s = '25c319f75e3fbed5a9f0497750ea12992b30d565'
n = 2
print("-".join([s[i:i+n] for i in range(0, len(s), n)]))

s = '25c319f75e3fbed5a9f0497750ea12992b30d565'
n = [8, 4, 4, 4, 4, 12] # edited for consistency - Coldspeed

print("-".join([s[0:8], s[8:12], s[12:16], s[16:20], s[20:24], s[24:32]]))

25c319f7-5e3f-bed5-a9f0-4977-50ea1299

>>> s = '25c319f75e3fbed5a9f0497750ea12992b30d565'
>>> n = [8, 4, 4, 4, 4, 12]
>>> print '-'.join([s[sum(n[:i]):sum(n[:i+1])] for i in range(len(n))])

25c319f7-5e3f-bed5-a9f0-4977-50ea12992b30

from itertools import islice

s = '25c319f75e3fbed5a9f0497750ea12992b30d565'
it = iter(s)
n = [8, 4, 4, 12]

s = '-'.join(''.join(islice(it, None, x)) for x in n)
print(s)
# 25c319f7-5e3f-bed5-a9f0497750ea

s += '-' + ''.join(it)
print(s)
# 25c319f7-5e3f-bed5-a9f0497750ea-12992b30d565

start = 0
d = []
for i in n:
   d.append(s[start:start+i])
   start += i
d.append(s[start:])
print('-'.join(d))
# 25c319f7-5e3f-bed5-a9f0497750ea-12992b30d565

s = '25c319f75e3fbed5a9f0497750ea12992b30d565'
n = [8, 4, 4, 12]

def make_chunks(s,n):
    result = []
    for length in n:
        result.append(s[:length])
        s = s[length:]
    if s:
        result.append(s)
    return '-'.join(result)

print(make_chunks(s,n))

s = '25c319f75e3fbed5a9f0497750ea12992b30d565'
n = [8, 4, 4, 4, 4, 12]
res=[]
for split in n:
    temp=s[:split]
    s=s[split:]
    res.append(temp)
print(res)

['25c319f7', '5e3f', 'bed5', 'a9f0', '4977', '50ea12992b30']

string = '25c319f75e3fbed5a9f0497750ea12992b30d565'
lengths = [8, 4, 4, 12]

import itertools
ends = list(itertools.accumulate(lengths))

starts = [sum(lengths[:i]) for i in range(len(lengths))]

dashed = '-'.join(string[end-length : end]
                  for end,length in zip(ends,lengths))