Split a file on any character in Python

I need to split a big text file on a certain character. I expect I am being thick about this, but split doesn’t quite do what I want because it includes the matching line, whereas I want to split right on the matching character.

My Python answer:

def readlines(filename, endings, chunksize=4096):
    """Returns a generator that splits on lines in a file with the given
    line-ending.
    """
    line = ''
    while True:
        buf = filename.read(chunksize)
        if not buf:
            yield line
            break

        line = line + buf

        while endings in line:
            idx = line.index(endings) + len(endings)
            yield line[:idx]
            line = line[idx:]

if __name__ == "__main__":
    import sys, os

    FORMFEED = chr(12) # ASCII 12
    basename = os.path.basename(sys.argv[1])
    for num, data in enumerate(readlines(open(sys.argv[1]), endings=FORMFEED)):
        filename = basename + '-' + str(num)
        open(filename, 'wb').write(data)

This is also useful when reading data exported from some old-fashioned Mac application like Filemaker 5 where the line-endings are ASCII 13 not ASCII 10.

This post was inspired by Lotus Notes version 8.5, which is so advanced that to save a message in a file on disk you have to export it as structured text. And if you want to save a whole bunch of messages as individual files you must forget that drag-and-drop was introduced with System 7, that would be too obvious.