How to insert a character after every 2 characters in a string

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Is there a pythonic way to insert an element into every 2nd element in a string?

I have a string: ‘aabbccdd’ and I want the end result to be ‘aa-bb-cc-dd’.

I am not sure how I would go about doing that.

>>> s="aabbccdd"
>>> '-'.join(s[i:i+2] for i in range(0, len(s), 2))
'aa-bb-cc-dd'

Assume the string’s length is always an even number,

>>> s="12345678"
>>> t = iter(s)
>>> '-'.join(a+b for a,b in zip(t, t))
'12-34-56-78'

The t can also be eliminated with

>>> '-'.join(a+b for a,b in zip(s[::2], s[1::2]))
'12-34-56-78'

The algorithm is to group the string into pairs, then join them with the - character.

The code is written like this. Firstly, it is split into odd digits and even digits.

>>> s[::2], s[1::2]
('1357', '2468')

Then the zip function is used to combine them into an iterable of tuples.

>>> list( zip(s[::2], s[1::2]) )
[('1', '2'), ('3', '4'), ('5', '6'), ('7', '8')]

But tuples aren’t what we want. This should be a list of strings. This is the purpose of the list comprehension

>>> [a+b for a,b in zip(s[::2], s[1::2])]
['12', '34', '56', '78']

Finally we use str.join() to combine the list.

>>> '-'.join(a+b for a,b in zip(s[::2], s[1::2]))
'12-34-56-78'

The first piece of code is the same idea, but consumes less memory if the string is long.

If you want to preserve the last character if the string has an odd length, then you can modify KennyTM’s answer to use itertools.izip_longest:

>>> s = "aabbccd"
>>> from itertools import izip_longest
>>> '-'.join(a+b for a,b in izip_longest(s[::2], s[1::2], fillvalue=""))
'aa-bb-cc-d'

or

>>> t = iter(s)
>>> '-'.join(a+b  for a,b in izip_longest(t, t, fillvalue=""))
'aa-bb-cc-d'

I tend to rely on a regular expression for this, as it seems less verbose and is usually faster than all the alternatives. Aside from having to face down the conventional wisdom regarding regular expressions, I’m not sure there’s a drawback.

>>> s="aabbccdd"
>>> '-'.join(re.findall('..', s))
'aa-bb-cc-dd'

This version is strict about actual pairs though:

>>> t = s + 'e'
>>> '-'.join(re.findall('..', t)) 
'aa-bb-cc-dd'

… so with a tweak you can be tolerant of odd-length strings:

>>> '-'.join(re.findall('..?', t))
'aa-bb-cc-dd-e'

Usually you’re doing this more than once, so maybe get a head start by creating a shortcut ahead of time:

PAIRS = re.compile('..').findall

out="-".join(PAIRS(in))

Or what I would use in real code:

def rejoined(src, sep='-', _split=re.compile('..').findall):
    return sep.join(_split(src))

>>> rejoined('aabbccdd', sep=':')
'aa:bb:cc:dd'

I use something like this from time to time to create MAC address representations from 6-byte binary input:

>>> addr = b'\xdc\xf7\x09\x11\xa0\x49'
>>> rejoined(addr[::-1].hex(), sep=':')
'49:a0:11:09:f7:dc'

Here is one list comprehension way with conditional value depending of modulus of enumeration, odd last character will be in group alone:

for s  in ['aabbccdd','aabbccdde']:
    print(''.join([ char if not ind or ind % 2 else '-' + char
                    for ind,char in enumerate(s)
                    ]
                  )
          )
""" Output:
aa-bb-cc-dd
aa-bb-cc-dd-e
"""

This one-liner does the trick. It will drop the last character if your string has an odd number of characters.

"-".join([''.join(item) for item in zip(mystring1[::2],mystring1[1::2])])

As PEP8 states:

Do not rely on CPython’s efficient implementation of in-place string concatenation for statements in the form a += b or a = a + b. This optimization is fragile even in CPython (it only works for some types) and isn’t present at all in implementations.

A pythonic way of doing this that avoids this kind of concatenation, and allows you to join iterables other than strings could be:

':'.join(f'{s[i:i+2]}' for i in range(0, len(s), 2))

And another more functional-like way could be:

':'.join(map('{}{}'.format, *(s[::2], s[1::2]))) 

This second approach has a particular feature (or bug) of only joining pairs of letters. So:

>>> s="abcdefghij"
'ab:cd:ef:gh:ij'

and:

>>> s="abcdefghi"
'ab:cd:ef:gh'


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