How to Empty a List

How to empty a list?

This actually removes the contents from the list, but doesn't replace the old label with a new empty list:

del lst[:]

Here's an example:

lst1 = [1, 2, 3]
lst2 = lst1
del lst1[:]
print(lst2)

For the sake of completeness, the slice assignment has the same effect:

lst[:] = []

It can also be used to shrink a part of the list while replacing a part at the same time (but that is out of the scope of the question).

Note that doing lst = [] does not empty the list, just creates a new object and binds it to the variable lst, but the old list will still have the same elements, and effect will be apparent if it had other variable bindings.

Different ways of clearing lists

Clearing a list in place will affect all other references of the same list.

For example, this method doesn't affect other references:

>>> a = [1, 2, 3]
>>> b = a
>>> a = []
>>> print(a)
[]
>>> print(b)
[1, 2, 3]

But this one does:

>>> a = [1, 2, 3]
>>> b = a
>>> del a[:]      # equivalent to   del a[0:len(a)]
>>> print(a)
[]
>>> print(b)
[]
>>> a is b
True

You could also do:

>>> a[:] = []

How to empty a list in C#?

It's really easy:

myList.Clear();

How do I empty an array in JavaScript?

Ways to clear an existing array A:

Method 1

(this was my original answer to the question)

A = [];

This code will set the variable A to a new empty array. This is perfect if you don't have references to the original array A anywhere else because this actually creates a brand new (empty) array. You should be careful with this method because if you have referenced this array from another variable or property, the original array will remain unchanged. Only use this if you only reference the array by its original variable A.

This is also the fastest solution.

This code sample shows the issue you can encounter when using this method:

var arr1 = ['a','b','c','d','e','f'];
var arr2 = arr1;  // Reference arr1 by another variable 
arr1 = [];
console.log(arr2); // Output ['a','b','c','d','e','f']

Method 2 (as suggested by Matthew Crumley)

A.length = 0

This will clear the existing array by setting its length to 0. It also works when using "strict mode" in ECMAScript 5 because the length property of an array is a read/write property.

Method 3 (as suggested by Anthony)

A.splice(0,A.length)

Using .splice() will work perfectly, but since the .splice() function will return an array with all the removed items, it will actually return a copy of the original array. Benchmarks suggest that this has no effect on performance whatsoever.

Method 4 (as suggested by tanguy_k)

while(A.length > 0) {
    A.pop();
}

This solution is not very succinct, and it is also the slowest solution, contrary to earlier benchmarks referenced in the original answer.

Performance

Of all the methods of clearing an existing array, methods 2 and 3 are very similar in performance and are a lot faster than method 4. See this benchmark.

As pointed out by Diadistis in their answer below, the original benchmarks that were used to determine the performance of the four methods described above were flawed. The original benchmark reused the cleared array so the second iteration was clearing an array that was already empty.

The following benchmark fixes this flaw: http://jsben.ch/#/hyj65. It clearly shows that methods #2 (length property) and #3 (splice) are the fastest (not counting method #1 which doesn't change the original array).

This has been a hot topic and the cause of a lot of controversy. There are actually many correct answers and because this answer has been marked as the accepted answer for a very long time, I will include all of the methods here.

Python: How to remove empty lists from a list?

Try

list2 = [x for x in list1 if x != []]

If you want to get rid of everything that is "falsy", e.g. empty strings, empty tuples, zeros, you could also use

list2 = [x for x in list1 if x]

How to clear a list in Dart?

.clear() will remove data from list with the same reference and foo = [] will have clear data with new reference

Note: .clear() is the best option

How to delete all the content of a list A without deleting the content of a list of lists that contains the list A.?

replace this statement indexList.clear(); with indexList = new ArrayList<Integer>();