dynamic class creation in python

Question

I want to create this class by dynamically:

class CreateAgent(show.ShowOne):
    """Create compute agent command"""

    log = logging.getLogger(__name__ + ".CreateAgent")

    def get_parser(self, prog_name):
        parser = super(CreateAgent, self).get_parser(prog_name)
        parser.add_argument(
            "os",
            metavar="<os>",
            help="Type of OS")
        parser.add_argument(
            "architecture",
            metavar="<architecture>",
            help="Type of architecture")
        parser.add_argument(
            "version",
            metavar="<version>",
            help="Version")
        parser.add_argument(
            "url",
            metavar="<url>",
            help="URL")
        parser.add_argument(
            "md5hash",
            metavar="<md5hash>",
            help="MD5 hash")
        parser.add_argument(
            "hypervisor",
            metavar="<hypervisor>",
            help="Type of hypervisor",
            default="xen")
        return parser

    def take_action(self, parsed_args):
        self.log.debug("take_action(%s)", parsed_args)
        compute_client = self.app.client_manager.compute
        args = (
            parsed_args.os,
            parsed_args.architecture,
            parsed_args.version,
            parsed_args.url,
            parsed_args.md5hash,
            parsed_args.hypervisor
        )
        agent = compute_client.agents.create(*args)._info.copy()
        return zip(*sorted(six.iteritems(agent)))

I went through many links but could find resources for dynamic class creation of classes with basic variables declarations and function.

class Foo(object):
    x = 10
    y = 20
    def get_x(self):
        return self.x
    def get_y(self):
        return self.y

Dynamic class creation can be done as:

Bar = type(
       'Bar',
       (object,),
       dict(
            x = 10,
            y = 20,
            get_x=lambda self:self.x,
            get_y=lambda self:self.y
        ) 
       )

But i don't know how to crete CreateAent class and define its functions dynamically..

This is the code that i have written:

def create_model(name, base=None, fields=None):
    model = type(name, base, fields)

    return model

if __name__ == '__main__':
    fields = {
        'os': str,
        'architecture': str,
        'version': int,
        'url': str,
        'md5hash': int,
        'hypervisor': int,
        }
    model = create_model('CreateAgent', ('Showone',), fields)
    model.os = "windows"
    print model.os

It gives me the following error:

TypeError: metaclass conflict: the metaclass of a derived class must be a (non-strict) subclass of the metaclasses of all its bases
I don't know how to define get_parser function.

PS: Some hints will be really helpful

generally you'd make some sort of a CreateAgentMaker function that returned a new class with your attributes. You can even use @classmethod to do it as CreateAgent.dynamic_from_string("name",(parents,), {"key":"value", "attri":"butes"}) — Adam Smith
– Adam Smith, Commented Jun 25, 2014 at 23:21
@AdamSmith I am mainly confused as to how to dynamically define class functions ? Can you help me with that — user3731594
– user3731594, Commented Jun 25, 2014 at 23:26
Give me a concrete example. What function you're trying to build from what arguments and we'll work on it. — Adam Smith
– Adam Smith, Commented Jun 25, 2014 at 23:29

Ethan Furman · Accepted Answer · 2014-06-26 01:02:51Z

2

In the type() command where you have (object,), you want to have

(show.ShowOne, object),

and where you have dict(...) you could also you an already existing dictionary:

def some_func(self):
    do_something()

cls_dict['some_func'] = some_func

and then your Bar creation would look like:

# towards the top of your file
import show

# somewhere else in your file...
Bar = type(
        'Bar',
        (show.ShowOne, object),
        cls_dict,
        )

edited Jun 26, 2014 at 1:02

answered Jun 25, 2014 at 23:31

Ethan Furman

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2 Comments

user3731594 Over a year ago

It gives error NameError: name 'show' is not defined

Ethan Furman Over a year ago

You have to import show prior to that line, same as you would for a normal class construction.

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