How to pass on argparse argument to function as kwargs?

Question

I have a class defined as follows

class M(object):
    def __init__(self, **kwargs):
        ...do_something

and I have the result of argparse.parse_args(), for example:

> args = parse_args()
> print args
Namespace(value=5, message='test', message_type='email', extra="blah", param="whatever")

I want to pass on the values of this namespace (except message_type) to create an instance of the class M. I have tried

M(args)

but got an error

TypeError: __init__() takes exactly 1 argument (2 given)

which I do not understand. How can I

1. remove the value message_type from the list in args
2. pass on the values as if I would type M(value=5, message='test', extra="blah", param="whatever") directly.

Solution

You need to pass in the result of vars(args) instead:

M(**vars(args))

The vars() function returns the namespace of the Namespace instance (its __dict__ attribute) as a dictionary.

Inside M.__init__(), simply ignore the message_type key.

Comments

[Alex]

Thats exactly what I need. Thanks. Except for removal of the certain key. I need it to be removed, I cannot just ignore it for reasons outside the scope of this question.

[Alex]

I suggest to do arg_dict = dict((k,v) for k,v in vars(args).items() if k!="message_type") to avoid possible undefined behaviour with vars.

[Martijn Pieters]

@Alex: what undefined behaviour with vars? You can filter out the message_type key that way, sure, but do that for it's own sake, not because vars() might have undefined behaviour (which it doesn't).

[Martijn Pieters]

@Alex: I'd rather make the M class more robust in the face of extra keyword arguments it doesn't support.

[Alex]

Note From the documentation of vars: The returned dictionary should not be modified: the effects on the corresponding symbol table are undefined. [2]. As for your suggestion, M can be one of many classes absolutely detached from what I am doing here. These classes cannot possibly know of some abundant keyword (and might give an error otherwise). Let's say, those classes cannot be modified.

[Martijn Pieters]

@Alex: The values are copied into the M.__init__ parameters, so the original mapping is never modified. If you need to regulate what keywords are passed in, use a whitelist instead.