[Python-Dev] PEP 479: Change StopIteration handling inside generators (original) (raw)

Terry Reedy tjreedy at udel.edu
Sun Nov 23 11:08:37 CET 2014

On 11/22/2014 5:23 PM, Chris Angelico wrote:

On Sun, Nov 23, 2014 at 8:03 AM, Ron Adam <ron3200 at gmail.com> wrote:

Making comprehensions work more like generator expressions would, IMO, imply making the same change to all for loops: having a StopIteration raised by the body of the loop quietly terminate the loop.

I'm not suggesting making any changes to generator expressions or for loops at all. They would continue to work like they currently do. But if you're suggesting making list comps react to StopIteration raised by their primary expressions, then to maintain the correspondence between a list comp and its expanded form, for loops would have to change too. Or should that correspondence be broken, in that single-expression loop constructs become semantically different from statement loop constructs?

The 2.x correspondence between list comp and for loops was already broken in 3.0 by moving execution to a new function to prevent name binding in comprehension from leaking the way they did in 2.x. We did not change for loops to match.

The following, which is an example of equivalence in 2.7, raises in 3.4 because 'itertools' is not defined.

def binder(ns, name, ob): ns[name] = ob

for mod in ['sys']: binder(locals(), mod, import(mod)) print(sys)

[binder(locals(), mod, import(mod)) for mod in ['itertools']] print(itertools)

Ceasing to leak any local bindings in 3.0 broke equivalence and code depending on such bindings. Ceasing to leak StopIteration would break equivalence a bit more, but only a bit, and code dependent on such leakage, which I expect is extremely rare.

-- Terry Jan Reedy

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