Rearranging List Elements

Python provides two list methods to rearrange the order of the elements. Both methods mutate (change) the original list.

reverse Example

The general syntax for this method is:

list_name.reverse()

reverse flips the order of the elements in a list. No arguments go inside the ().

Example

words = ['At', 'banana', 'orange', 'apple', 'zoo']

words.reverse()
print(words)

Output

['zoo', 'apple', 'orange', 'banana', 'At']

sort Examples

The general syntax for this method is:

list_name.sort()

sort arranges the elements of a list into increasing order (smallest to largest). However, all elements inside the list MUST be of the same data type. Numbers sort with other numbers, strings with other strings, booleans with other booleans, etc.

Example

Numerical sorting:

numbers = [2, 8, 10, 400, 30, 45.5]
mixed_types = [20, 8, 'm', 'a']

numbers.sort()
print(numbers)

mixed_types.sort()

Console Output

[2, 8, 10, 30, 45.5, 400]
Line 11:
   mixed_types.sort()
TypeError: '<' not supported between instances of 'str' and 'int'

For strings, sort arranges the elements in alphabetical order.

Example

When sorting strings, case matters!

letters = ['f', 'c', 'B', 'X', 'a']
letters_and_numbers = ['5', '3', 'm', '8', 'T']

letters.sort()
letters_and_numbers.sort()
print(letters)
print(letters_and_numbers)

Console Output

[ 'B', 'X', 'a', 'c', 'f' ]
['3', '5', '8', 'T', 'm']

From the alphabet song, we know that 'a' comes before 'B' (and certainly before 'X'), but Python treats capital and lowercase letters differently. Capital letters get sorted before lowercase.

Note that numerical strings come before all letters.

Sorting Numerical Strings

Let’s take a look at sorting a list of strings when each element is a string of digits.

Example

number_strings = ['2', '8', '10', '400', '30']

number_strings.sort()
print(number_strings)

Console Output

[ 10, 2, 30, 400, 8 ]

Wait… what? How is 8 larger than 400?

Note that even though we use digits, each element is still a string. Just like 'apple' comes before 'pear' because 'a' comes before 'p', the string '400' begins with a '4', which comes before any string starting with an '8'. Looking only at the first digit in each string, we see the expected progression (1, 2, 3, 4, 8).

Sort in Descending Order

To sort a list from the largest to smallest value, the general syntax is:

list_name.sort(reverse = True)

Example

Sort in descending order:

numbers = [2, 8, 10, 400, 30, 45.5]
letters = ['f', 'c', 'B', 'X', 'a']

numbers.sort(reverse = True)
letters.sort(reverse = True)
print(numbers)
print(letters)

Console Output

[400, 45.5, 30, 10, 8, 2]
['f', 'c', 'a', 'X', 'B']