Python reverse list [duplicate]

Question

I'm trying to reverse a string and using the below code but the resultant reverse list value is None.

The code:

str_a = 'This is stirng'
rev_word = str_a.split()
rev_word = rev_word.reverse()
rev_word = ''.join(rev_word)

It returns the TypeError. Why?

Already answered here? (Personally, I like the ''.join(reversed(s)) solution there.) — ron rothman
– ron rothman, Commented Sep 9, 2012 at 2:46
I wish #reverse returned a reference to self. I guess Guido van Rossum was not as into method chaining as Yukihiro Matsumoto. — Eric Walker
– Eric Walker, Commented May 12, 2014 at 1:00

Sean Johnson · Accepted Answer · 2013-04-01 21:11:53Z

123

This is my personal favorite way to reverse a string:

stra="This is a string"
revword = stra[::-1]

print(revword) #"gnirts a si sihT

or, if you want to reverse the word order:

revword = " ".join(stra.split()[::-1])

print(revword) #"string a is This"

:)

edited Apr 1, 2013 at 21:11

answered Sep 9, 2012 at 2:52

Sean Johnson

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3 Comments

thefourtheye Over a year ago

I know it works. But I was expecting "T", as we havnt mentioned anything for the start and end, I expected Python to start with 0 and move backwards. Please let me know how it works.

Sean Johnson Over a year ago

@thefourtheye see this stackoverflow: stackoverflow.com/questions/509211/the-python-slice-notation

thefourtheye Over a year ago

@SeanJohnson So, if we give -1 as the step value, by default it will traverse backwards. Is that right?

Mr Mystery Guest · Accepted Answer · 2016-10-10 14:56:00Z

53

.reverse() returns None. Therefore you should not be assigning it to a variable.

Use this instead:

stra = 'This is a string'
revword = stra.split()
revword.reverse()
revword=''.join(revword)

I've run the code on IDEOne for you so you can see the output. (Also notice that the output is stringaisThis; you may want to use ' '.join(revword), with a space, instead.)

Also note that the method you have provided only reverses the words, not the text. @ron.rothman provided a link that does detail how to reverse a string in its entirety.

edited Oct 10, 2016 at 14:56

Mr Mystery Guest

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answered Sep 9, 2012 at 2:43

Cat

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Comments

Paddy3118 · Accepted Answer · 2012-09-09 05:10:08Z

7

Various reversals on a string:

instring = 'This is a string'
reversedstring = instring[::-1]
print reversedstring        # gnirts a si sihT
wordsreversed = ' '.join(word[::-1] for word in instring.split())
print wordsreversed         # sihT si a gnirts
revwordorder = ' '.join(word for word in instring.split()[::-1])
print revwordorder          # string a is This
revwordandorder = ' '.join(word[::-1] for word in instring.split()[::-1])
print revwordandorder       # gnirts a si sihT

answered Sep 9, 2012 at 5:10

Paddy3118

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Comments

dm03514 · Accepted Answer · 2014-01-02 21:43:54Z

6

For future reference when an object has a method like [].reverse() it generally performs that action o n the object (ie. the list is sorted and returns nothing, None) in contrast to built in functions like sorted which perform an action on an object and returns a value (ie the sorted list)

edited Jan 2, 2014 at 21:43

answered Sep 9, 2012 at 5:20

dm03514

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Comments

user2665694 · Accepted Answer · 2012-09-09 05:01:58Z

4

>>> s = 'this is a string'
>>> s[::-1]
'gnirts a si siht'
>>> ''.join(reversed(s))
'gnirts a si siht'

answered Sep 9, 2012 at 5:01

user2665694

Comments

Konsole · Accepted Answer · 2014-05-08 08:43:55Z

0

The for loop iterates the string from end (last letter) to start (first letter)

>>> s = 'You can try this too :]'
>>> rev = ''
>>> for i in range(len(s) - 1, -1, -1):
...     rev += s[i]
>>> rev
']: oot siht yrt nac uoY'

edited May 8, 2014 at 8:43

Konsole

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answered May 8, 2014 at 7:38

Aziz Alto

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2 Comments

Konsole Over a year ago

Consider providing some explanation!

Aziz Alto Over a year ago

rev is an empty string var. The for loop iterates the original string backward, (from the end "last letter" to the beginning "first letter"). In every iteration, the corresponding letter is appended to rev. Let me know if this is helpful :]

user2443147 · Accepted Answer · 2014-08-01 15:42:20Z

0

Based on comments and other answers:

str_a = 'this is a string'
rev_word = ' '.join(reversed(str_a.split()))

Method chaining does work in Python after all...

answered Aug 1, 2014 at 15:42

user2443147

Comments

Aashish P · Accepted Answer · 2015-12-29 09:44:31Z

0

List reverse can be done using more than one way.
As mentioned in previous answers two are very prominent, one with reverse() function and two with slicing feature. I'm giving some insights on which one we should prefer. We should always use reverse() function for reversal of a Python list. Two reasons, one in-place reversal and two faster than other. I've some figures to support my answer,

In [15]: len(l)
Out[15]: 1000

In [16]: %timeit -n1 l.reverse()
1 loops, best of 3: 7.87 µs per loop

In [17]: %timeit -n1 l[::-1]
1 loops, best of 3: 10 µs per loop

For 1000 integer list, reverse() function performed better compared to slicing.

answered Dec 29, 2015 at 9:44

Aashish P

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