python – How can I create an object and add attributes to it?

The Question :

336 people think this question is useful

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

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 a for loop.

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

The Question Comments :
  • What do you mean by “didn’t work”? I assume it raised an AttributeError exception, right?
  • yeah. ‘object’ object has no attribute ‘somefield’
  • Why are you doing this? A generic “object” has no actual meaning. What is the meaning of the thing you are creating? Why is it not a proper class or namedtuple?
  • The example is not minimal and confusing for me or I just don’t see why you don’t work with some a = object() and you need obj.a = object(). Again I am talking about the example, in your actual code an object inside an object might be useful.

The Answer 1

231 people think this answer is useful

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.

The Answer 2

356 people think this answer is useful

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)

The Answer 3

146 people think this answer is useful

There is types.SimpleNamespace class in Python 3.3+:

obj = someobject
obj.a = SimpleNamespace()
for p in params:
    setattr(obj.a, p, value)
# obj.a.attr1

collections.namedtuple, typing.NamedTuple could be used for immutable objects. PEP 557 — Data Classes suggests a mutable alternative.

For a richer functionality, you could try attrs package. See an example usage.

The Answer 4

36 people think this answer is useful

The mock module is basically made for that.

import mock
obj = mock.Mock()
obj.a = 5

The Answer 5

33 people think this answer is useful

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()

The Answer 6

22 people think this answer is useful

Now you can do (not sure if it’s the same answer as evilpie):

MyObject = type('MyObject', (object,), {})
obj = MyObject()
obj.value = 42

The Answer 7

21 people think this answer is useful

You can also use a class object directly; it creates a namespace:

class a: pass
a.somefield1 = 'somevalue1'
setattr(a, 'somefield2', 'somevalue2')

The Answer 8

19 people think this answer is useful

Try the code below:

$ python
>>> class Container(object):
...     pass 
...
>>> x = Container()
>>> x.a = 10
>>> x.b = 20
>>> x.banana = 100
>>> x.a, x.b, x.banana
(10, 20, 100)
>>> dir(x)
['__class__', '__delattr__', '__dict__', '__doc__', '__format__', 
'__getattribute__', '__hash__', '__init__', '__module__', '__new__',
'__reduce__', '__reduce_ex__', '__repr__', '__setattr__',     '__sizeof__', 
'__str__', '__subclasshook__', '__weakref__', 'a', 'b', 'banana']

The Answer 9

9 people think this answer is useful

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.

The Answer 10

3 people think this answer is useful

These solutions are very helpful during testing. Building on everyone else’s answers I do this in Python 2.7.9 (without staticmethod I get a TypeError (unbound method…):

In [11]: auth = type('', (), {})
In [12]: auth.func = staticmethod(lambda i: i * 2)
In [13]: auth.func(2)
Out[13]: 4

The Answer 11

1 people think this answer is useful

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: Why can’t you add attributes to object in python?

The Answer 12

1 people think this answer is useful

If we can determine and aggregate all the attributes and values together before creating the nested object, then we could create a new class that takes a dictionary argument on creation.

# python 2.7

class NestedObject():
    def __init__(self, initial_attrs):
        for key in initial_attrs:
            setattr(self, key, initial_attrs[key])

obj = someobject
attributes = { 'attr1': 'val1', 'attr2': 'val2', 'attr3': 'val3' }
obj.a = NestedObject(attributes)
>>> obj.a.attr1
'val1'
>>> obj.a.attr2
'val2'
>>> obj.a.attr3
'val3'

We can also allow keyword arguments. See this post.

class NestedObject(object):
    def __init__(self, *initial_attrs, **kwargs):
        for dictionary in initial_attrs:
            for key in dictionary:
                setattr(self, key, dictionary[key])
        for key in kwargs:
            setattr(self, key, kwargs[key])


obj.a = NestedObject(attr1='val1', attr2='val2', attr3= 'val3')

The Answer 13

0 people think this answer is useful

Coming to this late in the day but here is my pennyworth with an object that just happens to hold some useful paths in an app but you can adapt it for anything where you want a sorta dict of information that you can access with getattr and dot notation (which is what I think this question is really about):

import os

def x_path(path_name):
    return getattr(x_path, path_name)

x_path.root = '/home/x'
for name in ['repository', 'caches', 'projects']:
    setattr(x_path, name, os.path.join(x_path.root, name))

This is cool because now:

In [1]: x_path.projects
Out[1]: '/home/x/projects'

In [2]: x_path('caches')
Out[2]: '/home/x/caches'

So this uses the function object like the above answers but uses the function to get the values (you can still use (getattr, x_path, 'repository') rather than x_path('repository') if you prefer).

The Answer 14

0 people think this answer is useful

I think the easiest way is through the collections module.

import collections
FinanceCtaCteM = collections.namedtuple('FinanceCtaCte', 'forma_pago doc_pago get_total')
def get_total(): return 98989898
financtacteobj = FinanceCtaCteM(forma_pago='CONTADO', doc_pago='EFECTIVO',
                                get_total=get_total)

print financtacteobj.get_total()
print financtacteobj.forma_pago
print financtacteobj.doc_pago

The Answer 15

-1 people think this answer is useful
di = {}
for x in range(20):
    name = '_id%s' % x
    di[name] = type(name, (object), {})
    setattr(di[name], "attr", "value")

The Answer 16

-2 people think this answer is useful

Other way i see, this way:

import maya.cmds

def getData(objets=None, attrs=None):
    di = {}
    for obj in objets:
        name = str(obj)
        di[name]=[]
        for at in attrs:
            di[name].append(cmds.getAttr(name+'.'+at)[0])
    return di

acns=cmds.ls('L_vest_*_',type='aimConstraint')
attrs=['offset','aimVector','upVector','worldUpVector']

getData(acns,attrs)

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