为什么在Python中遍历字典时必须调用.items()? [英] Why do you have to call .items() when iterating over a dictionary in Python?

问题描述

为什么必须调用 items() 进行迭代超过键，字典中的值对?即.

Why do you have to call items() to iterate over key, value pairs in a dictionary? ie.

dic = {'one': '1', 'two': '2'}
for k, v in dic.items():
    print(k, v)

为什么不是在字典上进行迭代的默认行为

Why isn't that the default behavior of iterating over a dictionary

for k, v in dic:
    print(k, v)

推荐答案

对于每个python容器C，期望是

For every python container C, the expectation is that

for item in C:
    assert item in C

会顺利通过-如果in(循环子句)的一种含义与另一种(存在检查)的含义完全不同，您不会感到惊讶吗?我一定会的！它自然适用于列表，集合，元组，...

will pass just fine -- wouldn't you find it astonishing if one sense of in (the loop clause) had a completely different meaning from the other (the presence check)? I sure would! It naturally works that way for lists, sets, tuples, ...

因此，当C是字典时，如果in要在for循环中生成键/值元组，那么根据最小惊讶的原理，in也必须采用这样的形式元组作为其在包含检查中的左侧操作数.

So, when C is a dictionary, if in were to yield key/value tuples in a for loop, then, by the principle of least astonishment, in would also have to take such a tuple as its left-hand operand in the containment check.

那会有用吗?实际上，这实际上是没有用的，基本上使if (key, value) in C成为if C.get(key) == value的代名词–我认为这是我执行或想要执行的检查的次数，比if k in C实际的意思少100倍. >，检查键 only 的存在并完全忽略该值.

How useful would that be? Pretty useless indeed, basically making if (key, value) in C a synonym for if C.get(key) == value -- which is a check I believe I may have performed, or wanted to perform, 100 times more rarely than what if k in C actually means, checking the presence of the key only and completely ignoring the value.

另一方面，只想在键上循环是很常见的，例如:

On the other hand, wanting to loop just on keys is quite common, e.g.:

for k in thedict:
    thedict[k] += 1

同时具有该值也无济于事:

having the value as well would not help particularly:

for k, v in thedict.items():
    thedict[k] = v + 1

实际上不太清晰和简洁. (请注意，items是用于获取键/值对的正确"方法的原始拼写:不幸的是，这是在此类访问器返回整个列表的时代，因此，为了支持只是迭代"，需要使用另一种拼写引入iteritems就是-在Python 3中，与以前的Python版本的向后兼容性约束大大减弱，它又变成了items.

actually somewhat less clear and less concise. (Note that items was the original spelling of the "proper" methods to use to get key/value pairs: unfortunately that was back in the days when such accessors returned whole lists, so to support "just iterating" an alternative spelling had to be introduced, and iteritems it was -- in Python 3, where backwards compatibility constraints with previous Python versions were much weakened, it became items again).

这篇关于为什么在Python中遍历字典时必须调用.items()?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！