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Tue Jul 2 11:45:27 CEST 2013 · https://mail.python.org/pipermail/python-dev/2013-July/127152.html

2013/7/2 Andrea Griffini <agriff at tin.it>

> I'm trying to understand how CPython implements closure variable capture
> and there is one minor point I can't understand.
>
> When a local is captured it gets allocated in co_cellvars and is accessed
> with (LOAD|STORE)_DEREF, and this is clear.
> However when a local is coming from a parameter it gets ALSO allocated in
> co_varnames even if the local slot apparently is not accesible because
> *_FAST opcodes are not generated.
>
> Is there a technical reason for this? It happens in CPython 2, 3 and even
> in PyPy...
>
>
co_varnames is also used in error messages, for example in the following
code:

>>> def f():
...     def g():
...         x
...     print x
...     x = 1
...     return g
...
>>> f()
UnboundLocalError: local variable 'x' referenced before assignment

This is also needed when x is a parameter of f(), for inspect.signature of
course,
but also because in python3 you can "del x".

-- 
Amaury Forgeot d'Arc
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[Python-Dev] Why are captured parameters also listed in co_varnames?