[Python-Dev] namespace for generator expressions
Guido van Rossum
guido at python.org
Mon Jan 26 23:11:10 EST 2004
> > A trivial example is:
> Are there non-trivial examples? The PEP suggests that they exist, but
> doesn't provide any.
> > iterators = 
> > for i in range(5):
> > iterators.append(x*2 for x in range(i))
> > print map(list,iterators)
> > If a listcomp is used, you get:
> > [,[0,],[0,2],[0,2,4],[0,2,4,6],[0,2,4,6,8]]
> > If genexprs do late binding, you get:
> > [[0,2,4,6,8],[0,2,4,6,8],[0,2,4,6,8],[0,2,4,6,8],[0,2,4,6,8]]
> Note that Armin Rigo suggested a small change here a few weeks ago that
> makes the list comp and the gen expr behave the same way.
I may be mistaken, but I had always assumed it should be the way Armin
> The range(i) part of the gen expr is evaluated at the point of
> definition. The gen expr, thus, translates to:
> def f(it):
> for x in it:
> yield x * 2
> As a result of this change, the only new scope is for the body of the
> target expression. I don't know how that effects the earlier examples.
There were definitely examples along the lines of using 'i' in the
> BTW is there good terminology for describing the parts of a list comp or
> gen expr? How about the iterator, the conditions, and the expression?
Expression is too generic; I suggest target expression.
--Guido van Rossum (home page: http://www.python.org/~guido/)
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