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

[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...

Andrea Griffini
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