On 22 February 2014 00:22, Rob Cliffe <rob.cli...@btinternet.com> wrote: > Thanks for looking into this Nick. > I confess I don't entirely understand the technical argument (my > understanding breaks down at "the subexpressions can't see the class level > variables", but I don't want to waste anybody's time expecting an > explanation, I can always look into it myself)
It's a relatively arcane scoping rule that only matters if you have non-trivial logic at class scope. The vast majority of Python programmers will never have to care, because they do the typical thing and their class bodies consist almost entirely of function definitions and relatively simple assignment statements. However, it's something that expression level name binding proposals need to take into account if they want to avoid side effects on the containing scope. Python's scoping rules are deliberately designed so that methods *can't* see class attributes directly, they have to access them via their first argument or the magic __class__ variable (Py3 only for the latter). For example: >>> class C: ... attr = "Hello" ... def m(self): ... print(self.attr) ... print(attr) ... >>> C().m() Hello Traceback (most recent call last): File "<stdin>", line 1, in <module> File "<stdin>", line 5, in m NameError: global name 'attr' is not defined The trick that makes it relevant here is that the "hidden" iteration variable in comprehensions is implemented by creating an implicit closure and then calling it. From the point of view of the scoping rules, that implicit closure behaves the same way as any other method definition, so most of the subexpressions can't see the class variables. The sole exception is the outermost iterator expression, because that is evaluated in the containing scope and then passed to the closure as an argument (this is all based on the way generator expressions work, and those *have* to be a closure, because they need to run in their own completely independent frame). As Chris later noted, you likely *could* still implement expression local name binding for an except expression without a full closure, it would just be rather difficult. Cheers, Nick. -- Nick Coghlan | ncogh...@gmail.com | Brisbane, Australia _______________________________________________ Python-Dev mailing list Python-Dev@python.org https://mail.python.org/mailman/listinfo/python-dev Unsubscribe: https://mail.python.org/mailman/options/python-dev/archive%40mail-archive.com
