Lists

A list in Python is similar to an array in Java or C: an ordered collection of objects. However, unlike lists in many other languages, Python lists can contain different types of elements; a list element can be any Python object, including Strings, Tuples, Lists, Dictionaries, Functions, Files and any kind of Numbers. You create a list by enclosing no elements or elements separated by commas in square brackets, like this:

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

Tip

I recommend that you do not use the ctypes.Array type available in Python, but if numerical calculations require it, consider NumPy, which is described in our Python for Data Science tutorial.

Indices

Elements can be extracted from a Python list using a notation similar to array indexing in C, starting with 0; asking for element 0 will return the first element of the list, asking for element 1 will return the second element, and so on. Here are a few examples:

>>> x = [1, "2.", 3.0, ["4a", "4b"], (5.1, 5.2)]
>>> x[0]
'1'
>>> x[1]
'2.'

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

>>> x[-1]
(5.1, 5.2)
>>> x[-2]
['4a', '4b']
>>> x[1:-1]
['2.', 3.0, ['4a', '4b']]
>>> x[0:3]
[1, '2.', 3.0]
>>> x[:3]
[1, '2.', 3.0]
>>> x[-4:-1]
['2.', 3.0, ['4a', '4b']]
>>> x[-4:]
['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.

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:

>>> x[0:3:2]
[1, [3.1, 3.2, 3.3]]
>>> x[::2]
[1, [3.1, 3.2, 3.3]]
>>> x[1::2]
['secondly', (5.1, 5.2)]

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

>>> x[3:0:-2]
[(5.1, 5.2), 'secondly']
>>> x[::-2]
[(5.1, 5.2), 'secondly']
>>> x[::-1]
[(5.1, 5.2), [3.1, 3.2, 3.3], 'secondly', 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.

Tip

To reverse the order, however, list.reverse() should be easier to read than a stride of -1, see also list.reverse().

See also

• Select and filter data with pandas

Changing lists

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:

>>> x = [1, "2.", 3.0, ["4a", "4b"], (5.1, 5.2)]
>>> x[1] = "secondly"
>>> x
[1, 'secondly', 3.0, ['4a', '4b'], (5.1, 5.2)]
>>> x[5:] = [6, 7]
>>> x
[1, 'secondly', 3.0, ['4a', '4b'], (5.1, 5.2), 6, 7]
>>> x[:0] = [-1, 0]
>>> x
[-1, 0, 1, 'secondly', 3.0, ['4a', '4b'], (5.1, 5.2), 6, 7]
>>> x[2:3] = []
>>> x
[-1, 0, 'secondly', 3.0, ['4a', '4b'], (5.1, 5.2), 6, 7]

Line 2

replaces the second element of the list.

Line 5

adds elements at the end of the list.

Line 8

adds elements at the beginning of the list.

Line 11

removes elements from the list.

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

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

You can also use the built-in functions (len(), max() and min()), some operators (in, not in, + and *), the del statement and the list methods (append, count, extend, index, insert, pop, remove, reverse, sort and sum) for lists:

>>> len(x)
4
>>> x[len(x) :] = [0, -1]
>>> x
[(5.1, 5.2), [3.1, 3.2, 3.3], 'secondly', 1, 0, -1]
>>> x.append(-2)
>>> x
[(5.1, 5.2), [3.1, 3.2, 3.3], 'secondly', 1, 0, -1, -2]
>>> y = [-3, -4, -5]
>>> x.append(y)
>>> x
[(5.1, 5.2), [3.1, 3.2, 3.3], 'secondly', 1, 0, -1, -2, [-3, -4, -5]]
>>> x[7:8] = []
>>> x
[(5.1, 5.2), [3.1, 3.2, 3.3], 'secondly', 1, 0, -1, -2]
>>> x.extend(y)
>>> x
[(5.1, 5.2), [3.1, 3.2, 3.3], 'secondly', 1, 0, -1, -2, -3, -4, -5]
>>> x + [-6, -7]
[(5.1, 5.2), [3.1, 3.2, 3.3], 'secondly', 1, 0, -1, -2, -3, -4, -5, -6, -7]
>>> x.reverse()
>>> x
[-5, -4, -3, -2, -1, 0, 1, 'secondly', [3.1, 3.2, 3.3], (5.1, 5.2)]

Line 1

shows the number of list elements.

