spyderlib.qt: Enable PyQt4 API #1 back again, but leave #2 the defaut.

Implement fallback logic choosing APIs: PyQt4 #2 -> PyQt4 #1 -> PySide.
Add 'pyqtv1' constant to explicitly select PyQt4 API #1.

bootstrap.py

@@ -43,7 +43,7 @@
                   default=False, help="Run Spyder in debug mode")
 options, args = parser.parse_args()
 
-assert options.gui in (None, 'pyqt', 'pyside'), \
+assert options.gui in (None, 'pyqt', 'pyqtv1', 'pyside'), \
        "Invalid GUI toolkit option '%s'" % options.gui
 
 # For testing purposes

spyderlib/qt/__init__.py

@@ -9,37 +9,48 @@
 
 import os
 
+
+def debug(msg):
+    # lightweight debug function
+    #print(msg)
+    pass
+
+
 os.environ.setdefault('QT_API', 'pyqt')
-assert os.environ['QT_API'] in ('pyqt', 'pyside')
+assert os.environ['QT_API'] in ('pyqt', 'pyqtv1', 'pyside')
 
 API = os.environ['QT_API']
-API_NAME = {'pyqt': 'PyQt4', 'pyside': 'PySide'}[API]
+API_NAME = None
 
 if API == 'pyqt':
     # Spyder 2.2 is compatible with both #1 and #2 PyQt API,
     # but to avoid issues with IPython and other Qt plugins
     # we choose to support only API #2 for 2.3+
-    import sip
     try:
+        import sip
         sip.setapi('QString', 2)
         sip.setapi('QVariant', 2)
+    except ImportError:
+        debug('qt: PyQt4 is not found. Fallback to PySide API')
+        API = os.environ['QT_API'] = 'pyside'
     except AttributeError:
-        # PyQt < v4.6. The actual check is done by requirements.check_qt()
-        # call from spyder.py
-        pass
+        # PyQt < 4.6. Fallback to API #1
+        debug('qt: Fallback to PyQt4 API #1')
+        API = os.environ['QT_API'] = 'pyqtv1'
 
+if API in ('pyqtv1', 'pyqt'):
     try:
         from PyQt4.QtCore import PYQT_VERSION_STR as __version__
     except ImportError:
-        # Switching to PySide
+        # No PyQt4. Fallback to PySide
+        debug('qt: Fallback to PySide API')
         API = os.environ['QT_API'] = 'pyside'
-        API_NAME = 'PySide'
     else:
         is_old_pyqt = __version__.startswith(('4.4', '4.5', '4.6', '4.7'))
         is_pyqt46 = __version__.startswith('4.6')
         import sip
         try:
-            API_NAME += (" (API v%d)" % sip.getapi('QString'))
+            API_NAME = ("PyQt4 (API v%d)" % sip.getapi('QString'))
         except AttributeError:
             pass
 
@@ -50,3 +61,4 @@
         raise ImportError("Spyder requires PySide or PyQt to be installed")
     else:
         is_old_pyqt = is_pyqt46 = False
+    API_NAME = 'PySide'