On Fri, Sep 7, 2018 at 4:38 AM, Franklin? Lee <leewangzhong+pyt...@gmail.com> wrote: > The following are equivalent and compile down to the same code: > a, b, c = lst > [a, b, c] = lst > > The left hand side is not an actual list (even though it looks like > one). The brackets are optional. The docs call the left hand side a > target list: > https://docs.python.org/3/reference/simple_stmts.html#assignment-statements > > "Target list" is not a real type. You can't construct such an object, > or hold one in memory. You can't make a class that emulates it > (without interpreter-specific hacks), because it is a collection of > its names, not a collection of values.
A target list is a syntactic element, like a name, or an operator, or a "yield" statement. You can't construct one, because it isn't an object type. It's not a "virtual type". It's a completely different sort of thing. > target_list.__iadd__ also does not exist, because target_list does not > exist. However, target_list can be thought of as a virtual type, a > type that the compiler compiles away. We can then consider > target_list.__iadd__ as a virtual operator, which the compiler will > understand but hide from the runtime. > > I was making the point that, because the __iadd__ in the example does > not refer to list.__iadd__, but rather a virtual target_list.__iadd__, > there is not yet a violation of the rule. What you're suggesting is on par with trying to say that: for += 5 should be implemented as: current_loop.__iadd__(5) where "current_loop" doesn't really exist, but it's a virtual type that represents a 'for' loop. That doesn't make sense, because there is no object in Python to represent the loop. There is no class/type that represents all loops, on which a method like this could be added. The word 'for' is part of the grammar, not the object model. And "target list" is the same. There's no way to attach an __iadd__ method to something that doesn't exist. So for your proposal to work, you would need to break that rule, and give a *different* meaning to this. ChrisA _______________________________________________ Python-ideas mailing list Python-ideas@python.org https://mail.python.org/mailman/listinfo/python-ideas Code of Conduct: http://python.org/psf/codeofconduct/
