How can I combine range() functions

Question

For some code I'm writing, I need to iterate from 1-30 skipping 6. What I tried naively is

a = range(1,6)
b = range(7,31)

for i in a+b:
    print i

Is there a way to do it more efficiently?

Answer 1

Use itertools.chain:

import itertools

a = range(1,6)
b = range(7,31)

for i in itertools.chain(a, b):
    print i

Or tricky flattening generator expressions:

a = range(1,6)
b = range(7,31)
for i in (x for y in (a, b) for x in y):
    print i

Or skipping in a generator expression:

skips = set((6,))
for i in (x for x in range(1, 31) if x not in skips):
    print i

Any of these will work for any iterable(s), not just ranges in Python 3 or listss in Python 2.

Answer 2

In python 2 you are not combining "range functions"; these are just lists. Your example works just well. But range always creates a full list in memory, so a better way if only needed in for loop could be to to use a generator expression and xrange:

range_with_holes = (j for j in xrange(1, 31) if j != 6)

for i in range_with_holes:
    ....

In generator expression the if part can contain a complex logic on which numbers to skip.

Another way to combine iterables is to use the itertools.chain:

range_with_holes = itertools.chain(xrange(1, 6), xrange(7, 31))

Or just skip the unwanted index

for i in range(1, 31):
    if i == 6:
        continue

    ...