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Python reversed() function

Updated on Jan 07, 2020

The reversed() function allows us to process the items in a sequence in reverse order. It accepts a sequence and returns an iterator.

Its syntax is as follows:

Syntax:

reversed(sequence) -> reverse iterator
Parameter Description
sequence A sequence list string, list, tuple etc.

Here are some examples:

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>>>
>>> reversed([44, 11, -90, 55, 3])
<list_reverseiterator object at 0x7f2aff2f91d0>
>>>
>>>
>>> list(reversed([44, 11, -90, 55, 3])) # reversing a list
[3, 55, -90, 11, 44]
>>>
>>>
>>> list(reversed((6, 1, 3, 9))) # reversing a tuple
[9, 3, 1, 6]
>>>
>>> list(reversed("hello")) # reversing a string
['o', 'l', 'l', 'e', 'h']
>>>

Try it out:

print( reversed([44, 11, -90, 55, 3]) )

print(list(reversed([44, 11, -90, 55, 3]))) # reversing a list

print( list(reversed((6, 1, 3, 9)))) # reversing a tuple

print(list(reversed("hello"))) # reversing a string

To produce the result at once we have wrapped reversed() in a list() call. This is required in both Python 2 and Python 3.

The argument passed to reversed()  must be a proper sequence. Trying to pass objects which do no maintain their orders like dict and set will result in a TypeError.

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>>>
>>> reversed({0, 4, -2, 12, 6})
Traceback (most recent call last):
  File "", line 1, in
TypeError: argument to reversed() must be a sequence
>>>
>>>
>>> reversed({'name': 'John', 'age': 20})
Traceback (most recent call last):
  File "", line 1, in
TypeError: argument to reversed() must be a sequence
>>>

Reversing User-defined objects #

To reverse user-defined objects the class must do one of the following:

  1. Implement __len__()  and __getitem__()  methods; or
  2. Implement __reversed__() method

In following listing, the CardDeck class implement __len__() and __getitem__() methods. As a result, we can apply the reversed() on the CardDeck instance.

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>>>
>>> from collections import namedtuple
>>>
>>> Card = namedtuple('Card', ['rank', 'suit'])
>>>
>>> class CardDeck:
...     suits = ('club', 'diamond', 'heart', 'spades')
...     ranks = tuple((str(i) for i in range(2, 11))) + tuple("JQKA")
...
...     def __init__(self):
...         self._cards = [Card(r, s) for s in self.suits for r in self.ranks ]
...
...     def __len__(self):
...         return len(self._cards)
...
...     def __getitem__(self, index):
...         return self._cards[index]
...
...    #  def __reversed__(self):   this is how you would define __reversed__() method
...    #     return self._cards[::-1]
...
...
>>>
>>> deck = CardDeck()
>>>
>>> deck
<__main__.CardDeck object at 0x7f2aff2feb00>
>>>
>>>
>>> deck[0], deck[-1] # deck before reversing
(Card(rank='2', suit='club'), Card(rank='A', suit='spades'))
>>>
>>>
>>> reversed_deck = list(reversed(deck))
>>>
>>>
>>> reversed_deck[0], reversed_deck[-1] # deck after reversing
(Card(rank='A', suit='spades'), Card(rank='2', suit='club'))
>>>

Try it out:

from collections import namedtuple

Card = namedtuple('Card', ['rank', 'suit'])

class CardDeck:
    suits = ('club', 'diamond', 'heart', 'spades')
    ranks = tuple((str(i) for i in range(2, 11))) + tuple("JQKA")

    def __init__(self):
        self._cards = [Card(r, s) for s in self.suits for r in self.ranks ]

    def __len__(self):
        return len(self._cards)

    def __getitem__(self, index):
        return self._cards[index]

    #  def __reversed__(self):   this is how you would define __reversed__() method
    #     return self._cards[::-1]


deck = CardDeck()

print(deck)

print( deck[0], deck[-1] ) # deck before reversing

reversed_deck = list(reversed(deck))

print(reversed_deck[0], reversed_deck[-1] ) # deck after reversing

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