Question

I find functools.partial to be extremely useful, but I would like to be able to freeze arguments out of order (the argument you want to freeze is not always the first one) and I’d like to be able to apply it to several methods on a class at once, to make a proxy object that has the same methods as the underlying object except with some of its methods parameters being frozen (think of it as generalizing partial to apply to classes). And I’d prefer to do this without editing the original object, just like partial doesn’t change its original function.

I’ve managed to scrap together a version of functools.partial called ‘bind’ that lets me specify parameters out of order by passing them by keyword argument. That part works:

>>> def foo(x, y):
...     print x, y
...
>>> bar = bind(foo, y=3)
>>> bar(2)
2 3

But my proxy class does not work, and I’m not sure why:

>>> class Foo(object):
...     def bar(self, x, y):
...             print x, y
...
>>> a = Foo()
>>> b = PureProxy(a, bar=bind(Foo.bar, y=3))
>>> b.bar(2)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: bar() takes exactly 3 arguments (2 given)

I’m probably doing this all sorts of wrong because I’m just going by what I’ve pieced together from random documentation, blogs, and running dir() on all the pieces. Suggestions both on how to make this work and better ways to implement it would be appreciated 😉 One detail I’m unsure about is how this should all interact with descriptors. Code follows.

from types import MethodType

class PureProxy(object):
    def __init__(self, underlying, **substitutions):
        self.underlying = underlying

        for name in substitutions:
            subst_attr = substitutions[name]
            if hasattr(subst_attr, "underlying"):
                setattr(self, name, MethodType(subst_attr, self, PureProxy))

    def __getattribute__(self, name):
        return getattr(object.__getattribute__(self, "underlying"), name)

def bind(f, *args, **kwargs):
    """ Lets you freeze arguments of a function be certain values. Unlike
    functools.partial, you can freeze arguments by name, which has the bonus
    of letting you freeze them out of order. args will be treated just like
    partial, but kwargs will properly take into account if you are specifying
    a regular argument by name. """
    argspec = inspect.getargspec(f)
    argdict = copy(kwargs)

    if hasattr(f, "im_func"):
        f = f.im_func

    args_idx = 0
    for arg in argspec.args:
        if args_idx >= len(args):
            break

        argdict[arg] = args[args_idx]
        args_idx += 1

    num_plugged = args_idx

    def new_func(*inner_args, **inner_kwargs):
        args_idx = 0
        for arg in argspec.args[num_plugged:]:
            if arg in argdict:
                continue
            if args_idx >= len(inner_args):
                # We can't raise an error here because some remaining arguments
                # may have been passed in by keyword.
                break
            argdict[arg] = inner_args[args_idx]
            args_idx += 1

        f(**dict(argdict, **inner_kwargs))

    new_func.underlying = f

    return new_func

Update: In case anyone can benefit, here’s the final implementation I went with:

from types import MethodType

class PureProxy(object):
    """ Intended usage:
    >>> class Foo(object):
    ...     def bar(self, x, y):
    ...             print x, y
    ...
    >>> a = Foo()
    >>> b = PureProxy(a, bar=FreezeArgs(y=3))
    >>> b.bar(1)
    1 3
    """

    def __init__(self, underlying, **substitutions):
        self.underlying = underlying

        for name in substitutions:
            subst_attr = substitutions[name]
            if isinstance(subst_attr, FreezeArgs):
                underlying_func = getattr(underlying, name)
                new_method_func = bind(underlying_func, *subst_attr.args, **subst_attr.kwargs)
                setattr(self, name, MethodType(new_method_func, self, PureProxy))

    def __getattr__(self, name):
        return getattr(self.underlying, name)

class FreezeArgs(object):
    def __init__(self, *args, **kwargs):
        self.args = args
        self.kwargs = kwargs

def bind(f, *args, **kwargs):
    """ Lets you freeze arguments of a function be certain values. Unlike
    functools.partial, you can freeze arguments by name, which has the bonus
    of letting you freeze them out of order. args will be treated just like
    partial, but kwargs will properly take into account if you are specifying
    a regular argument by name. """
    argspec = inspect.getargspec(f)
    argdict = copy(kwargs)

    if hasattr(f, "im_func"):
        f = f.im_func

    args_idx = 0
    for arg in argspec.args:
        if args_idx >= len(args):
            break

        argdict[arg] = args[args_idx]
        args_idx += 1

    num_plugged = args_idx

    def new_func(*inner_args, **inner_kwargs):
        args_idx = 0
        for arg in argspec.args[num_plugged:]:
            if arg in argdict:
                continue
            if args_idx >= len(inner_args):
                # We can't raise an error here because some remaining arguments
                # may have been passed in by keyword.
                break
            argdict[arg] = inner_args[args_idx]
            args_idx += 1

        f(**dict(argdict, **inner_kwargs))

    return new_func

Answer 1

You’re “binding too deep”: change def __getattribute__(self, name): to def __getattr__(self, name): in class PureProxy. __getattribute__ intercepts every attribute access and so bypasses everything that you’ve set with setattr(self, name, ... making those setattr bereft of any effect, which obviously’s not what you want; __getattr__ is called only for access to attributes not otherwise defined so those setattr calls become “operative” & useful.

In the body of that override, you can and should also change object.__getattribute__(self, "underlying") to self.underlying (since you’re not overriding __getattribute__ any more). There are other changes I’d suggest (enumerate in lieu of the low-level logic you’re using for counters, etc) but they wouldn’t change the semantics.

With the change I suggest, your sample code works (you’ll have to keep testing with more subtle cases of course). BTW, the way I debugged this was simply to stick in print statements in the appropriate places (a jurassic=era approach but still my favorite;-).