How to make a str var callable

Question

What i want is to make a string var callable. I just have a list with different models_names and i want to call their create methods like this way.

class Object_model_a:
     #def ...
class Object_model_b:
     #def ...
class Object_model_c:
     #def ...

list = ('Object_model_a', 'Object_model_b', 'Object_model_c')
x = list[0]() # this must create a new instance of Object_model_a

This is possible to develop using php like this way:

$hi = 'Hello'
$Hello = 'Hi!!'
echo $$hi
>> Hi!!

Anyone knows if this is possible using django?? This will simplify a lot my code.

Thanks a lot!

No, that won't simplify your code. Use [Factory](http://en.wikipedia.org/wiki/Factory_method_pattern) instead. — DrTyrsa, May 29 '12 at 11:46
"What i want is to make a string var callable" No, you don't. — Marcin, May 29 '12 at 11:49
What keeps you from storing the classes in the list? e.g. [Object_model_a,Object_model_b] etc. — devsnd, May 29 '12 at 12:00
The reason which i have to do this is because i'll never know the type of the model that this script will manage, and on future develop this will be able to get information about any other model. — jsujar, May 30 '12 at 07:44

Alasdair · Answer 1 · 2012-05-29T12:47:12.353

4

You could use Django's get_model helper function, which takes the app name and model name and returns the model class.

from django.db.models import get_model

list = ('Object_model_a', 'Object_model_b', 'Object_model_c')
model = get_model('my_app_name', list[0]) # returns Object_model_a class
instance = model() # creates a model instance

edited May 29 '12 at 12:47

answered May 29 '12 at 11:51

Alasdair

298,606
55
578
516

Marcin · Answer 2 · 2012-05-29T12:16:13.947

0

If for whatever reason you have a string that represents a particular class, the correct way to use that to instantiate an object of that class is to place your classes in a dict, keyed by the strings, then just invoke the class.

classes = dict((c.__name__, c) for c in (Object_model_a, Object_model_b, Object_model_c))

instance = classes['Object_model_a']()

Obviously, you don't need to use the classname as the key.

edited May 29 '12 at 12:16

answered May 29 '12 at 11:55

Marcin

48,559
18
128
201

Could you please also point out what a horrably terrible idea this is? – devsnd May 29 '12 at 12:01
@twall Well, if you do need to select classes by string, this is the right way to do it. This is occasionally useful. – Marcin May 29 '12 at 12:15
1

@twall: why "horrably terrible" ? this is a very common Python idiom. – bruno desthuilliers May 29 '12 at 12:28

score -1 · Answer 3 · answered May 30 '12 at 08:07

Thanks for all your comments, this is the way i've solved this requirement.

def create_object_instance(model_name, id=None):
    for model in get_models():
        if str(model.__name__).lower() == str(model_name).lower():
            if id:
                try:
                    return model.objects.get(id=id)              
                except ObjectsDoesNotExist:
                    return None
            else:
                return model.objects.all()
    return None

This is kind of terrible. If you want to allow an arbitrary model to be specified, why not use `get_model`? — Marcin, May 30 '12 at 08:14

How to make a str var callable

3 Answers3