create a decorator for adding default keyword arguments to a function

[mhansen]

The typical usage of this decorator would be to be applied above a :obj:cached_method or :obj:cached_function decorator so that the correct cached object is returned.

[nthiery]

Hi Mike!

I am not so sure about the name, although I can't propose much better than default_values, or set_default_values.

Could it be generalized to handle both positional and non positional arguments?

I'd suggest to have the doc by starting with what the thing actually does, followed by the typical usage. Speaking of which: could you add an example of this typical usage? (it was not clear to me).

[nthiery]

Oops, should have set the subject to needs work. Done.

[timdumol]

Sorry if I don't get this right, but doesn't functools.partial already fulfill this purpose?

[jason]

Do you mean something like this?

from functools import partial

def partial_dec(*args, **kwds):
    def p(f):
        return partial(f,*args,**kwds)
    return p
    
@partial_dec(b=2)
def f(a,b):
    return 10*a+b

f(4)

[timdumol]

Actually I meant something like this:

from functools import partial

@partial(partial, b=4)
def f(a,b):
    return 10*a + b

f(4)

[jason]

Cute. Very nice!

[jason]

So now can you use @wraps or something so that g? works correctly below?

from functools import partial, wraps

@partial(partial, b=4)
def g(a,b):
    """Docs"""
    return 10*a + b

g?

[timdumol]

This works, but it certainly isn't obvious:

from functools import partial, wraps

@partial(lambda x: wraps(x)(partial(partial, b = 4))(x))
def g(a,b):
    """Docs"""
    return 10*a + b

print(g(5))

g?

[jason]

and at that point, I'd say

@default_keywords...
def g...

is nicer. However, one might use partial in the above decorator. I think our discussion is evidence for the usefulness of the idea on this ticket.

[timdumol]

Yep. It's certainly much clearer. Using partial should deal with the positional and non-positional arguments thing.

[jason]

So we've agreed that we should create a partial decorator that allows something like:

from sage.misc.decorators (or wherever) import partial

@partial(*args, **kwds) # Same as calling partial(g, *args, **kwds) and wrapping with @wraps
def g(a,b):
   ...

just works as expected.

[timdumol]

Perhaps a name of curry [1] would be better, since it prevents name collision with functools.partial? Then again, it supersedes functools.partial anyways.

[1]  http://en.wikipedia.org/wiki/Currying

[nthiery]

Thanks much for pointing out functools.partial and functool.wrapper; I have several other use cases for them!

Replying to timdumol:

Perhaps a name of curry [1] would be better, since it prevents name collision with functools.partial? Then again, it supersedes functools.partial anyways. [1]  http://en.wikipedia.org/wiki/Currying

I prefer partial, since curry does not really encompass the specialization of named arguments. It's really functools.partial, but made into a decorator.

[timdumol]

I prefer partial, since curry does not really encompass the specialization of named arguments. It's really functools.partial, but made into a decorator.

Fair enough -- but just to clarify, functools.partial *is* a decorator, just that it doesn't update the documentation string.

[timdumol]

Adds module sage.misc.decorators with content specialize.

[timdumol]

Nevermind, functools.partial is not a decorator. My apologies.

This patch should do the trick. I named it specialize rather than partial, since partial conflicts with functools.partial.

[mhansen]

Looks good to me.

[mvngu]

same as previous but with username

[mvngu]

The patch  trac_5405-decorator-partial.2.patch is the same as Tim's patch, but with his username.

[mvngu]

Merged  trac_5405-decorator-partial.2.patch.