def read_file(filename):
    """
    Return the contents of the file with the given filename as a string

    >>> write_file('read_write_file.txt', 'Hello World')
    >>> read_file('read_write_file.txt')
    'Hello World'
    >>> os.unlink('read_write_file.txt')
    """
    with open(filename) as in_fh:
        return in_fh.read()