(I'm catching up on this thread from the end.)
On Sun, Nov 16, 2014 at 5:29 PM, Chris Angelico email@example.com wrote:
On Mon, Nov 17, 2014 at 11:03 AM, Steven D'Aprano firstname.lastname@example.org wrote:
On Mon, Nov 17, 2014 at 11:05:01AM +1300, Greg Ewing wrote:
Chris Angelico wrote:
if you call on someone else's generator, and that someone hasn't applied the __future__ directive, you'll be in the current situation of not being able to distinguish 'return' from 'raise StopIteration'. But for your own generators, you can guarantee that they're distinct.
This suggests that the use of a __future__ directive is not really appropriate, since it can affect code outside of the module with the __future__ directive in it.
I don't see how that is different from any other __future__ directive. They are all per-module, and if you gain access to an object from another module, it will behave as specified in the module that created it, not the module that imported it. How is this different?
Well, let's see. For feature in sorted(__future__.all_feature_names):
absolute_import: Affects implementation of a keyword barry_as_FLUFL: Not entirely sure what this one actually accomplishes. :) division: Changes the meaning of one operator. generators: Introduces a keyword nested_scopes: Alters the compilation of source to byte-code(?) print_function: Removes a keyword unicode_literals: Alters the type used for literals with_statement: Introduces a keyword
Apart from the joke, it seems that every __future__ directive is there to affect the compilation, not execution, of its module: that is, once a module has been compiled to .pyc, it shouldn't matter whether it used __future__ or not. Regardless of unicode_literals, you can create bytes literals with b'asdf' and unicode literals with u'asdf'. I'm not entirely sure about division (can you call on true-division without the future directive?), but in any case, it's all done at compilation time, as can be seen interactively:
Python 2.7.3 (default, Mar 13 2014, 11:03:55) [GCC 4.7.2] on linux2 Type "help", "copyright", "credits" or "license" for more information.
... return x/y, x//y, x%y ...
from __future__ import division def division2(x,y):
... return x/y, x//y, x%y ...
import dis dis.dis(division1)
2 0 LOAD_FAST 0 (x) 3 LOAD_FAST 1 (y) 6 BINARY_DIVIDE 7 LOAD_FAST 0 (x) 10 LOAD_FAST 1 (y) 13 BINARY_FLOOR_DIVIDE 14 LOAD_FAST 0 (x) 17 LOAD_FAST 1 (y) 20 BINARY_MODULO 21 BUILD_TUPLE 3 24 RETURN_VALUE
2 0 LOAD_FAST 0 (x) 3 LOAD_FAST 1 (y) 6 BINARY_TRUE_DIVIDE 7 LOAD_FAST 0 (x) 10 LOAD_FAST 1 (y) 13 BINARY_FLOOR_DIVIDE 14 LOAD_FAST 0 (x) 17 LOAD_FAST 1 (y) 20 BINARY_MODULO 21 BUILD_TUPLE 3 24 RETURN_VALUE
So to make this consistent with all other __future__ directives, there would need to be some kind of safe way to define this: perhaps an attribute on the generator object. Something like this:
... yield x ... raise StopIteration(42) ...
gen.__distinguish_returns__ = True g=gen(123) list(g)
Traceback (most recent call last): File "<stdin>", line 1, in <module> File "<stdin>", line 3, in gen StopIteration: 42
The attribute on the function would be what affects behaviour; the __future__ directive applies that attribute to all generator functions in its module (including genexprs). Once the __future__ directive becomes automatic, the attribute can and will be dropped - any code which interrogates it MUST be prepared to stop interrogating it once the feature applies to all modules.
Does that sound reasonable? Should it be added to the PEP?
I agree with you and Steven that this is a fine use of __future__. What a generator does with a StopIteration that is about to bubble out of its frame is up to that generator. I don't think it needs to be a flag on the *function* though -- IMO it should be a flag on the code object. (And the flag should somehow be transferred to the stack frame when the function is executed, so the right action can be taken when an exception is about to bubble out of that frame.)
One other small point: let's change the PEP to just propose RuntimeError, and move the "some other exception" to the "rejected ideas" section.