[Python] Convert List of Numbers to String Ranges

I'd like to know if there is a simple (or already created) way of doing the opposite of this: Generate List of Numbers from Hyphenated.... This link could be used to do:

>> list(hyphen_range('1-9,12,15-20,23'))
[1, 2, 3, 4, 5, 6, 7, 8, 9, 12, 15, 16, 17, 18, 19, 20, 23]:


I'm looking to do the opposite (note that 10 and 21 are included so it would be compatible with the range function, where range(1,10)=[1,2,3,4,5,6,7,8,9]):

>> list_to_ranges([1, 2, 3, 4, 5, 6, 7, 8, 9, 12, 15, 16, 17, 18, 19, 20, 23])
'1-10,12,15-21,23'


Eventually, I would like to have the output also incorporate a step where the last number of the output indicates the step:

>> list_to_ranges([1, 3, 5, 7, 8, 9, 10, 11])
'1-13:2,8,10'


Essentially, this would end up being kind of like an "inverse" range function

>> tmp = list_to_ranges([1, 3, 5])
>> print tmp
'1-7:2'
>> range(1, 7, 2)
[1, 3, 5]


My guess is that there is no really easy/simple way to do this, but I thought I would ask on here before I go make some brute force, long method.

EDIT

Using the code from an answer to this post as an example, I came up with a simple way to do the first part. But I think that identifying the patterns to do steps would be a bit harder.

from itertools import groupby
from operator import itemgetter

data = [ 1, 4,5,6, 10, 15,16,17,18, 22, 25,26,27,28]
print data, '\n'

str_list = []
for k, g in groupby(enumerate(data), lambda (i,x):i-x):
ilist = map(itemgetter(1), g)
print ilist
if len(ilist) > 1:
str_list.append('%d-%d' % (ilist[0], ilist[-1]+1))
else:
str_list.append('%d' % ilist[0])
print '\n', ','.join(str_list)


EDIT 2

Here is my attempt at including the step size...it is pretty close, but the first numbers get repeated. I think that with a little bit of tweaking of this, it will be close to what I want - or at least good enough.

import numpy as np
from itertools import groupby

def list_to_ranges(data):
data = sorted(data)
diff_data = np.diff(data).tolist()
ranges = []
i = 0
for k, iterable in groupby(diff_data, None):
rng = list(iterable)
step = rng[0]
if len(rng) == 1:
ranges.append('%d' % data)
elif step == 1:
ranges.append('%d-%d' % (data, data[i+len(rng)]+step))
else:
ranges.append('%d-%d:%d' % (data, data[i+len(rng)]+step, step))
i += len(rng)
return ','.join(ranges)

data = [1, 3, 5, 6, 7, 11, 13, 15, 16, 17, 18, 19, 22, 25, 28]
print data
data_str = list_to_ranges(data)
print data_str

_list = []
for r in data_str.replace('-',':').split(','):
r = [int(a) for a in r.split(':')]
if len(r) == 1:
_list.extend(r)
elif len(r) == 2:
_list.extend(range(r[0], r[1]))
else:
_list.extend(range(r[0], r[1], r[2]))
print _list
print list(set(_list))

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