How do I check if a list is empty?

if not a:
  print("List is empty")

Using the implicit booleanness of the empty list is quite pythonic.

The pythonic way to do it is from the PEP 8 style guide.

For sequences, (strings, lists, tuples), use the fact that empty sequences are false:
# Correct:
if not seq:
if seq:

# Wrong:
if len(seq):
if not len(seq):

I prefer it explicitly:

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.

How do I check if a list is empty?

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