[Python-Dev] Expected stability of PyCode_New() and types.CodeType() signatures

[Serhiy Storchaka]

31.05.19 11:46, Petr Viktorin пише:
> PEP 570 (Positional-Only Parameters) changed the signatures of 
> PyCode_New() and types.CodeType(), adding a new argument for "posargcount".
> Our policy for such changes seems to be fragmented tribal knowledge. I'm 
> writing to check if my understanding is reasonable, so I can apply it 
> and document it explicitly.
> 
> There is a surprisingly large ecosystem of tools that create code objects.
> The expectation seems to be that these tools will need to be adapted for 
> each minor version of Python.

I have a related proposition. Yesterday I have reported two bugs (and 
Pablo quickly fixed them) related to handling positional-only arguments. 
These bugs were occurred due to subtle changing the meaning of 
co_argcount. When we make some existing parameters positional-only, we 
do not add new arguments, but mark existing parameters. But co_argcount 
now means the only number of positional-or-keyword parameters. Most code 
which used co_argcount needs now to be changed to use 
co_posonlyargcount+co_argcount.

I propose to make co_argcount meaning the number of positional 
parameters (i.e. positional-only + positional-or-keyword). This would 
remove the need of changing the code that uses co_argcount.

As for the code object constructor, I propose to make posonlyargcount an 
optional parameter (default 0) added after existing parameters. 
PyCode_New() can be kept unchanged, but we can add new PyCode_New2() or 
PyCode_NewEx() with different signature.