python: validating function parameters

Question

I need to make sure all the named parameters were passed to a method (I don't want any defaults). Is the following the best way to enforce it?

class X:
 def func(self, **kwargs):
 if set(kwargs.keys() != ('arg1', 'arg2', 'arg3'):
 raise ArgException(kwargs)

• Glenn Maynard: Be sure that the exception you're raising is a subclass of TypeError. Why are you using kwargs at all if you want to do this? Just specify the variables normally.
• S.Lott: Bad, bad design. Use def func( self, arg1, arg2, arg3 ).
• max: @S Lott I would use what you suggest, but then I keep making mistakes in the order of the arguments. @Glenn Maynard I was using named parameters just to avoid mistakes in the order (I have 3-6 arguments in many of my functions). Not good?
• max: OMG I didn't know that I can use keyword arguments even when the function is defined as def func(self, a, b, c)... :)

Answer 1

For Python 2.x, Hugh's answer (i.e. just use named positional arguments) is your best bet.

For Python 3.x, you also have the option of requiring the use of keyword arguments (rather than merely allowing it) as follows:

class X(object):
 def func(self, *, arg1, arg2, arg3):
 pass

Answer 2

class X(object):
 def func(self, **kwargs):
 required = set(['arg1','arg2','arg3'])
 if not set(kwargs.keys()).issuperset(required):
 raise ArgException(kwargs)

although you could just let the interpreter take care of it:

class X(object):
 def func(self, arg1, arg2, arg3):
 pass

• max: +1 .. yes, see my comment to the question. I have no idea how I didn't know that..

Answer 3

Do you want to allow other arguments? If not:

class X(object):
 def func(self, arg1, arg2, arg3):
 pass

If yes:

class X(object):
 def func(self, arg1, arg2, arg3, **kwargs):
 pass

That's for all versions of Python.