On Mon, May 27, 2019 at 9:18 PM Guido van Rossum <gu...@python.org> wrote: > > Note that the weird, Action At A Distance behavior is also visible for > locals() called at module scope (since there, locals() is globals(), which > returns the actual dict that's the module's __dict__, i.e. the Source Of > Truth. So I think it's unavoidable in general, and we would do wise not to > try and "fix" it just for function locals. (And I certainly don't want to > mess with globals().)
I think it's worth distinguishing between two different types of weird Action At A Distance here: - There's the "justified" action-at-a-distance that currently happens at module scope, where locals().__setitem__ affects variable lookup, and variable mutation affects locals().__getitem__. This can produce surprising results if you pass locals() into something that's expecting a regular dict, but it's also arguably the point of an environment introspection API, and like you say, it's unavoidable and expected at module scope and when using globals(). - And then there's the "spooky" action-at-a-distance that currently happens at function scope, where calling locals() has the side-effect of mutating the return value from previous calls to locals(), and the objects returned from locals may or may not spontaneously mutate themselves depending on whether some other code registered a trace function. This is traditional, but extremely surprising if you aren't deeply familiar with internals of CPython's implementation. Of the four designs: [PEP] and [PEP-minus-tracing] both have "spooky" action-at-a-distance (worse in [PEP]), but they don't have "justified" action-at-a-distance. [proxy] adds "justified" action-at-a-distance, and removes "spooky" action at a distance. [snapshot] gets rid of both kinds of action-at-a-distance (at least in function scope). -n -- Nathaniel J. Smith -- https://vorpus.org _______________________________________________ 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
