🔒 partially copy an object | Python | Page 1

unkempt sandal Sep 19, 2024, 5:55 PM

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I want this:

class Obj:
  def __init__(self): 
    self.number = 10
    self.seq = [1,2,3]
    self.seq2 = [3,4,5]
  def calculate(self): 
    return sum(self.seq) + self.number + len(self.seq2)

obj = Obj()
# so I want to semi-copy `obj`. With the only difference that `seq` refers to a differnt object. So for example
obj2 = semicopy(obj)
# now if I say
obj2.seq = [0.1, 0.2, 0.3] # it will only change on obj2
# but if I say
obj1.seq2 = [0.3, 0.4, 0.5] # it will change on both because it isnt copied

pearl topazBOT Sep 19, 2024, 5:55 PM

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unkempt sandal I want this: ```py class Obj: def __init__(self): self.number = 10 se...

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@unkempt sandal

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pliant socket Sep 19, 2024, 5:57 PM

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that should already be the case

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a new list will be constructed for each object

unkempt sandal Sep 19, 2024, 5:57 PM

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pliant socket a new list will be constructed for each object

I want seq2 to be the same list

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not a new copy

pliant socket Sep 19, 2024, 5:57 PM

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wait what

unkempt sandal Sep 19, 2024, 5:57 PM

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but not like that, I want it to just be a reference to obj.seq2

pliant socket Sep 19, 2024, 5:57 PM

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you just said you wanted it to be a different object

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"With the only difference that seq refers to a differnt object"

unkempt sandal Sep 19, 2024, 5:58 PM

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pliant socket you just said you wanted it to be a different object

yeah I need obj2 to be a partially different object, like all variables are the same, and changing them will change them on obj1, except seq

pliant socket Sep 19, 2024, 5:58 PM

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so seq should be a shared list between all the instances?

unkempt sandal Sep 19, 2024, 5:59 PM

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well I can do that

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actually

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yeah

pliant socket Sep 19, 2024, 5:59 PM

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yeah, then use a class var

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otherwise this sounds like an xy problem

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!xy

pearl topazBOT Sep 19, 2024, 5:59 PM

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xy-problem

The XY problem can be summarised as asking about your attempted solution, rather than your actual problem.

Often programmers will get distracted with a potential solution they've come up with, and will try asking for help getting it to work. However, it's possible this solution either wouldn't work as they expect, or there's a much better solution instead.

For more information and examples, see http://xyproblem.info/.

unkempt sandal Sep 19, 2024, 5:59 PM

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no, I can do that by copying the object, and then setting that list to a copy, all other things refer to same objects

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I need it to be a reference to a variable

pliant socket Sep 19, 2024, 6:00 PM

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you can't reference a variable

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only its data

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a variable is already a reference to data

unkempt sandal Sep 19, 2024, 6:04 PM

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pliant socket otherwise this sounds like an xy problem

okay I have an array and it works like this: I have a bunch of transforms, where I set some value to the array, they get applied to that value. For example transform that multiplies that value by 2, meaning whatever value I set to my array, it gets multiplied by 2. And when I access my array, opposite transform gets applied so it gets divided by 2. Now there is a lot of other logic on both transformed and untransformed arrays

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and I want to create a new array which is just one of my arrays but with an additional transform on top

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But I want it to still refer to same underlying data

pliant socket Sep 19, 2024, 6:05 PM

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I'd create an inherited class then. It's fundamentally different behaviour

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and then you could make your new list a property

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!e

class Obj:

    def __init__(self):
        self.seq = [1, 2, 3]


class TransObj(Obj):

    def __init__(self, seq):
        self._seq = seq

    @property
    def seq(self):
        return [i * 2 for i in self._seq]



obj = Obj()
trans_obj = TransObj(obj.seq)

print(trans_obj.seq)

pearl topazBOT Sep 19, 2024, 6:07 PM

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pliant socket !e ```py class Obj: def __init__(self): self.seq = [1, 2, 3] clas...

:white_check_mark: Your 3.12 eval job has completed with return code 0.

