我正在阅读 python cookbook 2nd ,一个样本在&#34; chap 2.13使用C ++ - 就像iostream语法“,我试着理解< em> self 在代码中工作。
class IOManipulator(object):
def __init__(self, function=None):
self.function = function
def do(self, output):
self.function(output)
def do_endl(stream):
stream.output.write('\n')
stream.output.flush()
endl = IOManipulator(do_endl)
# my understanding, all above is about make a newline and flush sys.stdout,
class OStream(object):
def __init__(self, output=None):
if output is None:
import sys
output = sys.stdout
self.output = output
self.format = '%s'
def __lshift__(self, thing):
if isinstance(thing, IOManipulator):
thing.do(self)
# It make no sense to me, why the function belongs to
# another class's instance need call (self)
else:
self.output.write(self.format % thing)
self.format = '%s' # <- is it necessary? seems to be not.
return self # <- why return a "self" here?
# If comment this line out,
# python raise a TypeError when execute to the "<< 1"
# TypeError: unsupported operand type(s) for <<: 'NoneType' and 'int'
def example_main():
cout = OStream()
cout << "The average of " << 1 << " and " << 3 << " is " << (1+3)/2 << endl
if __name__ == '__main__':
example_main()
# emits:
#> The average of 1 and 3 is 2
&#34>自我&#34;是<__main__.OStream object at 0x7fc28cd92410>
,我知道它是OStream类的一个实例,也许可以作为C指针。
答案 0 :(得分:2)
我会回答你在评论中提出的问题:
if isinstance(thing, IOManipulator):
thing.do(self)
# It make no sense to me, why the function belongs to
# another class's instance need call (self)
您将self
(输出流)传递给thing
(这将是IOManipulator
,例如endl
操作),以便IOManipulator
class可以在输出流上执行函数(参见下面的IOManipulator.do
)。
def do(self, output):
self.function(output)
对于最大的混淆,此代码段中的第一个self
不是您在self
中传递给它的OStream
!您传递给self
的{{1}}设置为thing.do
变量。
output
您可以在此处返回return self # <- why return a "self" here?
实例,以便进行连锁操作。请注意,python将行OStream
解析为a << b << c
。 (a << b) << c
部分需要返回其更新后的自我,以便能够对其进行(a << b)
并且有意义。如果您对<< c
发表评论,最终会得到return self
,因为该函数会返回None << c
。
None
我不确定作者对此的意图,似乎没必要。行self.format
也可以写为self.output.write(self.format % thing)
。
作为旁注:这可能是一个如何实现新运算符的示例,但是这个特定的运算符非常非pythonic:它会导致非常丑陋和令人困惑的代码。在现实世界中,尝试使用语言已有的功能。