[Python-Dev] Why are captured parameters also listed in co_varnames? (original) (raw)
Amaury Forgeot d'Arc amauryfa at gmail.com
Tue Jul 2 11:45:27 CEST 2013
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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 cocellvars 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 covarnames 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".
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