Lists and Tuples in Python

Lists Are Ordered

A list is not merely a collection of objects. It is an ordered collection of objects. The order in which you specify the elements when you define a list is an innate characteristic of that list and is maintained for that list’s lifetime. (You will see a Python data type that is not ordered in the next tutorial on dictionaries.)

Lists that have the same elements in a different order are not the same:

>>> a = ['foo', 'bar', 'baz', 'qux']
>>> b = ['baz', 'qux', 'bar', 'foo']
>>> a == b
False
>>> a is b
False

>>> [1, 2, 3, 4] == [4, 1, 3, 2]
False

Lists Can Contain Arbitrary Objects

A list can contain any assortment of objects. The elements of a list can all be the same type:

>>> a = [2, 4, 6, 8]
>>> a
[2, 4, 6, 8]

Or the elements can be of varying types:

>>> a = [21.42, 'foobar', 3, 4, 'bark', False, 3.14159]
>>> a
[21.42, 'foobar', 3, 4, 'bark', False, 3.14159]

Lists can even contain complex objects, like functions, classes, and modules, which you will learn about in upcoming tutorials:

>>> int
<class 'int'>
>>> len
<built-in function len>
>>> def foo():
...     pass
...
>>> foo
<function foo at 0x035B9030>
>>> import math
>>> math
<module 'math' (built-in)>

>>> a = [int, len, foo, math]
>>> a
[<class 'int'>, <built-in function len>, <function foo at 0x02CA2618>,
<module 'math' (built-in)>]

A list can contain any number of objects, from zero to as many as your computer’s memory will allow:

>>> a = []
>>> a
[]

>>> a = [ 'foo' ]
>>> a
['foo']

>>> a = [0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20,
... 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40,
... 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60,
... 61, 62, 63, 64, 65, 66, 67, 68, 69, 70, 71, 72, 73, 74, 75, 76, 77, 78, 79, 80,
... 81, 82, 83, 84, 85, 86, 87, 88, 89, 90, 91, 92, 93, 94, 95, 96, 97, 98, 99, 100]
>>> a
[0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20,
21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39,
40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58,
59, 60, 61, 62, 63, 64, 65, 66, 67, 68, 69, 70, 71, 72, 73, 74, 75, 76, 77,
78, 79, 80, 81, 82, 83, 84, 85, 86, 87, 88, 89, 90, 91, 92, 93, 94, 95, 96,
97, 98, 99, 100]

(A list with a single object is sometimes referred to as a singleton list.)

List objects needn’t be unique. A given object can appear in a list multiple times:

>>> a = ['bark', 'meow', 'woof', 'bark', 'cheep', 'bark']
>>> a
['bark', 'meow', 'woof', 'bark', 'cheep', 'bark']

List Elements Can Be Accessed by Index

Individual elements in a list can be accessed using an index in square brackets. This is exactly analogous to accessing individual characters in a string. List indexing is zero-based as it is with strings.

Consider the following list:

>>> a = ['foo', 'bar', 'baz', 'qux', 'quux', 'corge']

The indices for the elements in a are shown below:

Here is Python code to access some elements of a:

>>> a[0]
'foo'
>>> a[2]
'baz'
>>> a[5]
'corge'

Virtually everything about string indexing works similarly for lists. For example, a negative list index counts from the end of the list:

>>> a[-1]
'corge'
>>> a[-2]
'quux'
>>> a[-5]
'bar'

Slicing also works. If a is a list, the expression a[m:n] returns the portion of a from index m to, but not including, index n:

>>> a = ['foo', 'bar', 'baz', 'qux', 'quux', 'corge']

>>> a[2:5]
['baz', 'qux', 'quux']

Other features of string slicing work analogously for list slicing as well:

• Both positive and negative indices can be specified:

  Python

  >>> a[-5:-2]
  ['bar', 'baz', 'qux']
  >>> a[1:4]
  ['bar', 'baz', 'qux']
  >>> a[-5:-2] == a[1:4]
  True
  

