2013/7/2 Andrea Griffini <agriff@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