Name resolution and the (wrong?) LEGB rule

Chris Angelico rosuav at gmail.com
Thu Dec 8 12:24:37 EST 2016


On Fri, Dec 9, 2016 at 1:51 AM, Marco Buttu <marco.buttu at gmail.com> wrote:
> "if a particular name:object mapping cannot be found in the local
> namespaces, the namespaces of the enclosed scope are being searched next. If
> the search in the enclosed scope is unsuccessful, too, Python moves on to
> the global namespace, and eventually, it will search the built-in namespace
> (side note: if a name cannot found in any of the namespaces, a NameError
> will is raised)."
>
> AFAIK, Python has static scoping: the scope for a name is given during the
> bytecode compilation. This means that before executing the program, Python
> already know in which namespace to look for an object. So, it seems to me
> that the LEGB rule is wrong,

It isn't wrong, but there are some parts of it that can be resolved at
compile time. Once a function is compiled, it cannot normally gain or
lose local names. There are a few situations that can mess with that
(a star import or 'exec' statement, in Python 2), and when the
compiler detects one of those, it has to avoid the usual optimization.

The "usual optimization" is exactly what you describe: that different
bytecodes represent Local, Enclosing, and Global/Built-in scope
lookups. (Globals can be created or removed at run-time, so there's no
optimization possible there.) But in terms of language specifications,
the lookup rules are the same; it's just that the CPython compiler
takes advantage of information that it can see ("these are the only
locals for this function") to speed up execution.

ChrisA


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