python create object and add attributes to it

Question

Hi,

I want to create a dynamic object (inside another object) in python and then add attributes to it.

I tried:

obj = someobject
obj.a = object()
setattr(obj.a, 'somefield', 'somevalue')

but this didn't work.

Any ideas?

edit:

I am setting the attributes from a for loop which loops through a list of values. e.g.

params = ['attr1', 'attr2', 'attr3']
obj = someobject
obj.a = object()

for p in params:
   obj.a.p # where p comes from for loop variable

so in the above example I would get obj.a.attr1, obj.a.attr2, obj.a.attr3.

I used the setattr function because I didn't know how to do obj.a.NAME FROM FOR LOOP

How would I set the attribute based on the value of p in the example above?

Thanks

Answer 1

The built-in object can be instantiated but can't have any attributes set on it. (I wish it could, for this exact purpose.) It doesn't have a __dict__ to hold the attributes.

I generally just do this:

class Object(object):
    pass

a = Object()
a.somefield = somevalue

When I can, I give the Object class a more meaningful name, depending on what kind of data I'm putting in it.

Some people do a different thing, where they use a sub-class of dict that allows attribute access to get at the keys. (d.key instead of d['key'])

Edit: For the addition to your question, using setattr is fine. You just can't use setattr on object() instances.

params = ['attr1', 'attr2', 'attr3']
for p in params:
    setattr(obj.a, p, value)

Answer 2

as docs say:

Note: object does not have a __dict__, so you can’t assign arbitrary attributes to an instance of the object class.

You could just use dummy-class instance.

Answer 3

There are a few ways to reach this goal. Basically you need an object which is extendable.

obj.a = type('Test', (object,) {})  
obj.a.b = 'fun'  

obj.b = lambda:None

class Test:
  pass
obj.c = Test()

Answer 4

You could use my ancient Bunch recipe, but if you don't want to make a "bunch class", a very simple one already exists in Python -- all functions can have arbitrary attributes (including lambda functions). So, the following works:

obj = someobject
obj.a = lambda: None
setattr(obj.a, 'somefield', 'somevalue')

Whether the loss of clarity compared to the venerable Bunch recipe is OK, is a style decision I will of course leave up to you.

Answer 5

Which objects are you using? Just tried that with a sample class and it worked fine:

class MyClass:
  i = 123456
  def f(self):
    return "hello world"

b = MyClass()
b.c = MyClass()
setattr(b.c, 'test', 123)
b.c.test

And I got 123 as the answer.

The only situation where I see this failing is if you're trying a setattr on a builtin object.

Update: From the comment this is a repetition of: http://stackoverflow.com/questions/1285269/why-cant-you-add-attributes-to-object-in-python