metadecorators.py

from functools import wraps

def predecoration(decoration=None, has_params=False):
    "Decorates a function as a pre-decorator"

    def metadecorator(decoration):
        "Takes a decoration to pre-apply to a func"
        def cap_args(*dec_args, **dec_kwds):
            "Captures decorator arguments"
            def decorator_with_args(func):
                "The Decorator - takes a func to pre-decorate"
                @wraps(func)
                def decoration_then_func(*args, **kwds):
                    "Runs decoration, then func"
                    # the decoration will be passed its arguments along with the func arguments
                    decoration_args = {}
                    if dec_args: decoration_args['dec_args'] = dec_args
                    if dec_kwds: decoration_args['dec_kwds'] = dec_kwds
                    if args: decoration_args['args'] = args
                    if kwds: decoration_args['kwds'] = kwds

                    # run decoration, and if return value is truthy, modify arguments to func
                    decoration_return = decoration(**decoration_args)
                    if decoration_return:
                        if type(decoration_return) == tuple:
                            if len(decoration_return) == 2 and type(decoration_return[0]) == tuple and type(decoration_return[1]) == dict:
                                args = decoration_return[0]
                                kwds.update(decoration_return[1])
                            else:
                                args = decoration_return
                        elif type(decoration_return) == dict:
                            kwds.update(decoration_return)
                        else:
                            args = (decoration_return,)
                            
                    return func(*args, **kwds)
                return decoration_then_func
            return decorator_with_args
        return cap_args
        
    return metadecorator if has_params else metadecorator(decoration)((), {})



# examples

@predecorator
def debug_input(*input):
    print input

@debug_input
def foo(x,y):
    return x+y

@predecorator(has_params=True)
def accepts(args, dec_args):
    for (a,t) in zip(args, dec_args):
        assert isinstance(a, t), "arg %r does not match %s" % (a,t)
    
@accepts(int, int)
def bar(x,y):
    return x+y

bar(1,1) # return 2
bar(1.0, 1) # throws AssertError