Python：将地图对象转换为列表会使地图对象为空？ [英] Python: casting map object to list makes map object empty?

问题描述

我有一个地图对象，我想将其打印为列表，但此后继续用作地图对象。其实我想打印长度以便转换为列表，但是如果我仅按如下方式打印内容，也会出现此问题：

I have a map object that I want to print as a list but continue using as a map object afterwards. Actually I want to print the length so I cast to list but the issue also happens if I just print the contents as follows:

print("1",my_map)
print("1",list(my_map))
print("2",my_map)
print("2",list(my_map))

这给了我以下输出。

1 <map object at 0x7fd2962a75f8>
1 [(1000.0, 1.0, 0.01, 0.01, 0.01, 0.01, 0.01)]
2 <map object at 0x7fd2962a75f8>
2 []

为什么会发生这种情况，以及如何避免继续使用地图及其内容？

Why is this happening and how can I avoid it to continue using the map and its contents?

推荐答案

map 对象是返回的生成器调用 map（） 内置功能。它打算仅迭代一次（例如，将其传递到 list（）），然后再使用。尝试对其进行第二次迭代将导致序列为空。

A map object is a generator returned from calling the map() built-in function. It is intended to be iterated over (e.g. by passing it to list()) only once, after which it is consumed. Trying to iterate over it a second time will result in an empty sequence.

如果要保存映射的值以供重用，则需要转换 map 对象映射到另一个序列类型，例如 list ，然后保存结果。因此，请更改您的内容：

If you want to save the mapped values to reuse, you'll need to convert the map object to another sequence type, such as a list, and save the result. So change your:

my_map = map(...)

至

my_map = list(map(...))

之后，上面的代码应该可以按预期工作。

After that, your code above should work as you expect.

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