python2.7 - Questions about Python 2.7 stdout redirection

Question

Code first

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'

Problem: As I wrote in the comments, the redirected output of sys.stdout was not implemented when calling TestWriter.write(), but the subsequent print proved that the standard output has been redirected to the file f object.
When tracking breakpoints, self.stream is also displayed as an f object
Solve the doubts! ! !

Answer 1

def __init__(self, stream=sys.stdout)

When Python creates each function, each parameter will be bound, and the default value will not be reloaded as the value changes

# 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

But if the default parameter of the binding parameter is bound to the address, it is different. The address remains unchanged, but the content can change.

# 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 interprets each statement sequentially, so TestWriter的构造器参数stdout is not redirected.

The above are all my guesses

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

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)

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