问题描述

我正在从一本书中学习 Python，我遇到了这个例子:

I'm learning Python from a book, and I came across this example:

M = [[1,2,3],
     [4,5,6],
     [7,8,9]]

G = (sum(row) for row in M) # create a generator of row sums
next(G) # Run the iteration protocol

由于我是一个绝对的初学者，并且作者没有对示例或 next() 函数提供任何解释，所以我不明白代码在做什么.

Since I'm an absolute beginner, and the author hasn't provided any explanation of the example or the next() function, I don't understand what the code is doing.

推荐答案

表达式 (sum(row) for row in M) 创建了所谓的 generator.此生成器将为 M 中的每一行计算一次表达式 (sum(row)).但是，生成器还没有做任何事情，我们只是设置好了.

The expression (sum(row) for row in M) creates what's called a generator. This generator will evaluate the expression (sum(row)) once for each row in M. However, the generator doesn't do anything yet, we've just set it up.

语句next(G) 实际上运行 M 上的生成器.因此，如果您运行一次 next(G)，您将获得第一行的总和.如果你再次运行它，你会得到第二行的总和，以此类推.

The statement next(G) actually runs the generator on M. So, if you run next(G) once, you'll get the sum of the first row. If you run it again, you'll get the sum of the second row, and so on.

>>> M = [[1,2,3],
...      [4,5,6],
...      [7,8,9]]
>>> 
>>> G = (sum(row) for row in M) # create a generator of row sums
>>> next(G) # Run the iteration protocol
6
>>> next(G)
15
>>> next(G)
24

另见:

• 关于生成器的文档
• 关于 yield 表达式的文档(包含一些关于生成器的信息)

See also:

• Documentation on generators
• Documentation on yield expressions (with some info about generators)

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