为什么在python中迭代一个字典时必须调用.iteritems（）？ [英] Why do you have to call .iteritems() when iterating over a dictionary in python?

问题描述

为什么要调用 iteritems（）来迭代字典中的键值？即

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

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

为什么不在字典中迭代的默认行为

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

将通过正确 - 不会你发现令人惊讶的是，如果 >（循环子句）与另一个（存在检查）有完全不同的意义？我肯定会！对于列表，集合，元组，...自然可以这样做。

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, ...

所以，当 C 是一个字典，如果中的在中为循环产生键/值元组，那么以最不惊讶的原则， 中的也必须在遏制检查中使用左手操作数的元组。

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.

如何有用会是吗如果C.get（key）== value if（key，value）在C 中是的同义词$ c> - 这是一个支票，我相信我可能已经执行了，或者想要执行，比C 如果在C 中的k实际意味着，检查密钥的存在，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，想要在密钥上循环很常见，例如：

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

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

有价值也不会有帮助：

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

实际上有些不太清楚，不太简洁。 （请注意，项目是使用正确方法获取键/值对的原始拼写：不幸的是，在这些访问者返回整个列表的日子里，所以要支持只是迭代，必须引入一个替代的拼写，而在Python 3中，这是一个与之前的Python版本的向后兼容性约束很大程度的削弱的 iteritems 它再次成为项目。

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).

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