各位说说下面的程序应该输出什么？
class A:
        def __init__(self,m={}):
                self.x=m

while True:
        a=A()
        print a.x
        a.x['1']=2

-- 
devdoer
devdoer at gmail.com
http://devdoer.blog.sohu.com/
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第一行是空字典，后面就不是空的了

就是告诉你函数参数的缺省值，不是每次都重新生成新对象，而是一直用一个

在 07-3-30，bird devdoer<devdoer at gmail.com> 写道：
> 各位说说下面的程序应该输出什么？
> class A:
>         def __init__(self,m={}):
>                 self.x=m
>
> while True:
>         a=A()
>         print a.x
>         a.x['1']=2
>
> --
> devdoer
>  devdoer at gmail.com
> http://devdoer.blog.sohu.com/
> _______________________________________________
> python-chinese
> Post: send python-chinese at lists.python.cn
> Subscribe: send subscribe to
> python-chinese-request at lists.python.cn
> Unsubscribe: send unsubscribe to
> python-chinese-request at lists.python.cn
> Detail Info:
> http://python.cn/mailman/listinfo/python-chinese
>

函数的参数问题
在 Python Tutorial 有这样一段话


Important warning: The default value is evaluated only once. This
makes a difference when the default is a mutable object such as a
list, dictionary, or instances of most classes. For example, the
following function accumulates the arguments passed to it on
subsequent calls:


def f(a, L=[]):
    L.append(a)
    return L

print f(1)
print f(2)
print f(3)

This will print


[1]
[1, 2]
[1, 2, 3]

If you don't want the default to be shared between subsequent calls,
you can write the function like this instead:


def f(a, L=None):
    if L is None:
        L = []
    L.append(a)
    return L



On 3/30/07, bird devdoer <devdoer at gmail.com> wrote:
> 各位说说下面的程序应该输出什么？
> class A:
>         def __init__(self,m={}):
>                 self.x=m
>
> while True:
>         a=A()
>         print a.x
>         a.x['1']=2
>

很特别，跟直观的理解很不一致。
一般的理解都是创建a对象时，缺省参数是个空字典对象。但是现在看来，缺省的字典对象只是构造一次。

在07-3-30，helium <helium.sun at gmail.com> 写道：
>
> 第一行是空字典，后面就不是空的了
>
> 就是告诉你函数参数的缺省值，不是每次都重新生成新对象，而是一直用一个
>
> 在 07-3-30，bird devdoer<devdoer at gmail.com> 写道：
> > 各位说说下面的程序应该输出什么？
> > class A:
> >         def __init__(self,m={}):
> >                 self.x=m
> >
> > while True:
> >         a=A()
> >         print a.x
> >         a.x['1']=2
> >
> > --
> > devdoer
> >  devdoer at gmail.com
> > http://devdoer.blog.sohu.com/
> > _______________________________________________
> > python-chinese
> > Post: send python-chinese at lists.python.cn
> > Subscribe: send subscribe to
> > python-chinese-request at lists.python.cn
> > Unsubscribe: send unsubscribe to
> > python-chinese-request at lists.python.cn
> > Detail Info:
> > http://python.cn/mailman/listinfo/python-chinese
> >
> _______________________________________________
> python-chinese
> Post: send python-chinese at lists.python.cn
> Subscribe: send subscribe to python-chinese-request at lists.python.cn
> Unsubscribe: send unsubscribe to  python-chinese-request at lists.python.cn
> Detail Info: http://python.cn/mailman/listinfo/python-chinese




-- 
devdoer
devdoer at gmail.com
http://devdoer.blog.sohu.com/
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多谢给出资料，以后用缺省参数时需要特别注意了。

2007/3/30, lubiao <lubiao.py at gmail.com>:
>
> 函数的参数问题
> 在 Python Tutorial 有这样一段话
>
>
> Important warning: The default value is evaluated only once. This
> makes a difference when the default is a mutable object such as a
> list, dictionary, or instances of most classes. For example, the
> following function accumulates the arguments passed to it on
> subsequent calls:
>
>
> def f(a, L=[]):
>     L.append(a)
>     return L
>
> print f(1)
> print f(2)
> print f(3)
>
> This will print
>
>
> [1]
> [1, 2]
> [1, 2, 3]
>
> If you don't want the default to be shared between subsequent calls,
> you can write the function like this instead:
>
>
> def f(a, L=None):
>     if L is None:
>         L = []
>     L.append(a)
>     return L
>
>
>
> On 3/30/07, bird devdoer <devdoer at gmail.com> wrote:
> > 各位说说下面的程序应该输出什么？
> > class A:
> >         def __init__(self,m={}):
> >                 self.x=m
> >
> > while True:
> >         a=A()
> >         print a.x
> >         a.x['1']=2
> >
> _______________________________________________
> python-chinese
> Post: send python-chinese at lists.python.cn
> Subscribe: send subscribe to python-chinese-request at lists.python.cn
> Unsubscribe: send unsubscribe to  python-chinese-request at lists.python.cn
> Detail Info: http://python.cn/mailman/listinfo/python-chinese




-- 
devdoer
devdoer at gmail.com
http://devdoer.blog.sohu.com/
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ÔÚ07-3-30£¬bird devdoer <devdoer在gmail.com> дµÀ£º
>
> ¸÷λ˵˵ÏÂÃæµÄ³ÌÐòÓ¦¸ÃÊä³öʲô£¿
> class A:
>         def __init__(self,m={}):
>                 self.x=m
>
> while True:
>         a=A()
>         print a.x
>         a.x['1']=2
>
> --
> devdoer
> devdoer在gmail.com
> http://devdoer.blog.sohu.com/
> _______________________________________________
> python-chinese
> Post: send python-chinese在lists.python.cn
> Subscribe: send subscribe to python-chinese-request在lists.python.cn
> Unsubscribe: send unsubscribe to  python-chinese-request在lists.python.cn
> Detail Info: http://python.cn/mailman/listinfo/python-chinese
>
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