Resources

  • list in the standard types.
  • lists in a tutorial on the docs - that covers the methods on a list and how to use a list as a stack of queue.

The list keyword

x = [1, 2, 3]
type(x)
# list
isinstance(x, list)
# True

y = (1, 2, 3)
list(y)
# [1, 2, 3]

Methods

Common:

  • .append - add an element at the end, one argument only. 
      x.append('def')
  • .extend - add multiple elements from an iterable at the end of a list. 
      x.extend(['ghi','jkl'])
  • .insert - add an element at a given position. 
      x.insert(0, 'def')
  • .pop - remove an element from the end or given position. 
      el = x.pop() # end
  el = x.pop(0) # start

Less common:

  • .remove - remove a target value. 
      x.remove('def')
  • .index 
      x.count('abc')
  # 0
  • .count 
      x.count('abc')
  # 1
  • .copy
  • .sort - see also sorted(my_list)
  • .reverse - see also reversed(my_list)
  • .clear - empty the lit.

Create

Empty array

>>> []
[]
>>> list()
[]

Null list

This is only safe for primitive types.

>>> x = [None]*10
[None, None, None, None, None, None, None, None, None, None]
>>> x[0] = 'abc'
>>> x
['abc', None, None, None, None, None, None, None, None, None]

For data structures, this gives unexpected results as each element points to the same variable.

>>> x = [[]]*10
>>> x
[[], [], [], [], [], [], [], [], [], []]

>>> # Append a value inside the first item.
>>> x[0].append('abc')
>>> # It gets added to all.
>>> x
[['abc'], ['abc'], ['abc'], ['abc'], ['abc'], ['abc'], ['abc'], ['abc'], ['abc'], ['abc']

You can’t use this either.

[list()]*10

You’ll need a lambda.

Or list comprehension. With [] or list() inside.

>>> x = [[] for _ in range(10)]
>>> x[0].append('abc')
>>> x
[['abc'], [], [], [], [], [], [], [], [], []]

With values

>>> [10, 13]
[10, 13]

>>> list([10, 13])
[10, 13]

You must pass an iterable to list. Passing a single str or int will fail.

>>> list(1)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: 'int' object is not iterable

Slices and index

x = ['abc', 'def', 'ghi', 'jkl']

Get item.

>>> x[1]
'def'

Get a range.

>>> x[:]  # all
['abc', 'def', 'ghi', 'jkl']
>>> x[:1]  # first
['abc']
>>> x[1:2] # second
['def']
>>> x[1:3] # second and third, using index 1 and 2
['def', 'ghi']
>>> x[-1]  # last
'jkl'
>>> x[-2]  # second last
'ghi'
>>> x[-2:]  # last 2
['ghi', 'jkl']

Assign.

>>> x[0] = 'ABC'
>>> x
['ABC', 'def', 'ghi', 'jkl']

Overwrite a range using an iterable.

>>> x[1:3] = ['DEF', 'GHI']  # Replace items at index 1 and 2.
>>> x
['abc', 'DEF', 'GHI', 'jkl']

Insert elements between elements. Here inserting at the start.

>>> x[0:0] = [123, 456]
>>> x
[123, 456, 'abc', 'def', 'ghi', 'jkl']

Copy

See Copy cheatsheet.

Cast to list

Tuple to list.

>>> x = (10, 13)
>>> list(x)
[10, 13]

Set to list.

>>> x = {1, 1, 2}  # Or set((1, 1, 2))
{1, 2}
>>> x
{1, 2}
>>> list(x)
[1, 2]

String to list.

>>> list('abc')
['a', 'b', 'c']