def guess_lineno(file):
    """Guess current line number in a file.

    Guessing is done in a very crude way - scanning file from beginning
    until current offset and counting newlines. Only meant to be used in
    exceptional cases - generating line number for error message.
    """
    offset = file.tell()
    file.seek(0)
    startpos = 0
    lineno = 1
    while True:
        line = file.readline()
        if not line:
            break
        endpos = file.tell()
        if startpos <= offset < endpos:
            break
        lineno += 1
    file.seek(offset)
    return lineno
