Python how to handle split when delimiter not present?

First, put the result of the split in a list:

split_build_descriptor = buildDescriptor.split("#")

Then check how many elements it has:

if len(split_build_descriptor) == 1:
    buildfile = split_build_descriptor[0]
    target = ''
elif len(split_build_descriptor) == 2:
    buildfile, target = split_build_descriptor
else:
    pass  # handle error; there's two #s

I'd use the obvious approach:

    buildfile, target = buildDescriptor.split("#") if \
                        "#" in buildDescriptor else \
                        (buildDescriptor, "")

Note that this will also throw an Exception when there is more than one "#" in buildDescriptor (which is generally a GOOD thing!)

>>> buildfile, _, target = "hello#world".partition("#")
>>> buildfile, target
('hello', 'world')
>>> buildfile, _, target = "hello".partition("#")
>>> buildfile, target
('hello', '')

You can do this in Python 3:

input_string = 'this is a test'
delimiter = '#'

slots = input_string.split(delimiter)
if slots[0] == input_string:
  print('no %s found' % delimiter)
else:
  print('%s found right after \"%s\"' % (delimiter, slots[0]))