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