Python:使用地图功能 [英] Python : Using the map function

问题描述

我在使用 map 函数时遇到了麻烦.当我要打印创建的列表时，解释器显示指针:

I'm having trouble with the map function. When I want to print the created list, the interpreter shows the pointer:

>>> squares = map(lambda x: x**2, range(10))
>>> print(squares)
<map object at 0x0000000002A086A0>

出什么问题了?

推荐答案

问题是未创建列表. map 返回特定类型的生成器，Python 3不是列表(而是一个映射对象"，如您所见).你可以尝试

The problem is that a list is not created. map returns a specific type of generator in Python 3 that is not a list (but rather a 'map object', as you can see). You can try

print(list(squares))

或者只是使用列表理解来首先获取列表(无论如何在这里似乎效果更好):

Or just use a list comprehension to obtain a list in the first place (which seems to work better here anyway):

squares = [x**2 for x in range(10)]

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map 用于返回Python 2.x中的列表，并且

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map used to return a list in Python 2.x, and the change that was made in Python 3 is described in this section of the documentation:

• map()和 filter()返回迭代器.如果您确实需要列表，可以使用例如快速修复的方法. list(map(...))，但是更好的解决方案通常是使用列表理解功能(尤其是当原始代码使用 lambda 时)，或者重写代码，因此根本不需要列表.为函数的副作用而调用 map()尤其棘手；正确的转换是使用常规的 for 循环(因为创建列表只是浪费).

• map() and filter() return iterators. If you really need a list, a quick fix is e.g. list(map(...)), but a better fix is often to use a list comprehension (especially when the original code uses lambda), or rewriting the code so it doesn’t need a list at all. Particularly tricky is map() invoked for the side effects of the function; the correct transformation is to use a regular for loop (since creating a list would just be wasteful).

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