Question

I don’t know if this will make sense, but…

I’m trying to dynamically assign methods to an object.

#translate this
object.key(value)
#into this
object.method({key:value})

To be more specific in my example, I have an object (which I didn’t write), lets call it motor, which has some generic methods set, status and a few others. Some take a dictionary as an argument and some take a list. To change the motor’s speed, and see the result, I use:

motor.set({'move_at':10})
print motor.status('velocity')

The motor object, then formats this request into a JSON-RPC string, and sends it to an IO daemon. The python motor object doesn’t care what the arguments are, it just handles JSON formatting and sockets. The strings move_at and velocity are just two of what might be hundreds of valid arguments.

What I’d like to do is the following instead:

motor.move_at(10)
print motor.velocity()

I’d like to do it in a generic way since I have so many different arguments I can pass. What I don’t want to do is this:

# create a new function for every possible argument
def move_at(self,x)
    return self.set({'move_at':x})
def velocity(self)
    return self.status('velocity')
#and a hundred more...

I did some searching on this which suggested the solution lies with lambdas and meta programming, two subjects I haven’t been able to get my head around.

UPDATE:

Based on the code from user470379 I’ve come up with the following…

# This is what I have now....
class Motor(object):
    def set(self,a_dict):
        print "Setting a value", a_dict
    def status(self,a_list):
        print "requesting the status of", a_list
        return 10

# Now to extend it....
class MyMotor(Motor):
    def __getattr__(self,name):
        def special_fn(*value):
            # What we return depends on how many arguments there are.
            if len(value) == 0: return self.status((name))
            if len(value) == 1: return self.set({name:value[0]})
        return special_fn
    def __setattr__(self,attr,value): # This is based on some other answers
        self.set({attr:value})


x = MyMotor()
x.move_at = 20 # Uses __setattr__
x.move_at(10)  # May remove this style from __getattr__ to simplify code.
print x.velocity()

output:

Setting a value {'move_at': 20}
Setting a value {'move_at': 10}
10

Thank you to everyone who helped!

Answer 1

What about creating your own __getattr__ for the class that returns a function created on the fly? IIRC, there’s some tricky cases to watch out for between __getattr__ and __getattribute__ that I don’t recall off the top of my head, I’m sure someone will post a comment to remind me:

def __getattr__(self, name):
    def set_fn(self, value):
        return self.set({name:value})
    return set_fn

Then what should happen is that calling an attribute that doesn’t exist (ie: move_at) will call the __getattr__ function and create a new function that will be returned (set_fn above). The name variable of that function will be bound to the name parameter passed into __getattr__ ("move_at" in this case). Then that new function will be called with the arguments you passed (10 in this case).

Edit

A more concise version using lambdas (untested):

def __getattr__(self, name):
    return lambda value: self.set({name:value})