How do I assign a property to an instance in Python?

Question

Using python, one can set an attribute of a instance via either of the two methods below:

>>> class Foo(object):
    pass

>>> a = Foo()
>>> a.x = 1
>>> a.x
1
>>> setattr(a, 'b', 2)
>>> a.b
2

One can also assign properties via the property decorator.

>>> class Bar(object):
    @property
    def x(self):
        return 0


>>> a = Bar()
>>> a.x
0

My question is, how can I assign a property to an instance?

My intuition was to try something like this...

>>> class Doo(object):
    pass

>>> a = Doo()
>>> def k():
    return 0

>>> a.m = property(k)
>>> a.m
<property object at 0x0380F540>

... but, I get this weird property object. Similar experimentation yielded similar results. My guess is that properties are more closely related to classes than instances in some respect, but I don't know the inner workings well enough to understand what's going on here.

score 7 · Accepted Answer · edited May 23 '17 at 11:49

7

It is possible to dynamically add properties to a class after it's already created:

class Bar(object):
    def x(self):
        return 0

setattr(Bar, 'x', property(Bar.x))

print Bar.x
# <property object at 0x04D37270>
print Bar().x
# 0

However, you can't set a property on an instance, only on a class. You can use an instance to do it:

class Bar(object):
    def x(self):
        return 0

bar = Bar()

setattr(bar.__class__, 'x', property(bar.__class__.x))

print Bar.x
# <property object at 0x04D306F0>
print bar.x
# 0

See How to add property to a class dynamically? for more information.

edited May 23 '17 at 11:49

Community

1
1

answered Aug 19 '11 at 06:58

agf

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1

I'm pretty sure they need this to be per-instance. What you propose would affect all instances. – Ross Patterson Aug 19 '11 at 07:01
Argh! 1 minute before me!! :) +1 – rubik Aug 19 '11 at 07:02
3

@Ross - I know, it says that in the answer. It simply isn't possible to add a property to an instance. See the thread I linked. – agf Aug 19 '11 at 07:02

Ross Patterson · Answer 2 · 2011-08-19T07:10:14.263

Properties use descriptors which only work on classes and thus for all instances. But you could use a combination of a descriptor on a class that would consult a per-instance function.

>>> class Foo(object):
...     @property
...     def x(self):
...         if 'x' in self.__dict__:
...             return self.__dict__['x'](self)
... 
>>> a = Foo()
>>> def k(self):
...     return 0
... 
>>> a.__dict__['x'] = k
>>> a.x
0

score 1 · Answer 3 · answered Aug 19 '11 at 07:01

You can assign the property directly to the class object:

>>> class Foo(object):
    pass    
>>> a = Foo()
>>> a.__class__
__main__.Foo    
>>> setattr(a.__class__, 'm', property(lambda self: 0))
>>> a.m
0 
>>> a.m = 24
AttributeError: can't set attribute

score 0 · Answer 4 · answered Jun 21 '17 at 23:38

Here we have taken @agf's solution and used a lambda function to define the class property.

class A(object):
    pass

a = A()
a.__class__.f = property(lambda self: 57)
a.f  # 57

The following post provides more context: https://crosscompute.com/n/jAbsB6OIm6oCCJX9PBIbY5FECFKCClyV/_/Assign%20a%20class%20property%20to%20an%20instance

How do I assign a property to an instance in Python?

4 Answers4

Linked