• Omitting the first index starts the slice at the beginning of the list, and omitting the second index extends the slice to the end of the list:

  Python

  >>> print(a[:4], a[0:4])
  ['foo', 'bar', 'baz', 'qux'] ['foo', 'bar', 'baz', 'qux']
  >>> print(a[2:], a[2:len(a)])
  ['baz', 'qux', 'quux', 'corge'] ['baz', 'qux', 'quux', 'corge']
  
  >>> a[:4] + a[4:]
  ['foo', 'bar', 'baz', 'qux', 'quux', 'corge']
  >>> a[:4] + a[4:] == a
  True
  

• You can specify a stride—either positive or negative:

  Python

  >>> a[0:6:2]
  ['foo', 'baz', 'quux']
  >>> a[1:6:2]
  ['bar', 'qux', 'corge']
  >>> a[6:0:-2]
  ['corge', 'qux', 'bar']
  

• The syntax for reversing a list works the same way it does for strings:

  Python

  >>> a[::-1]
  ['corge', 'quux', 'qux', 'baz', 'bar', 'foo']
  

• The [:] syntax works for lists. However, there is an important difference between how this operation works with a list and how it works with a string.

  If s is a string, s[:] returns a reference to the same object:

  Python

  >>> s = 'foobar'
  >>> s[:]
  'foobar'
  >>> s[:] is s
  True
  

  Conversely, if a is a list, a[:] returns a new object that is a copy of a:

  Python

  >>> a = ['foo', 'bar', 'baz', 'qux', 'quux', 'corge']
  >>> a[:]
  ['foo', 'bar', 'baz', 'qux', 'quux', 'corge']
  >>> a[:] is a
  False
  

Several Python operators and built-in functions can also be used with lists in ways that are analogous to strings:

• The in and not in operators:

  Python

  >>> a
  ['foo', 'bar', 'baz', 'qux', 'quux', 'corge']
  
  >>> 'qux' in a
  True
  >>> 'thud' not in a
  True
  

• The concatenation (+) and replication (*) operators:

  Python

  >>> a
  ['foo', 'bar', 'baz', 'qux', 'quux', 'corge']
  
  >>> a + ['grault', 'garply']
  ['foo', 'bar', 'baz', 'qux', 'quux', 'corge', 'grault', 'garply']
  >>> a * 2
  ['foo', 'bar', 'baz', 'qux', 'quux', 'corge', 'foo', 'bar', 'baz',
  'qux', 'quux', 'corge']
  

• The len(), min(), and max() functions:

  Python

  >>> a
  ['foo', 'bar', 'baz', 'qux', 'quux', 'corge']
  
  >>> len(a)
  6
  >>> min(a)
  'bar'
  >>> max(a)
  'qux'
  

It’s not an accident that strings and lists behave so similarly. They are both special cases of a more general object type called an iterable, which you will encounter in more detail in the upcoming tutorial on definite iteration.

By the way, in each example above, the list is always assigned to a variable before an operation is performed on it. But you can operate on a list literal as well:

>>> ['foo', 'bar', 'baz', 'qux', 'quux', 'corge'][2]
'baz'

>>> ['foo', 'bar', 'baz', 'qux', 'quux', 'corge'][::-1]
['corge', 'quux', 'qux', 'baz', 'bar', 'foo']

>>> 'quux' in ['foo', 'bar', 'baz', 'qux', 'quux', 'corge']
True

>>> ['foo', 'bar', 'baz'] + ['qux', 'quux', 'corge']
['foo', 'bar', 'baz', 'qux', 'quux', 'corge']

>>> len(['foo', 'bar', 'baz', 'qux', 'quux', 'corge'][::-1])
6

For that matter, you can do likewise with a string literal:

>>> 'If Comrade Napoleon says it, it must be right.'[::-1]
'.thgir eb tsum ti ,ti syas noelopaN edarmoC fI'

Lists Can Be Nested

You have seen that an element in a list can be any sort of object. That includes another list. A list can contain sublists, which in turn can contain sublists themselves, and so on to arbitrary depth.

