python2.7 - Python 2.7 stdout重定向的疑問

Question

先上程式碼

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'

問題：就如我註解裡寫的，呼叫TestWriter.write()的時候沒有實​​作sys.stdout的重定向輸出，但之後的print證明了標準輸出已經重定向到了檔案f物件。
斷點追蹤的時候，self.stream也顯示為f物件
求解惑！ ！ ！

###

Answer 1

def __init__(self, stream=sys.stdout)

Python在建立每個函數時，每個參數都會被綁定，預設值不會隨著值的改變而重新載入

# 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

但如果綁定參數預設參數綁定的是位址，那就不一樣，位址不變，內容可以變．

# 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會順序解釋每個語句，所以TestWriter的构造器参数stdout沒有被重定向。

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)

運行結果