对于可变函数参数的默认值好用途？ [英] Good uses for mutable function argument default values?

问题描述

这是一个常见的​​错误是设置一个可变对象作为函数参数的默认值。下面是来自这个优秀的写了由David Goodger ：

It is a common mistake to set a mutable object as the default value of an argument in a function. Here's an example taken from this excellent write-up by David Goodger:

>>> def bad_append(new_item, a_list=[]):
        a_list.append(new_item)
        return a_list
>>> print bad_append('one')
['one']
>>> print bad_append('two')
['one', 'two']

为什么出现这种情况的解释是<一个href=\"http://python.net/~goodger/projects/pycon/2007/idiomatic/handout.html#default-parameter-values\">here.

而现在我的问题：是否有一个良好的用例此语法结果我的意思是，如果大家谁遇到这让同样的错误，调试它，理解问题和？从上面设法避免它，有什么用是有这样的语法？

And now for my question: Is there a good use-case for this syntax?
I mean, if everybody who encounters it makes the same mistake, debugs it, understands the issue and from thereon tries to avoid it, what use is there for such syntax?

推荐答案

您可以与函数调用使用它的缓存值：

You can use it to cache values between function calls:

def get_from_cache(name, cache={}):
    if name in cache: return name
    cache[name] = result = expensive_calculation()
    return result

但通常那种事情做一个更好的类，你就可以有额外的属性来清除缓存等。

but usually that sort of thing is done better with a class as you can then have additional attributes to clear the cache etc.

这篇关于对于可变函数参数的默认值好用途？的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！