Pythonic way to insert every 2 elements in a string

>>> 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'

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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'