Wrap sed in Python to provide ed-style line editing in a simple function call.

editor.py

"Wrapper to provide ed-style line editing."
# Ron DuPlain <ron.duplain@gmail.com>

from subprocess import Popen, PIPE


def edit(text, command):
    r"""Edit given text using ed-style line editing, using system's sed command.

    Examples:
    >>> edit('Hello, world!', 's/world/friends/')
    u'Hello, friends!'
    >>> edit('Hello, world!', 'bad_input')
    Traceback (most recent call last):
       ...
    EditorException: sed: can't find label for jump to `ad_input'
    >>>

    Boring empty cases are handled gracefully:
    >>> edit('Hello, world!', '')
    u'Hello, world!'
    >>> edit('', 's/world/friends/')
    u''
    >>> edit('', '')
    u''
    >>>

    This is a full ed implementation:
    >>> edit('Hello hello, world! HELLO!', 's/hello/greetings/gi')
    u'greetings greetings, world! greetings!'
    >>> edit('Hello, world!', r's/\(.*\)/You said, "\1"/')
    u'You said, "Hello, world!"'
    >>>

    A note on backslash escaping for sed, remember that Python has it's own
    escaping. For example, "backslash one" must either be \\1 or provided in a
    raw string, r'\1'.

    >>> edit('Hello, world!', 's/\\(.*\\)/\\U\\1/')
    u'HELLO, WORLD!'
    >>> edit('Hello, world!', r's/\(.*\)/\U\1/')
    u'HELLO, WORLD!'
    >>>
    """
    output = sed_wrapper(text, command)
    if not output:
        return u''
    return unicode(output)


class EditorException(RuntimeError):
    "An error occured while processing the editor command."


def sed_wrapper(text, command):
    """Wrap sed to provide full ed-style line-editing.

    Being the stream editor, open sed in a subprocess and communicate with it
    using stdio, raising a sensible exception if the command call does not
    succeed.

    Note this wrapper deals with shell injection attempts:
    >>> sed_wrapper('Hello, world!', 's/world/friends/; ls')
    Traceback (most recent call last):
       ...
    EditorException: sed: -e expression #1, char 20: extra characters after command
    >>> sed_wrapper('Hello, world!', '; ls')
    Traceback (most recent call last):
       ...
    EditorException: sed: -e expression #1, char 4: extra characters after command
    >>>
    """
    arguments = ['sed', command]
    sed = Popen(arguments, stdin=PIPE, stdout=PIPE, stderr=PIPE)
    sed.stdin.write(text)
    sed.stdin.close()
    returncode = sed.wait()
    if returncode != 0:
        # Unix integer returncode, where 0 is success.
        raise EditorException(sed.stderr.read().strip())
    return sed.stdout.read()


if __name__ == '__main__':
    # Test with `pyflakes editor.py && python editor.py`.
    # If you do not have pyflakes, get it! `pip install pyflakes`
    # See output for errors. No output means that all tests pass.
    import doctest
    doctest.testmod()