Python-打印地图对象问题 [英] Python - Printing Map Object Issue

问题描述

我在玩地图对象，并发现如果事先执行list()，则该对象不会打印.当我只查看地图时，打印就可以了.为什么?

I was playing with the map object and noticed that it didn't print if I do list() beforehand. When I viewed only the map beforehand, the printing worked. Why?

推荐答案

地图返回一个迭代器，并且您只能使用一次迭代器.

map returns an iterator and you can consume an iterator only once.

示例:

>>> a=map(int,[1,2,3])
>>> a
<map object at 0x1022ceeb8>
>>> list(a)
[1, 2, 3]

>>> next(a)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
StopIteration

>>> list(a)
[]

另一个示例，我使用第一个元素并创建一个包含其余元素的列表

Another example where I consume the first element and create a list with the rest

>>> a=map(int,[1,2,3])
>>> next(a)
1 
>>> list(a)
[2, 3]

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