A generator based file line reader, optimised for reading in order, with a list like interface.

filereader.py

def get_generator(fh):
    while True:
        data = fh.readline()
        if not data:
            break
        yield data

class GenFileReader(object):
    def __init__(self, fn):
        self._fn = fn
        self._fh = open(self._fn)
        self._rewind()

    def _rewind(self):
        self._fh.seek(0)
        self._generator = get_generator(self._fh)
        self._index = 0

    def _getitem_slice(self, slice):
        start, stop, step = slice.start, slice.stop, slice.step
        start = 0 if start is None else start
        stop = float("inf") if stop is None else stop
        step = 1 if step is None else step
        if stop is not None and stop < start:
            start, stop = stop, start
            step *= -1
        r = []
        ii = start
        while ii <= stop:
            try:
                r.append(self[ii])
                ii += step
            except IndexError:
                break
        return r

    def _getitem_int(self, index):
        if index < 0:
            raise IndexError("Negative index used")
        if index == 0:
            self._rewind()
        if index < self._index:
            self._rewind()
        data = ""
        while self._index <= index:
            try:
                data = next(self._generator)
                self._index += 1
            except StopIteration:
                raise IndexError
        return data.strip()

    def __getitem__(self, index):
        if isinstance(index, slice):
            return self._getitem_slice(index)
        elif isinstance(index, int):
            return self._getitem_int(index)

    def __del__(self):
        self._fh.close()