Python惰性迭代器

Question

我试图了解如何以及何时对迭代器表达式求值。以下似乎是一个懒惰的表达:

g = (i for i in range(1000) if i % 3 == i % 2)

这个，但是在构造上失败了:

g = (line.strip() for line in open('xxx', 'r') if len(line) > 10)

我没有名为“xxx”的文件。不过，既然这东西懒，为什么这么快就失败了？

谢谢。

EDI:哇，我偷懒了!

g = (line.strip() for i in range(3) for line in open(str(i), 'r'))

Answer 1

来自documentation :

Variables used in the generator expression are evaluated lazily in a separate scope when the next() method is called for the generator object (in the same fashion as for normal generators). However, the in expression of the leftmost for clause is immediately evaluated in the current scope so that an error produced by it can be seen before any other possible error in the code that handles the generator expression.