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

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

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'

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'

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])])

I added tests to @SilentGhost's answer

def insert_between_every_n_characters(original: str, inserted: str, step: int) -> str:
    """
    Insert a string between every N characters.

    >>> insert_between_every_n_characters('aabbccdd', '--', 1)
    'a--a--b--b--c--c--d--d'

    >>> insert_between_every_n_characters('aabbccdd', '-', 2)
    'aa-bb-cc-dd'

    >>> insert_between_every_n_characters('aabbccd', ':', 3)
    'aab:bcc:d'

    >>> insert_between_every_n_characters('aabbccdda', ':', 3)
    'aab:bcc:dda'

    >>> insert_between_every_n_characters('a', '-', 2)
    'a'

    >>> insert_between_every_n_characters('', '-', 2)
    ''
    """
    if step <= 0:
        raise ValueError(f"step must be greater than zero. Got: {step}")
    return inserted.join(original[i : i + step] for i in range(0, len(original), step))

Using only generators is possible with current python.

Pros of this solution:

• 'length' is specified once and can be changed without needing to update the formula.

• Doesn't use indices (I'm calling it, they're unpythonic)

  from itertools import batched
  
  s = 'aabbccdd'
  r = '-'.join(''.join(b) for b in batched(s, 2))
  

Batched returns 2 chars from the string. They are 'joined' with a null char. Each resultant string is 'joined' with a '-' to those that came before until the string is exhausted.

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'