python dict.update vs.下标添加单个键/值对 [英] python dict.update vs. subscript to add a single key/value pair

问题描述

每个学期，我至少有一个Python学生使用dict.update()添加单个键/值对，即:

Every semester I have at least one Python student who uses dict.update() to add a single key/value pair, viz.:

mydict.update({'newkey':'newvalue'})

代替

mydict['newkey'] = 'newvalue'

我没有教授这种方法，也不知道他们在哪里找到这种方法的示例，但是我告诉他们不要这样做，因为它效率较低(大概创建了一个新的2元素字典)，并且非标准.

I don't teach this method and I don't know where they're finding examples of this, but I tell them not to do it because it's less efficient (presumably creates a new 2-element dict) and because it's nonstandard.

老实说，我可以理解使用可见方法而不是使用这种语法的愿望-也许感觉与其他方法调用更加一致.但我认为这似乎是一种新手方法.

Honestly, I can understand the desire to use a visible method rather than this syntax - it perhaps feels more consistent with other method calls. But I think it looks like a newbie approach.

在这一点上，有没有人可以提供任何智慧?

Is there any wisdom anyone has to offer on this point?

推荐答案

基准测试表明您怀疑其性能影响是正确的:

A benchmark shows your suspicions of its performance impact appear to be correct:

$ python -m timeit -s 'd = {"key": "value"}' 'd["key"] = "value"'
10000000 loops, best of 3: 0.0741 usec per loop
$ python -m timeit -s 'd = {"key": "value"}' 'd.update(key="value")'
1000000 loops, best of 3: 0.294 usec per loop
$ python -m timeit -s 'd = {"key": "value"}' 'd.update({"key": "value"})'
1000000 loops, best of 3: 0.461 usec per loop

也就是说，它在我的机器上慢了大约六倍.但是，如果您需要最佳性能，那么Python已经不是您要使用的语言，因此，我只建议您使用这种情况下最易读的语言.在许多情况下，这是[]的方式，尽管update在这种情况下可能更具可读性:

That is, it's about six times slower on my machine. However, Python is already not a language you'd use if you need top performance, so I'd just recommend use of whatever is most readable in the situation. For many things, that would be the [] way, though update could be more readable in a situation like this:

configuration.update(
    timeout=60,
    host='example.com',
)

…或类似的东西.

…or something like that.

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