Re: [Python-Dev] Why are captured parameters also listed in co_varnames?

2 Jul 2013


      2013/7/2 Andrea Griffini 
...
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