How to do an inverse `range`, i.e. create a compact range based on a set of numbers?

Question

Python has a range method, which allows for stuff like:

>>> range(1, 6)
[1, 2, 3, 4, 5]

What I’m looking for is kind of t

Answer 1

Since 9000 beat me to it, I'll just post the second part of the code, that prints pcre-like ranges from the previously computed output plus the added type check:

for i in output:
    if not isinstance(i, int) or i < 0:
        raise Exception("Only positive ints accepted in pcre_ranges")
result = [ str(x[0]) if x[0] == x[1] else '%s-%s' % (x[0], x[1]) for x in output ]
print result

Output: ['1-3', '5-9', '20-23', '50']