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