About MAKE_FUNCTION opcode in Python 3

[Eric Snow]

On Tue, Sep 20, 2011 at 1:59 PM, Arnaud Delobelle <arnodel at gmail.com> wrote:
> Since Python 3.0 we have keyword only arguments in functions (see PEP
> 3102).  Looking at the documentation for the dis module (where opcodes
> are documented), I see the following for MAKE_FUNCTION [1]
>
> """
> MAKE_FUNCTION(argc)
> Pushes a new function object on the stack. TOS is the code associated
> with the function. The function object is defined to have argc default
> parameters, which are found below TOS.
> """
>
> No mention of default values for keyword only arguments.  Now let's
> try (Python 3.2):
>
>>>> def foo():
> ...   def bar(x, y, *args, z=1, t=2, **kwargs): pass
> ...
>>>> dis.dis(foo)
>  2           0 LOAD_CONST               1 ('z')
>              3 LOAD_CONST               2 (1)
>              6 LOAD_CONST               3 ('t')
>              9 LOAD_CONST               4 (2)
>             12 LOAD_CONST               5 (<code object bar at
> 0x1005ec8b0, file "<stdin>", line 2>)
>             15 MAKE_FUNCTION          512
>             18 STORE_FAST               0 (bar)
>             21 LOAD_CONST               0 (None)
>             24 RETURN_VALUE
>
> MAKE_FUNCTION has an argc of 512.  So it seems that since Python 3.0:
>
> * the number of default values for normal arguments is argc & 0xFF
> * the number of default values for keyword only arguments is argc >> 8
>
> Can anyone confirm this?  I can then open a ticket on bugs.python.org

You're mostly right.

http://hg.python.org/cpython/file/default/Python/ceval.c#l2684

  2684             int posdefaults = oparg & 0xff;
  2685             int kwdefaults = (oparg>>8) & 0xff;
  2686             int num_annotations = (oparg >> 16) & 0x7fff;

-eric

>
> [1] http://docs.python.org/dev/library/dis.html#opcode-MAKE_FUNCTION
>
> --
> Arnaud
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>