python2.7 - Python 2.7 stdout重定向的疑问

Question

先上代码

import sys


class TestWriter(object):
    def __init__(self, stream=sys.stdout):
        super(TestWriter, self).__init__()
        self.stream = stream

    def write(self, line):
        self.stream.write(line)

tmp = sys.stdout
f = open('d:\stdout.txt', 'w')
try:
    sys.stdout = f
    adpt = TestWriter() //如果这里我把f当参数传入，则执行结果如预期。
    adpt.write('asdfwe')  // 预期字符串写入文本，单事实上字符串输出到了屏幕。
    print 'this is import from print' //如预期的输入到了文本
except Exception, e:
    sys.stdout = tmp
    print e
finally:
    sys.stdout = tmp
    f.close()
print 'finish'

问题：就如我注释里写的，调用TestWriter.write()的时候没有实现sys.stdout的重定向输出，但之后的print证明了标准输出已经重定向到了文件f对象。
断点跟踪的时候，self.stream也显示为f对象
求解惑！！！

Answer 1

def __init__(self, stream=sys.stdout)

Python在创建每个函数时，每个参数都会被绑定，默认值不会随着值的改变而重新加载

# coding: utf-8

D = 2 
class Test:
    def __init__(self, a=D):
        print a


if __name__ == '__main__':
    D = 3 
    t = Test()
    print D

inner function:  2
outer function: 3

但如果绑定参数默认参数绑定的是地址，那就不一样，地址不变，内容可以变．

# coding: utf-8

D = [3] 
class Test:
    def __init__(self, a=D):
        print "inner function: ", a


if __name__ == '__main__':
    D[0] = 2
    t = Test()
    print "outer function:", D
   
inner function:  [2]
outer function: [2]

Answer 2

In contrast, in Python, execution begins at the top of one file and proceeds in a well-defined order through each statement in the file, ...

http://stackoverflow.com/ques...

python会顺序解释每条语句，所以TestWriter的构造器参数stdout没有被重定向。

以上都是我猜的

=====================================================================

import sys

class A:
    def __init__(self, stream=sys.stdout):
        print(stream)

f = open('test.txt', 'w')

a = A()

sys.stdout = f

print(sys.stdout)

运行结果