Consider this (admittedly contrived) example:

>>> x = ['a', ['bb', ['ccc', 'ddd'], 'ee', 'ff'], 'g', ['hh', 'ii'], 'j']
>>> x
['a', ['bb', ['ccc', 'ddd'], 'ee', 'ff'], 'g', ['hh', 'ii'], 'j']

The object structure that x references is diagrammed below:

x[0], x[2], and x[4] are strings, each one character long:

>>> print(x[0], x[2], x[4])
a g j

But x[1] and x[3] are sublists:

>>> x[1]
['bb', ['ccc', 'ddd'], 'ee', 'ff']

>>> x[3]
['hh', 'ii']

To access the items in a sublist, simply append an additional index:

>>> x[1]
['bb', ['ccc', 'ddd'], 'ee', 'ff']

>>> x[1][0]
'bb'
>>> x[1][1]
['ccc', 'ddd']
>>> x[1][2]
'ee'
>>> x[1][3]
'ff'

>>> x[3]
['hh', 'ii']
>>> print(x[3][0], x[3][1])
hh ii

x[1][1] is yet another sublist, so adding one more index accesses its elements:

>>> x[1][1]
['ccc', 'ddd']
>>> print(x[1][1][0], x[1][1][1])
ccc ddd

There is no limit, short of the extent of your computer’s memory, to the depth or complexity with which lists can be nested in this way.

All the usual syntax regarding indices and slicing applies to sublists as well:

>>> x[1][1][-1]
'ddd'
>>> x[1][1:3]
[['ccc', 'ddd'], 'ee']
>>> x[3][::-1]
['ii', 'hh']

However, be aware that operators and functions apply to only the list at the level you specify and are not recursive. Consider what happens when you query the length of x using len():

>>> x
['a', ['bb', ['ccc', 'ddd'], 'ee', 'ff'], 'g', ['hh', 'ii'], 'j']
>>> len(x)
5

>>> x[0]
'a'
>>> x[1]
['bb', ['ccc', 'ddd'], 'ee', 'ff']
>>> x[2]
'g'
>>> x[3]
['hh', 'ii']
>>> x[4]
'j'

x has only five elements—three strings and two sublists. The individual elements in the sublists don’t count toward x’s length.

You’d encounter a similar situation when using the in operator:

>>> 'ddd' in x
False
>>> 'ddd' in x[1]
False
>>> 'ddd' in x[1][1]
True

'ddd' is not one of the elements in x or x[1]. It is only directly an element in the sublist x[1][1]. An individual element in a sublist does not count as an element of the parent list(s).

Lists Are Mutable

Most of the data types you have encountered so far have been atomic types. Integer or float objects, for example, are primitive units that can’t be further broken down. These types are immutable, meaning that they can’t be changed once they have been assigned. It doesn’t make much sense to think of changing the value of an integer. If you want a different integer, you just assign a different one.

By contrast, the string type is a composite type. Strings are reducible to smaller parts—the component characters. It might make sense to think of changing the characters in a string. But you can’t. In Python, strings are also immutable.

The list is the first mutable data type you have encountered. Once a list has been created, elements can be added, deleted, shifted, and moved around at will. Python provides a wide range of ways to modify lists.

>>> a = ['foo', 'bar', 'baz', 'qux', 'quux', 'corge']
>>> a
['foo', 'bar', 'baz', 'qux', 'quux', 'corge']

>>> a[2] = 10
>>> a[-1] = 20
>>> a
['foo', 'bar', 10, 'qux', 'quux', 20]

>>> s = 'foobarbaz'
>>> s[2] = 'x'
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: 'str' object does not support item assignment