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):

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

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

To Daren Thomas's answer:

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.

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.

I prefer the following:

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

Method 1 (preferred):

if not a:
   print ("Empty")

Method 2:

if len(a) == 0:
   print("Empty")

Method 3:

if a == []:
  print ("Empty")

To check whether a list is empty or not you can use two following ways. But remember, we should avoid the way of explicitly checking for a type of sequence (it's a less Pythonic way):

def enquiry(list1):
    return len(list1) == 0

# ––––––––––––––––––––––––––––––––

list1 = []

if enquiry(list1):
    print("The list isn't empty")
else:
    print("The list is Empty")

# Result: "The list is Empty".

The second way is a more Pythonic one. This method is an implicit way of checking and much more preferable than the previous one.

def enquiry(list1):
    return not list1

# ––––––––––––––––––––––––––––––––

list1 = []

if enquiry(list1):
    print("The list is Empty")
else:
    print("The list isn't empty")

# Result: "The list is Empty"

Many answers have been given, and a lot of them are pretty good. I just wanted to add that the check

not a

will also pass for None and other types of empty structures. If you truly want to check for an empty list, you can do this:

if isinstance(a, list) and len(a)==0:
    print("Received an empty list")

print('not empty' if a else 'empty')

a little more practical:

a.pop() if a else None

and the shortest version:

if a: a.pop() 

We could use a simple if else:

item_list=[]
if len(item_list) == 0:
    print("list is empty")
else:
    print("list is not empty")

Simple way is checking the length is equal zero.

if len(a) == 0:
    print("a is empty")

From python3 onwards you can use

a == []

to check if the list is empty

EDIT : This works with python2.7 too..

I am not sure why there are so many complicated answers. It's pretty clear and straightforward

The truth value of an empty list is False whereas for a non-empty list it is True.

What brought me here is a special use-case: I actually wanted a function to tell me if a list is empty or not. I wanted to avoid writing my own function or using a lambda-expression here (because it seemed like it should be simple enough):

foo = itertools.takewhile(is_not_empty, (f(x) for x in itertools.count(1)))

And, of course, there is a very natural way to do it:

foo = itertools.takewhile(bool, (f(x) for x in itertools.count(1)))

Of course, do not use bool in if (i.e., if bool(L):) because it's implied. But, for the cases when "is not empty" is explicitly needed as a function, bool is the best choice.