evaluated function defaults: stored where?

[John Machin]

David Isaac wrote:
> Default parameter values are evaluated once when the function definition is
> executed.
> Where are they stored?

A good bet for where to start looking for the storage would be as an 
attribute of the function object. From this point, there are two paths:

(a) Make a function and inspect it:

 >>> def fun(a,b=42,c=666):
...    pass
...
 >>> dir(fun)
['__call__', '__class__', '__delattr__', '__dict__', '__doc__', 
'__get__', '__getattribute__', '__hash__', '__init__', '__module__',
  '__name__', '__new__', '__reduce__', '__reduce_ex__', '__repr__', 
'__setattr__', '__str__', 'func_closure', 'func_code', 'func_defa
ults', 'func_dict', 'func_doc', 'func_globals', 'func_name']
 >>> fun.func_defaults
(42, 666)
 >>>


(b) Become familiar with the general structure of the manuals.
By process of elimination, the Python Reference Manual would be a good 
place to start for a question of this type.
Inside that manual, this is a likely candidate:
"""
3.2 The standard type hierarchy
Below is a list of the types that are built into Python.
"""

I'll leave you to read further ...

>  (A guess: in a dictionary local to the function.)

As you've seen the default values are contained in a tuple. So where are 
the keys of the mapping {'b': 42, 'c': 666} that you expected? Suppose 
*you* do the extra research and report back ...

> Where is this documented?

See above.

> 
> As a Python newbie I found this behavior quite surprising.

Surprisingly good, bad or just different? Do you have alternate 
non-surprising behaviours in mind?

> Is it common in many other languages?

That makes two of us who don't know and haven't done any research :-)
My impression based on a limited number of different languages that I've 
worked with or looked at is that Python's apparatus is the best I've 
seen; YMMV.

> Is it unsurprising if I look at it right?

Yes; in general this is true across many domains for a very large number 
of referents of "it" :-)

Cheers,
John