Line 3

appends a new list to the end of the list.

Line 6

appends a new element to the end of the list with append.

Line 10

appends not the elements of the y list to the end of the list with append, but the element y list.

Line 16

appends the elements of the y list with extend.

Line 19

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

Line 21

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

List operations

Sorting lists

Lists can be sorted using the built-in Python sort method list.sort():

>>> x = [5, 3, -3, 3.1, 0, 1]
>>> x.sort()
>>> x
[-3, 0, 1, 3, 3.1, 5]

With this method, sorting is performed on the spot, meaning that the list to be sorted is changed. If you want the original list to remain unchanged, you have two options:

1. You can use the built-in function sorted(), which is described in more detail later.

2. You can create a copy of the list and sort the copy:

   >>> x = [5, 3, -3, 3.1, 0, 1]
   >>> y = x[:]
   >>> y.sort()
   >>> y
   [-3, 0, 1, 3, 3.1, 5]
   >>> x
   [5, 3, -3, 3.1, 0, 1]
   

Strings and lists of lists can also be sorted:

>>> hipy_list = ["Say", "hi", "to", "all", "Pythonistas", "!"]
>>> hipy_list.sort()
>>> hipy_list
['!', 'Pythonistas', 'Say', 'all', 'hi', 'to']
>>> ll = [[5.1, 5.2], [4.0, 5.0], [4.0, 3.0], [3.3, 3.2, 3.1]]
>>> ll.sort()
>>> ll
[[3.3, 3.2, 3.1], [4.0, 3.0], [4.0, 5.0], [5.1, 5.2]]

When comparing complex objects, the sub-lists are first sorted by the first element and then by the second element in ascending order.

list.sort() can also sort in reverse order with reverse=True. A separate key function can also be used to determine how the elements of a list are to be sorted.

However, the standard key method used by list.sort() requires that all elements in the list are of comparable type. In a list that contains both numbers and strings, an Exception is therefore thrown:

>>> x
[-5, -4, -3, -2, -1, 0, 1, 'secondly', [3.1, 3.2, 3.3], (5.1, 5.2)]
>>> x.sort()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: '<' not supported between instances of 'str' and 'int'

User-defined sorting

Note

You must be able to define Functions for user-defined sorting. The processing of Strings will also be covered in more detail later.

Python usually sorts words lexicographically – upper case before lower case. However, we want to sort a list of words by the number of characters in each word in ascending order instead:

>>> def ascending_number_chars(string):
...     return len(string)
...
>>> hipy_list = ["Say", "hi", "to", "all", "Pythonistas", "!"]
>>> new_list = hipy_list[:]
>>> hipy_list.sort()
>>> hipy_list
['!', 'Pythonistas', 'Say', 'all', 'hi', 'to']
>>> new_list.sort(key=ascending_number_chars)
>>> new_list
['!', 'hi', 'to', 'Say', 'all', 'Pythonistas']

The sorted function

Lists have an inbuilt method for sorting themselves list.sort(). However, other iterables in Python, such as the keys of Dictionaries, do not have a sorting method. However, Python offers the built-in sorted() function for this purpose, which returns a sorted list from any iterable. sorted() uses the same Parameters key and reverse as the list.sort() method:

>>> x
[5, 3, -3, 3.1, 0, 1]
>>> y = sorted(x)
>>> y
[-3, 0, 1, 3, 3.1, 5]
>>> z = sorted(x, reverse=True)
>>> z
[5, 3.1, 3, 1, 0, -3]

List membership

The in and not in, which return a Boolean value, make it easy to check whether a value is contained in a list.

List concatenation

The + operator can be used to create a list from two existing lists, whereby the initial lists remain unchanged:

>>> x = [3, -3, 0, 1]
>>> y = [3.1]
>>> z = x + y
>>> z
[3, -3, 0, 1, 3.1]

List initialisation

You can use the * operator to create a list of a certain size and certain values. This is a common method for working with lists whose size is known in advance and which do not cause any memory reallocation overhead. You should therefore prefer append in such cases in order to enlarge the list at the start of the programme:

>>> x = [None] * 4
>>> x
[None, None, None, None]

The operator for list multiplications * repeats the copying of the elements of a list the specified number and merges all copies into a new list. A list with a single instance of None is usually used for list multiplication, but the list can be anything:

