問題描述

先上代碼

import sysclass 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.stdoutf = open(’d:stdout.txt’, ’w’)try: sys.stdout = f adpt = TestWriter() //如果這里我把f當(dāng)參數(shù)傳入，則執(zhí)行結(jié)果如預(yù)期。 adpt.write(’asdfwe’) // 預(yù)期字符串寫入文本，單事實(shí)上字符串輸出到了屏幕。 print ’this is import from print’ //如預(yù)期的輸入到了文本except Exception, e: sys.stdout = tmp print efinally: sys.stdout = tmp f.close()print ’finish’

問題：就如我注釋里寫的，調(diào)用TestWriter.write()的時候沒有實(shí)現(xiàn)sys.stdout的重定向輸出，但之后的print證明了標(biāo)準(zhǔn)輸出已經(jīng)重定向到了文件f對象。斷點(diǎn)跟蹤的時候，self.stream也顯示為f對象求解惑！！！

問題解答

def __init__(self, stream=sys.stdout)

Python在創(chuàng)建每個函數(shù)時，每個參數(shù)都會被綁定，默認(rèn)值不會隨著值的改變而重新加載

# coding: utf-8D = 2 class Test: def __init__(self, a=D):print aif __name__ == ’__main__’: D = 3 t = Test() print Dinner function: 2outer function: 3

但如果綁定參數(shù)默認(rèn)參數(shù)綁定的是地址，那就不一樣，地址不變，內(nèi)容可以變．

# coding: utf-8D = [3] class Test: def __init__(self, a=D):print 'inner function: ', aif __name__ == ’__main__’: D[0] = 2 t = Test() print 'outer function:', D inner function: [2]outer function: [2]回答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會順序解釋每條語句，所以TestWriter的構(gòu)造器參數(shù)stdout沒有被重定向。

以上都是我猜的

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import sysclass A: def __init__(self, stream=sys.stdout):print(stream)f = open(’test.txt’, ’w’)a = A()sys.stdout = fprint(sys.stdout)

運(yùn)行結(jié)果