为什么Python中禁止对字符串使用sum函数？

At first glance, it makes total sense to assume any object with an __add__ method should work with sum()—and strings do implement __add__ for concatenation:

"abc" + "def"  # Returns "abcdef"

But here's the key detail that trips things up: sum() uses a default starting value of 0 (an integer). You can confirm this with quick tests:

sum([])       # Output: 0
sum([], 7)    # Output: 7

So when you run sum(["abc", "def"]), Python is actually doing this under the hood:

1. Start with the default value 0
2. Try to add the first string: 0 + "abc"

That's exactly why you get the error:

sum(["abc", "def"])  # Throws TypeError: unsupported operand type(s) for +: 'int' and 'str'

For what it's worth, you can force sum() to work for strings by passing an empty string as the starting value:

sum(["abc", "def"], "")  # Returns "abcdef"

But a quick heads-up—don’t use this in real code. str.join() is purpose-built for string concatenation and is way more efficient, especially with large lists.

内容的提问来源于stack exchange，提问作者patapouf_ai