Without regex reduce multiple spaces in a string to one in Python

Question

While answering this question I had a doubt regarding how to reduce multiple spaces in a string to one in Python without using regex. Say the string is as follows:

s = "Mary    had    a little lamb"

The first solution that came to my mind was

' '.join(s.split())

But this will not conserve leading and trailing spaces if the string was something like

s = "    Mary    had    a little lamb  "

In order to conserve leading and trailing spaces (if present), I came up with a slightly different approach where I added a dummy character to the starting and ending of the string (in this case a '-') which I later removed after the split. The code is as follows:

' '.join(('-' + s + '-').split())[1:-1]

Is there any other built in function or another pythonic way to do this?

Edit: By conserving leading and trailing spaces, I meant that in case there are multiple spaces at the beginning and/or at the end of the string, these multiple spaces should also be reduced to a single space

I liked your solution. Other than that I'd go with a regex. Though yours definitely requires a comment, a regex might not. — keyser
– keyser, Commented Jan 11, 2014 at 14:34
when I try your solution with the dummy characters, it doesn't conserve the leading/trailing whitespace, I don't understand.. — Totem
– Totem, Commented Jan 11, 2014 at 15:29
@Totem, A correction: The solution that I mentioned would reduce multiple spaces at the beginning and/or at the end of the string to a single space. It will not conserve all the spaces — Pulimon
– Pulimon, Commented Jan 11, 2014 at 16:31

Jon Clements · Accepted Answer · 2014-01-11 16:46:12Z

3

You could use something like:

from itertools import groupby

s = "    Mary    had    a little lamb  "
res = ''.join(' ' if is_space else ''.join(chars) for is_space, chars in groupby(s, str.isspace))
#  Mary had a little lamb

answered Jan 11, 2014 at 16:46

Jon Clements

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2 Comments

DSM Over a year ago

I was just writing this up, although I decided against str.isspace because that would pick up tabs too.

Jon Clements Over a year ago

@DSM yes... I was being lazy... although... lambda ch: ch == ' ' is not really much more effort :p

alexanderlukanin13 · Accepted Answer · 2014-01-11 15:00:39Z

0

You can use reduce:

>>> reduce(lambda x, y: x if x[-1]==' ' and y==' ' else x+y, s) if s else ''
' Mary had a little lamb '

edited Jan 11, 2014 at 15:00

answered Jan 11, 2014 at 14:41

alexanderlukanin13

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2 Comments

Pulimon Over a year ago

Hi alexander, I'm using python3.3 seems like reduce() is not present in 3.x version

Ashwini Chaudhary Over a year ago

@Pulimon It is moved to functools in Python3: from functools import reduce

loa_in_ · Accepted Answer · 2014-01-11 15:21:24Z

0

>>> s = "    Mary    had    a little lamb  "
>>> s
'    Mary had a little lamb  '
>>> len0 = len(s)
>>> s = s.rstrip()
>>> len1 = len(s)
>>> s = s.lstrip()
>>> s = " ".join(s.split())
>>> print s
Mary had a little lamb
>>> s = s.rjust(len1).ljust(len0)
>>> s
'    Mary had a little lamb  '

This code will preserve spaces. I know it's not required, but I'll just leave this here.

edited Jan 11, 2014 at 15:21

answered Jan 11, 2014 at 14:58

loa_in_

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1 Comment

Pulimon Over a year ago

Hi loa_in_: By leading and trailing spaces I meant that if there are multiple spaces at the beginning or at the end of a string, those multiple spaces should also be reduced to a single space. In this solution you conserve all the spaces

Ray · Accepted Answer · 2014-01-11 16:58:27Z

0

This appears to be fine for me.

test_str = "    Mary    had    a little lamb  "
while "  " in test_str:
    test_str = test_str.replace("  ", " ")

print test_str # yield ' Mary had a little lamb '

answered Jan 11, 2014 at 16:58

Ray

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Comments

Anand · Accepted Answer · 2018-03-21 22:13:46Z

0

(len(s) - len(s.lstrip()) * ' ' + ' '.join(s.lstrip().rstrip().split()) + (len(s) - len(s.rstrip()) * ' '

edited Mar 21, 2018 at 22:13

answered Feb 27, 2018 at 23:41

Anand

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1 Comment

Mr. T Over a year ago

This doesn't preserve leading and trailing spaces, as OP asked for in the question.

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