python sum 函数 - 需要`start`参数解释

问题描述

我正在尝试了解内置 sum() 函数的工作原理，但是，start 参数让我失去了理智:

I am trying to understand the working of the built-in sum() function, but, the start parameter has evaporated my mind:

a=[[1, 20], [2, 3]]
b=[[[[[[1], 2], 3], 4], 5], 6]
>>> sum(b,a)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: can only concatenate list (not "int") to list
>>> sum(a,b)
[[[[[[1], 2], 3], 4], 5], 6, 1, 20, 2, 3]

>>> a=[1,2]
>>> b=[3,4]
>>> sum(a,b)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: can only concatenate list (not "int") to list
>>> sum(b,a)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: can only concatenate list (not "int") to list

我对此感到目瞪口呆，不知道发生了什么.以下是 python 文档的内容:http://docs.python.org/library/functions.html#sum.这并没有给出关于如果开始不是字符串而不是整数怎么办?"的任何解释

I am just dumbfounded by this and don't have any idea what is happening. Here is what the python docs have to say: http://docs.python.org/library/functions.html#sum. This does not give any explanation on 'what if the start is not a string and not an integer?'

推荐答案

Sum 做了这样的事情

Sum does something like this

def sum(values, start = 0):
    total = start
    for value in values:
        total = total + value
    return total

sum([1,2],[3,4]) 扩展了类似 [3,4] + 1 + 2 的内容，您可以看到它试图将数字和列表相加.

sum([1,2],[3,4]) expands something like [3,4] + 1 + 2, which you can see tries to add numbers and lists together.

为了使用 sum 生成列表，值应该是列表的列表，而 start 可以只是一个列表.您会在失败的示例中看到该列表至少包含一些整数，而不是所有列表.

In order to use sum to produce lists, the values should be a list of lists, whereas start can be just a list. You'll see in your failing examples that the list contains at least some ints, rather then all lists.

您可能会想到将 sum 与列表一起使用的通常情况是将列表列表转换为列表

The usual case where you might think of using sum with lists is to convert a list of lists into a list

sum([[1,2],[3,4]], []) == [1,2,3,4]

但实际上你不应该这样做，因为它会很慢.

But really you shouldn't do that, as it'll be slow.

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