Python: What is the best way to check if a list is empty?

Question

For example, if passed the following:

a = []

How do I check to see if a is empty?

Answer 1

if not a:
  print "List is empty"

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

Answer 2

I have seen the below as preferred, as it will catch the null list as well:

if not a:
    print "The list is empty or null"

Answer 3

An empty list is itself considered false in true value testing (see python documentation):

a = []
if a:
     print "not empty"

@Daren Thomas

EDIT: Another point against testing the empty list as False: What about polymorphism? You shouldn't depend on a list being a list. It should just quack like a duck - how are you going to get your duckCollection to quack ''False'' when it has no elements?

Your duckCollection should implement __nonzero__ or __len__ so the if a: will work without problems.

Answer 4

I prefer the following:

if a == []:
   print "The list is empty."

Readable and you don't have to worry about calling a function like len() to iterate through the variable. Although I'm not entirely sure what the BigO notation of something like this is... but Python's so blazingly fast I doubt it'd matter unless a was gigantic.

Answer 5

The pythonic way to do it is from the style guide:

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)

Answer 6

len() is an O(1) operation for Python lists, strings, dicts, and sets. Python internally keeps track of the number of elements in these containers.

JavaScript has a similar notion of truthy/falsy.

Answer 7

It's silly to compare if a==[] because as mentioned, it breaks polymorphism, worse, extra object creation, a sin, even if it's very fast. len IS the preferred way, because it's standard and any inherited class should support it.