:
             if 'API' in str(e):
-                # API mismatch, give more meaningful message
-                raise ImportError("""
+                # PyQt4 already imported, and APIv2 couldn't be set
+                # Give more meaningful message, and warn instead of raising
+                warn("""
     Assigning the ETS variable `QT_API=pyqt` implies PyQt's v2 API for
     QString and QVariant, but PyQt has already been imported
-    with v1 APIs.  You must unset QT_API to work with PyQt4
+    with v1 APIs.  You should unset QT_API to work with PyQt4
     in its default mode.
 """)
+                # allow it to still work
+                from PyQt4 import QtCore, QtGui
             else:
                 raise