Python Mistake

Python lesson I learned today - use "is None" when you want to check if something exists. I had heard this advice before but it was so easy for me to be a little slutty with my usage of "if not x:".

Here's where it hurt:

def get_notes(qs=None):
    if not qs:
        qs = self.notes.all()
...

The optional qs parameter is for a Django QuerySet ORM object which was intended for some subset of the user's notes like only files ( self.notes.filter(type='file') ). Versatile!

However, if the QuerySet was an empty result set, the "if not qs" evaluated to False. Then qs became the set of all notes. Exactly NOT what I wanted! Wow. Terrible.

The correct condition is "if qs is None" like so:

def get_notes(qs=None):
    if qs is None: # no predefined queryset
        qs = self.notes.all()
...

Anyways, I hope this helps someone else.