Question

I am trying to understand how and when iterator expressions get evaluated. The following seems to be a lazy expression:

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

This one, however fails on construction:

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

I do not have the file named ‘xxx’. However, since this thing is lazy, why is it failing so soon?

Thanks.

EDI: Wow, I made a lazy one!

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

Answer 1

From the 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.