Access class when used as class-level variable in Python

Question

I do apologize if this question is already answered on SO or if my problem could be solved by a simple Google search, but I don't know the terminology to describe it, other than the question title, which didn't turn up any results.

Because I don't know the terminology, the best I can do is give an example of what I want.

class MyClassProperty():
    def __init__(self):
        # somehow access MyObject class
        pass

class MyObject():
    var = MyClassProperty()

MyClassProperty will not only need to reference the MyObject class, otherwise it would be simple. The end goal is to be able to automatically add methods and variables to the MyObject class when a MyClassProperty is instantiated at class level.

I have seen frameworks and libraries do this before, the one that first comes to mind is Kivy, with its properties.

Thanks for any help you can give, even if the extent of that help is to tell me this is impossible.

You need to access class methods of MyObject or you actually want to act on an instance of MyObject inside MyClassProperty — Stefano Sanfilippo
– Stefano Sanfilippo, Commented Jan 19, 2014 at 14:57
@Nabla it's not clear what is the relation between the two classes, inheritance might not be appropriate. — Stefano Sanfilippo
– Stefano Sanfilippo, Commented Jan 19, 2014 at 14:58
@StefanoSanfilippo is correct, there is no relation between the two classes except that MyClassProperty is used as a class variable for MyObject. I could act on an instance if that is easier, but I would want to act on all instances. I thought it would be easier to add methods and variables to the MyObject class that would be automatically added to instances — KFox
– KFox, Commented Jan 19, 2014 at 15:01
Can you give some examples on what kind of methods and variables would be added? — poke
– poke, Commented Jan 19, 2014 at 15:04
You can raise an Exception when MyClassProperty inits, inspect the stacktrace and add MyObject to an internal list of objects instantiating MyClassProperty, but it is ugly. Explicit is better than implicit... Why not just use the "mixin" pattern? — Paulo Scardine
– Paulo Scardine, Commented Jan 19, 2014 at 15:06

Vlad · Accepted Answer · 2014-01-19 15:07:03Z

1

It looks like you might want traits.

The only existing Python implementation I could find on a cursory search is here: http://code.enthought.com/projects/traits.

Multiple inheritance might also work for you.

class Some(object):
  a = 97
  b = 98
  def f(self):
    return self.a + self.b + self.c

class Other(object):
  c = 99

class Thing(Some, Other, object):
  pass

o = Thing()
p = Thing()
p.a, p.b, p.c = 65, 66, 67

print o.a, o.b, o.c, o.f() # 97 98 99 294
print p.a, p.b, p.c, p.f() # 65 66 67 198

edited Jan 19, 2014 at 15:07

answered Jan 19, 2014 at 15:00

Vlad

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1 Comment

Paulo Scardine Over a year ago

@KFox: I would like to know what solution you found acceptable, traits or inheritance?

Paulo Scardine · Accepted Answer · 2014-01-19 16:34:14Z

Explicit is better than implicit. The idiomatic way to handle composition is using "mixins":

class MyFooMixin():
    def foo(self):
        print("I have a foo")

class MyBarMixin():
    def bar(self):
        print("This is a bar, where is my beer?")

class MyObject(MyFooMixin, MyBarMixin):
    pass

>>> x = MyObject()
>>> x.foo()
I have a foo
>>> x.bar()
This is a bar, where is my beer?

That said, at some implementations you can inspect the current frame. This is a naive approach to get you started (I'm sure it is wrong but I don't feel like digging all the frame structure stuff in order to write a proper implementation):

class Foo(object):
    _instantiators = []
    def __init__(self):
        try:
            raise Exception("foo")
        except Exception as foo:
            for frame in reversed(inspect.stack()):
                try:
                    for line in frame[4]:
                        if line.startswith('class '):
                            class_name = re.match(r'class\s+(\S+?)\(', line).group(1)
                            self._instantiators.append(frame[0].f_globals[class_name])
                            break
                except (IndexError, TypeError):
                    pass

class Bar(object):
    foo = Foo()

if __name__ == '__main__':
    bar = Bar()
    print Foo._instantiators

This abominable hack will print:

[<class '__main__.Bar'>]

This is probably wrong in so many ways, but Python is very hackable (my second programming language was Lisp, after BASIC, in Python I miss lispy features like hygienic macros and tail call optimization).

This is very close to what I want, but not quite. I would prefer to have the variable at class level though. I'll keep this in mind as a backup.

OBu · Accepted Answer · 2014-01-19 15:06:47Z

0

I'm nor sure whether I got you correctly, but is this what you want?

class MyObject(object):

    def __init__(self):
        pass

    def my_class_property(self):
        self.property_a = []
        self.property_b = 42

my_object = MyObject()
print my_object.property_a #fails
my_object.my_class_property()
print my_object.property_a
print my_object.property_b

answered Jan 19, 2014 at 15:06

OBu

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