visual programming langua - Page 403

How can I use functools.partial on multiple methods on an object, and freeze parameters out of order

- by Joseph Garvin

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 parameter being frozen (think of it as generalizing partial to apply to classes). 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

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