Friday Finking: initialising values and implied tuples

[Chris Angelico]

On Tue, Apr 6, 2021 at 5:36 AM Rob Cliffe via Python-list
<python-list at python.org> wrote:
>
>
>
> On 05/04/2021 18:33, Chris Angelico wrote:
> >
> > Firstly, anything with any variable at all can involve a lookup, which
> > can trigger arbitrary code (so "variables which do not occur on the
> > LHS" is not sufficient).
> Interesting.  I was going to ask: How could you make a variable lookup
> trigger arbitrary code?
> Then I saw your post in the "Yield after the return in Python function"
> thread.  (Took me a while to understand it.)  So I ask:
> Can you make a variable lookup trigger arbitrary code, other than in
> code passed to eval/exec/compile?

Hmm. When you're defining a class, the metaclass can set the namespace
dictionary, does that count?

class Wat(dict):
    def __getitem__(self, name):
        print("HEY! You're looking for %r!" % name)
        return 42
    def __setitem__(self, name, value):
        print("Okay, I'm setting %r to %r." % (name, value))
        # I'm totally not.
class WutFace(type):
    @classmethod
    def __prepare__(cls, name, bases):
        return Wat()
class Gotcha(metaclass=WutFace):
    a = b + 1

But for top-level code in a module, or for function locals, I'm not
sure of any way to do this. It might be possible to mess with
sys.modules[__name__] but the __dict__ attribute can't be changed, so
you might need to first subclass the module. In general, though, even
if you can't find an example of how to do it, you have to assume that
it could happen somewhere, some time.

(It might also be possible to mess with the builtins module, which
would effectively catch any name lookups that aren't resolved locally
or at module level. Again, not sure if that would count.)

ChrisA