Josh Rosenberg <shadowranger+pyt...@gmail.com> added the comment:
Aaron: Your understanding of how LEGB works in Python is a little off.
Locals are locals for the *entire* scope of the function, bound or unbound;
deleting them means they hold nothing (they're unbound) but del can't actually
stop them from being locals. The choice of whether to look something up in the
L, E or GB portions of LEGB scoping rules is a *static* choice made when the
function is defined, and is solely about whether they are assigned to anywhere
in the function (without an explicit nonlocal/global statement to prevent them
becoming locals as a result).
Your second example can be made to fail just by adding a line after the print:
def doSomething():
print(x)
x = 1
and it fails for the same reason:
def doSomething():
x = 10
del x
print(x)
fails; a local is a local from entry to exit in a function. Failure to assign
to it for a while doesn't change that; it's a local because you assigned to it
at least once, along at least one code path. del-ing it after assigning doesn't
change that, because del doesn't get rid of locals, it just empties them.
Imagine how complex the LOAD_FAST instruction would get if it needed to handle
not just loading a local, but when the local wasn't bound, had to choose
*dynamically* between:
1. Raising UnboundLocalError (if the value is local, but was never assigned)
2. Returning a closure scoped variable (if the value was local, but got del-ed,
and a closure scope exists)
3. Raising NameError (if the closure scope variable exists, but was never
assigned)
4. Returning a global/builtin variable (if there was no closure scope variable
*or* the closure scope variable was created, but explicitly del-ed)
5. Raising NameError (if no closure, global or builtin name exists)
That's starting to stretch the definition of "fast" in LOAD_FAST. :-)
----------
nosy: +josh.r
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<https://bugs.python.org/issue17792>
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