为什么 map 在 Python3中返回映射对象而不是列表？

我对了解 Python3.x 的新语言设计很感兴趣。

我很喜欢 Python 2.7中的函数 map:

Python 2.7.12
In[2]: map(lambda x: x+1, [1,2,3])
Out[2]: [2, 3, 4]

然而，在 Python 3. x 中，情况发生了变化:

Python 3.5.1
In[2]: map(lambda x: x+1, [1,2,3])
Out[2]: <map at 0x4218390>

我知道是怎么回事，但我找不到原因。为什么语言设计者要做出这样的选择，在我看来，这会带来很大的痛苦。这是为了和开发人员在列表理解方面进行“掰手腕”吗？

我的意思是，列表可以很自然地被认为是 函授员; 我也曾经被认为是这样想的:

fmap :: (a -> b) -> f a -> f b
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Because it returns an iterator, it omit storing the full size list in the memory. So that you can easily iterate over it in the future not making any pain to memory. Possibly you even don't need a full list, but the part of it, until your condition is reached.

You can find this docs useful, iterators are awesome.

An object representing a stream of data. Repeated calls to the iterator’s __next__() method (or passing it to the built-in function next()) return successive items in the stream. When no more data are available a StopIteration exception is raised instead. At this point, the iterator object is exhausted and any further calls to its __next__() method just raise StopIteration again. Iterators are required to have an __iter__() method that returns the iterator object itself so every iterator is also iterable and may be used in most places where other iterables are accepted. One notable exception is code which attempts multiple iteration passes. A container object (such as a list) produces a fresh new iterator each time you pass it to the iter() function or use it in a for loop. Attempting this with an iterator will just return the same exhausted iterator object used in the previous iteration pass, making it appear like an empty container.

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In Python 3 many functions (not just map but zip, range and others) return an iterator rather than the full list. You might want an iterator (e.g. to avoid holding the whole list in memory) or you might want a list (e.g. to be able to index).

However, I think the key reason for the change in Python 3 is that while it is trivial to convert an iterator to a list using list(some_iterator) the reverse equivalent iter(some_list) does not achieve the desired outcome because the full list has already been built and held in memory.

For example, in Python 3 list(range(n)) works just fine as there is little cost to building the range object and then converting it to a list. However, in Python 2 iter(range(n)) does not save any memory because the full list is constructed by range() before the iterator is built.

Therefore, in Python 2, separate functions are required to create an iterator rather than a list, such as imap for map (although they're not quite equivalent), xrange for range, izip for zip. By contrast Python 3 just requires a single function as a list() call creates the full list if required.

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最佳答案

I think the reason why map still exists at all when generator expressions also exist, is that it can take multiple iterator arguments that are all looped over and passed into the function:

>>> list(map(min, [1,2,3,4], [0,10,0,10]))
[0,2,0,4]

That's slightly easier than using zip:

>>> list(min(x, y) for x, y in zip([1,2,3,4], [0,10,0,10]))

Otherwise, it simply doesn't add anything over generator expressions.

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Guido answers this question here: "since creating a list would just be wasteful".

He also says that the correct transformation is to use a regular for loop.

Converting map() from 2 to 3 might not just be a simple case of sticking a list( ) around it. Guido also says:

"If the input sequences are not of equal length, map() will stop at the termination of the shortest of the sequences. For full compatibility with map() from Python 2.x, also wrap the sequences in itertools.zip_longest(), e.g.

map(func, *sequences)

becomes 

list(map(func, itertools.zip_longest(*sequences)))

"


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