Lists

Python has a powerful built-in list type:

1 []
2 [1]
3 [1, "2.", 3.0, ["4a", "4b"], (5.1,5.2)]

A list can contain a mixture of other types as elements, including strings, tuples, lists, dictionaries, functions, file objects and any kind of number.

A list can be indexed from the front or the back. You can also refer to a sub-segment of a list using slice notation:

 1 >>> x = [1, "2.", 3.0, ["4a", "4b"], (5.1,5.2)]
 2 >>> x[0]
 3 '1'
 4 >>> x[1]
 5 '2.'
 6 >>> x[-1]
 7 (5.1, 5.2)
 8 >>> x[-2]
 9 ['4a', '4b']
10 >>> x[1:-1]
11 ['2.', 3.0, ['4a', '4b']]
12 >>> x[0:3]
13 [1, '2.', 3.0]
14 >>> x[:3]
15 [1, '2.', 3.0]
16 >>> x[-4:-1]
17 ['2.', 3.0, ['4a', '4b']]
18 >>> x[-4:]
19 ['2.', 3.0, ['4a', '4b'], (5.1, 5.2)]
Lines 2 and 4

Index from the beginning using positive indices starting with 0 as the first element.

Lines 6 and 8

Index from the back using negative indices starting with -1 as the last element.

Lines 10 and 12

Slice with [m:n], where m is the inclusive start point and n is the exclusive end point.

Lines 14, 16 and 18

A [:n] slice starts at the beginning and an [m:] slice goes to the end of a list.

You can use this notation to add, remove and replace elements in a list or to get an element or a new list that is a slice of it, for example:

 1 >>> x = [1, "2.", 3.0, ["4a", "4b"], (5.1,5.2)]
 2 >>> x[1] = "zweitens"
 3 >>> x[2:3] = []
 4 >>> x
 5 [1, 'zweitens', ['4a', '4b'], (5.1, 5.2)]
 6 >>> x[2] = [3.1, 3.2, 3.3]
 7 >>> x
 8 [1, 'zweitens', [3.1, 3.2, 3.3], (5.1, 5.2)]
 9 >>> x[2:]
10 [[3.1, 3.2, 3.3], (5.1, 5.2)]
Line 3

The size of the list increases or decreases if the new slice is larger or smaller than the slice it replaces.

Slices also allow a step-by-step selection between the start and end indices. The default value for an unspecified stride is 1, which takes every element from a sequence between the indices. With a stride of 2, every second element is taken and so on:

1>>> x[0:3:2]
2[1, [3.1, 3.2, 3.3]]
3>>> x[::2]
4[1, [3.1, 3.2, 3.3]]
5>>> x[1::2]
6['zweitens', (5.1, 5.2)]

The stride value can also be negative. A -1 stride means counting from right to left:

1>>> x[3:0:-2]
2[(5.1, 5.2), 'zweitens']
3>>> x[::-2]
4[(5.1, 5.2), 'zweitens']
5>>> x[::-1]
6[(5.1, 5.2), [3.1, 3.2, 3.3], 'zweitens', 1]
Line 1

To use a negative increment, the start slice should be larger than the end slice.

Line 3

The exception is if you omit the start and end indices.

Line 5

A stride of -1 reverses the order.

Some functions of the slice notation can also be executed with special operations, which improves the readability of the code:

1>>> x.reverse()
2>>> x
3[(5.1, 5.2), [3.1, 3.2, 3.3], 'zweitens', 1]

You can also use the following built-in functions (len, max and min), some operators (in, + and *), the del statement and the list methods (append, count, extend, index, insert, pop, remove, reverse and sort) for lists:

1 >>> len(x)
2 4
3 >>> x + [0, -1]
4 [(5.1, 5.2), [3.1, 3.2, 3.3], 'zweitens', 1, 0, -1]
5 >>> x.reverse()
6 >>> x
7 [1, 'zweitens', [3.1, 3.2, 3.3], (5.1, 5.2)]
Line 3

The operators + and * each create a new list, leaving the original list unchanged.

Line 5

The methods of a list are called using the attribute notation for the list itself: :samp:`{LIST}.METHOD(ARGUMENTS).

Some of these operations repeat functions that can be performed using slice notation, but they improve the readability of the code.

See also

Summary

data type

mutable

ordered

indexed

duplicates

list

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