枚举的实现细节是什么? [英] What is the implementation detail for enumerate?

问题描述

Python具有enumerate()来遍历带有索引的对象.我怀疑解释器是否仅出于跟踪事物所在的目的而创建了许多int对象. PEP页面表示以下内容，但我不太了解发生了什么在引擎盖下:

Python has enumerate() to iterate over objects with an index.I doubt that interpreters create a lot of int objects for the sole purpose of keeping track of where things are. The PEP page says the following, but I do not really understand what is going on under the hood:

它为所有可迭代对象提供与iteritems()提供给字典相同的优点-紧凑，可读，可靠的索引符号.

It provides all iterables with the same advantage that iteritems() affords to dictionaries -- a compact, readable, reliable index notation.

那这里的魔力是什么?

推荐答案

enumerate()是迭代器；它只会动态产生索引int值 ；它并不能完全产生它们.

enumerate() is an iterator; it only produces the index int value on the fly; it does not produce them all up front.

您可以尝试阅读 enumobject.c源代码，但基本上可以像这样将其翻译成Python:

You can try to read the enumobject.c source code, but it basically can be translated to Python like this:

def enumerate(iterable, start=0):
    count = start
    for elem in iterable:
        yield count, elem
        count += 1

yield关键字使它成为生成器函数 ，您需要遍历生成器(或在生成器上调用next())以使该函数前进以生成数据，一次调用一次yield.

The yield keyword makes this a generator function, and you need to loop over the generator (or call next() on it) to advance the function to produce data, one yield call at a time.

Python还会实习int值，介于-5和256(含)之间的所有值都是单例，因此上面的代码直到达到257时才产生新的int对象.

Python also interns int values, all values between -5 and 256 (inclusive) are singletons, so the above code doesn't even produce new int objects until you reach 257.

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