可变函数参数默认值的好用途?
问题描述
将可变对象设置为函数中参数的默认值是 Python 中的常见错误.下面是一个示例，取自 David Goodger 的这篇出色的文章:
It is a common mistake in Python 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']
发生这种情况的原因是这里.
The explanation why this happens is here.
现在我的问题是:这种语法有很好的用例吗?
我的意思是，如果遇到它的每个人都犯了同样的错误，调试它，理解问题并从那里尝试避免它，那么这种语法有什么用?
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?
解决方案
你可以使用它来缓存函数调用之间的值:
You can use it to cache values between function calls:
def get_from_cache(name, cache={}):
if name in cache: return cache[name]
cache[name] = result = expensive_calculation()
return result
但通常这类事情用类做得更好，因为你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.
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