[2, 4, 6]

pliant socket Sep 19, 2024, 6:07 PM

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something like this

unkempt sandal Sep 19, 2024, 6:08 PM

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pliant socket I'd create an inherited class then. It's fundamentally different behaviour

I wanted it to be dynamic, but I have an idea

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so then I need to create a new object with the same class as the original object, so I do this, new object = existing object.__class__.__new__(existing object.__class__), and then I would copy all existing objects stuff over except this new thing, but isn't that slow for a lot of attributes, is there maybe a better way to do this

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no that won't work

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and then I could say new object.getattr = function that gets it from existing object unless its that attribute that i want to copy

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okay

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obj2.__getattr__ = replace_getattr this part doesnt replace anything

pliant socket Sep 19, 2024, 6:31 PM

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unkempt sandal I wanted it to be dynamic, but I have an idea

is my result not dynamic? If object 1 changes, so does the trans obj

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!e

class Obj:

    def __init__(self):
        self.seq = [1, 2, 3]


class TransObj(Obj):

    def __init__(self, seq):
        self._seq = seq

    @property
    def seq(self):
        return [i * 2 for i in self._seq]



obj = Obj()
trans_obj = TransObj(obj.seq)

print(trans_obj.seq)
obj.seq.append(10)
print(trans_obj.seq)

pearl topazBOT Sep 19, 2024, 6:31 PM

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pliant socket !e ```py class Obj: def __init__(self): self.seq = [1, 2, 3] clas...

:white_check_mark: Your 3.12 eval job has completed with return code 0.

001 | [2, 4, 6]
002 | [2, 4, 6, 20]

unkempt sandal Sep 19, 2024, 6:41 PM

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pliant socket is my result not dynamic? If object 1 changes, so does the trans obj

but lets say obj also has seq2, I want trans_obj.seq2 to reference obj.seq2, and the same for all other methods and attributes. Only seq must be different.

unkempt sandal Sep 19, 2024, 6:44 PM

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pliant socket !e ```py class Obj: def __init__(self): self.seq = [1, 2, 3] clas...

this is what I want

obj.seq = [1,2,3]
obj2 = obj.with_transforms(Rescaled(2))
obj2.seq # [2,4,6]
obj2.seq.append(8)
obj.seq # [1,2,3,4]

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okay

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I have a good example

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so lets say I have a dictionary

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dict(a = [1,2,3], b = [4,5,6])

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I want to create a new dictionary where b is a copy but a is not

pliant socket Sep 19, 2024, 6:48 PM

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are the amount of sequences dynamic?

unkempt sandal Sep 19, 2024, 7:08 PM

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pliant socket are the amount of sequences dynamic?

yes

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from functools import partial


class Wrap:
    def __init__(self, obj):
        self.obj = obj
        self.x =7
    def __getattr__(self, attr):
        attr = getattr(self.obj, attr)
        if callable(attr):
            return partial(attr, self)
        return attr

class A:
    x = 5
    def stuff(self): return self.x

w = Wrap(A)

w.stuff()

this seems to work (for some reason)

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says 7 correctly

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I don't understand why it works tho

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but I won't complain

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the only issue is that I have some callables that are not methods and it is going to be annoyhing to filter them

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I found a fix

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if attr not in self.obj.dict: do the partial thing

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there is still an issue which is that dunder methods i have to define them manually

unkempt sandal Sep 19, 2024, 7:42 PM

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full version works even nested

import functools

class AttrHook:
    def __init__(self, obj):
        self.obj = obj
        self.x = 9

    def __getattr__(self, name):
        attr = getattr(self.obj, name)
        if callable(attr) and name not in self.obj.__dict__:
            return functools.partial(attr, self)
        return attr

class A:
    def __init__(self): self.x = 0
    def stuff(self, *args, **kwargs): 
        print(f'{self = }')
        print(f'{args = }')
        print(f'{kwargs = }')
        n = args[0]
        return self.x, n

w = AttrHook(A)
w2 = AttrHook(w)
w2.stuff(6) # says (9, 6) so it works

pearl topazBOT Sep 19, 2024, 8:42 PM

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#🔒 partially copy an object