Split lines of a text file by a rule (delimiter) and add the delimiter either before or after matching chunks of text.

gen_groups.py

def gen_groups(iterable, test, to_beginning=True):
    group = []
    for line in iterable:
        # If the line is a delimiter
        if test(line):
            # and you want delimiters to start groups
            if to_beginning:
                # And there is already a group that has 
                # been accumulating non-delimiters
                if group:
                    # Yield the group
                    yield group 
                # Set group to a list of just the current line
                # since to_beginning is true and the line is
                # a delimiter. Now ready to accumulate more lines.
                group = [line]
            # Line is a delimiter, but we want to put delimiters
            # at the end of the group
            else:
                # Empty list + non-empty list = non-empty list, or
                # return existing group with delimiter tagged onto
                # the end
                yield group + [line]
                # Reset group for next run
                group = []
        # Line is not a delimiter...
        else:
            # ...so add it to the previous group
            group.append(line)
    # Yield anything left that didn't get yielded before.
    if group:
        yield group