Python check list is empty

The pythonic way to do it is from the PEP 8 style guide (where Yes means “recommended” and No means “not recommended”):

For sequences, (strings, lists, tuples), use the fact that empty sequences are false.

Yes: if not seq:
     if seq:

No:  if len(seq):
     if not len(seq):

Another way to do it:

if len(li) == 0:
    print('the list is empty')

This way it's 100% clear that li is a sequence (list) and we want to test its size. My problem with if not li: ... is that it gives the false impression that li is a boolean variable.

That does make some sense, but following PEP 8 style, it is ugly and unpythonic. If distinguishing list from boolean is important, you should add a comment, not more codes.