python - How to use a decorator to check arguments -

i have several functions want give default arguments. since these defaults should newly-initialized objects, can't set them defaults in argument list. introduces lot of clunky code default them none , check in every single function

def functionwithdefaults(foo=none):   if foo none:     foo = makenewobject() 

to reduce repeated code, made decorator initialize parameters.

@initializedefaults(makenewobject, 'foo') def functionwithdefaults(foo=none):   # if foo wasn't passed in, equal makenewobject() 

the decorator wrote such:

def initializedefaults(initializer, *parameters_to_initialize):   """returns decorator initialize parameters uninitialized.    args:     initializer: function called no arguments generate         values parameters need initialized.     *parameters_to_initialize: each arg name of parameter         represents user key.   """   def decorator(func):     def _initializedefaults(*args, **kwargs):       # gets list of names of variables in func.  so, if       # function being decorated takes 2 arguments, foo , bar,       # func_arg_names ['foo', 'bar'].  way, can tell whether or       # not parameter passed in positional argument.       func_arg_names = inspect.getargspec(func).args       num_args = len(args)       parameter in parameters_to_initialize:         # don't set default if parameter passed positional arg.         if num_args <= func_arg_names.index(parameter):           # don't set default if parameter passed keyword arg.           if parameter not in kwargs or kwargs[parameter] none:             kwargs[parameter] = initializer()       return func(*args, **kwargs)     return _initializedefaults   return decorator 

that works great if need decorate once, breaks multiple decorations. specifically, suppose want initialize foo , bar separate variables:

@initializedefaults(makenewobject, 'foo') @initializedefaults(makeotherobject, 'bar') def functionwithdefaults(foo=none, bar=none):   # foo should makenewobject() , bar should makeotherobject() 

that breaks because in second decorator, func_arg_defaults gets arguments of first decorator rather functionwithdefaults. result, can't tell whether or not parameter has been passed in positionally.

is there clean way solve problem?

things don't work:

• functools.wraps(func) makes __name__ of function decorated function, doesn't change arguments (unless there's different way it).
• inspect.getcallargs doesn't raise exception in second decorator
• using func.func_code.co_varnames doesn't provide function's arguments in second decorator

i use 1 decorator, seems less clean, , feels there should cleaner solution.

thanks!

this might best advice you'd decorators in python: use wrapt.decorator.

it make task of writing decorators simpler (you don't need write function inside function inside function), , avoid errors makes when writing simple decorators. learn are, might want read graham dumpleton's blog posts titled "how implemented python decorator wrong" (he convincing)