Python doctest 结果死活写不到文件里面去

系统:Ubuntu
python:2.7.6
我看别人都能写到文件里面, 我只能输出到终端中，难道我的打开方式有问题
这个是我看的官方实例，也不行

def factorial(n):
    """Return the factorial of n, an exact integer >= 0.

    If the result is small enough to fit in an int, return an int.
    Else return a long.

   >>> # comments are ignored
    >>> # comments are ignored
    >>> x = 12
    >>> x
    
    >>> if x == 13:
    ...     print "yes"
    ... else:
    ...     print "no"
    ...     print "NO"
    ...     print "NO!!!"
    ...


    """

    import math
    if not n >= 0:
        raise ValueError("n must be >= 0")
    if math.floor(n) != n:
        raise ValueError("n must be exact integer")
    if n+1 == n:  # catch a value like 1e300
        raise OverflowError("n too large")
    result = 1
    factor = 2
    while factor <= n:
        result *= factor
        factor += 1
    return result


if __name__ == "__main__":
    import doctest
    doctest.testmod()