Per PEP 8, equality comparisons to None should use the "is" or "is not" keywords. However, if x is a {QVariant}NULL, the equality statement with the "is" keyword returns FALSE whereas the "==" comparison returns TRUE.  

Hence where cond = {QVariant}NULL:

    if cond is None  # Returns False
    if cond == None  # Returns True

This can cause some unexpected code anomalies (as it did for me) when adjusting code for PEP 8 compliance.