preserve type of newlines in textfile while doing something with it

preserve_newline_type.py

# open textfile
filename = 'text.txt'
with open(filename, 'rU') as f:
    lines = f.readlines()
    newline = f.newlines
    if not newline:
        raise IOError, "no newlines in %s, possible no textfile" % filename
    if isinstance(newline, tuple):
        raise IOError, "more than one type of newlines in %s" % filename

# now do something with lines ...

# save textfile while preserving original newlines
with open('text_processed.txt', 'wb') as f:
    f.write(newline.join(lines))