evaluated function defaults: stored where?

[David Isaac]

Default parameter values are evaluated once when the function definition is
executed.
Where are they stored? (A guess: in a dictionary local to the function.)
Where is this documented?

As a Python newbie I found this behavior quite surprising.
Is it common in many other languages?
Is it unsurprising if I look at it right?

Thanks,
Alan Isaac

[Raymond Hettinger]

[Alan Isaac]

Default parameter values are evaluated once when the function definition is
executed. Where are they stored?

def f(a, b=10, c=20): .. pass f.func_defaults

(10, 20)

Where is this documented?

http://docs.python.org/ref/types.html#l2h-78

Raymond Hettinger

[alex23]

David Isaac wrote:

As a Python newbie I found this behavior quite surprising.

It can be even more surprising if a default value is mutable:

def foo(a, b=[]): .... b.append(a)
.... return b
foo(3,[1,2]) [1, 2, 3]
foo('c',['a','b']) ['a', 'b', 'c']
foo(1) [1]

So far, everything is behaving much as you'd expect, but then:
foo(2) [1, 2]
foo(3) [1, 2, 3]

The parameter is bound to the list at creation time and, being mutable,
is modifiable each time the function is called.

This can be avoided by modifying your function slightly:
foo(3, [1,2]) [1, 2, 3]
foo('c', ['a','b']) ['a', 'b', 'c']
foo(1) [1]
foo(2) [2]
foo(3)

[3]

-alex23

[alex23]

I wrote: (some guff on default parameters)

Of course, it helps if I actually include the alternative function's
code:

def foo(a, b = None):

.... if b == None: b = []
.... b.append(a)
.... return b

Sorry about that :)

-alex23

[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

[Raymond Hettinger]

> > Is it unsurprising if I look at it right?

[John Machin's QOTW]

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

There's a Quote of the Week in there somewhere.

[David Isaac]

Alan Isaac wrote:

Default parameter values are evaluated once when the function definition is executed. Where are they stored? ... Where is this documented?

Forgive any poor phrasing: I'm not a computer science type.
At http://www.network-theory.co.uk/docs/pytut/tut_26.html we read:
"The execution of a function introduces a new symbol table
used for the local variables of the function. More precisely,
all variable assignments in a function store the value in the local
symbol table; whereas variable references first look in the local
symbol table, then in the global symbol table, and then in the table of
built-in names."

But the default values of function parameters seem rather like a static
attributes of a class.
Is that a good way to think of them?
If so, are they somehow accessible?
How? Under what name?

Thanks,
Alan Isaac