>>> initial_list = [[1, 2, 3, 4]]
>>> arr = initial_list * 4
>>> arr
[[1, 2, 3, 4], [1, 2, 3, 4], [1, 2, 3, 4], [1, 2, 3, 4]]

Minimum or maximum of a list

You can use max() and min() to find the largest and smallest element of a list. You will probably use max() and min() mainly for numeric lists, but you can also use them for lists with arbitrary elements; however, if the comparison of these types does not make sense, this will result in an error:

>>> x = [5, 3, -3, 3.1, 0, 1]
>>> max(x)
5
>>> hipy_list = ["Say", "hi", "to", "all", "Pythonistas", "!"]
>>> max(hipy_list)
'to'
>>> max(x + hipy_list)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: '>' not supported between instances of 'str' and 'int'

When comparing complex objects, the sub-lists are first analysed according to the first element and then according to the second element (and so on).

>>> ll = [[1.0, 1.1], [1.0, 1.1, 1.2], [0.9, 1.3]]
>>> max(ll)
[1.0, 1.1, 1.2]

Search in a list

If you want to know where a value can be found in a list, you can use the index method. It searches a list for a list element with a specific value and returns the position of this list element:

>>> x = [5, 3, 3.0, -3, 3.1, 0, 1]
>>> x.index(3)
1
>>> x.index(3.0)
1
>>> x.index(5.0)
0
>>> x.index(6)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
ValueError: 6 is not in list

Line 8–11

Attempting to find the position of an element that is not in the list results in an error. This can be avoided by testing the list with the in or not-in list operators before using index.

Matches in lists

count also searches a list for a specific value, but returns the number of occurrences in the list and not the position:

>>> x = [5, 3, 3.0, -3, 3.1, 0, 1]
>>> x.count(3)
2
>>> x.count(5)
1
>>> x.count(6)
0

Nested lists and deepcopy

Lists can be nested, for example to display two-dimensional matrices. The elements of these matrices can be referenced using two-dimensional indices:

>>> ll = [[5.1, 5.2], [4.0, 5.0], [4.0, 3.0], [3.3, 3.2]]
>>> ll[0]
[5.1, 5.2]
>>> ll[0][1]
5.2

As expected, this mechanism can be transferred to more dimensions:

>>> sub = [0]
>>> sup = [sub, 1]
>>> sup
[[0], 1]
>>> sub[0] = 1
>>> sup
[[1], 1]
>>> sup[0][0] = 2
>>> sub
[2]
>>> sup
[[2], 1]

However, if sub is set to a different list, the connection between sub and sup is interrupted:

>>> sub = [3]
>>> sup
[[2], 1]

You can get a copy of a list by creating a full slice (x[:]) or by using + or * (for example, x + [] or x * 1). All three create a so-called flat copy of the list, which is probably what you want in most cases. However, if your list contains other lists that are nested within it, you may want to create a deep copy. You can do this with the copy.deepcopy() function of the copy module:

>>> shallow = sup[:]
>>> shallow
[[2], 1]

The shallow copy does not copy the elements of the list but only refers to the original elements. Changing one of these elements affects both shallow and sup:

>>> shallow[1] = 2
>>> shallow
[[2], 2]
>>> sup
[[2], 1]
>>> shallow[0][0] = 0
>>> sup
[[0], 1]

However, deepcopy is independent of the original list and no change to it has any effect on the original list:

>>> import copy
>>> deep = copy.deepcopy(sup)
>>> deep
[[0], 1]
>>> deep[0][0] = 1
>>> deep
[[1], 1]
>>> sup
[[0], 1]

Checks

• What does len() return for each of the following cases:

  • [3]

  • []

  • [[1, [2, 3], 4], "5 6"]

• How would you use len() and slices to determine the second half of a list if you don’t know how long it is?

• How could you move the last two entries of a list to the beginning without changing the order of the two?

• Which of the following cases triggers an exception?

  • min(["1", "2", "3"])

  • max([1, 2, "3"])

  • [1,2,3].count("1")

• If you have a list l, how can you remove a certain value i from it?

• If you have a nested list ll, how can you get a copy nll of this list in which you can change the elements without changing the contents of ll?

• Make sure that the my_collection object is a list before you try to append data to it.