instance of object versus object value python

Question

I having difficulties to understand the instance of an object in a list. How to save the value of an object into a list without saving the instance? This is not possible isnt it? The colde below works but i would like to avoid to use .value as i might have several parameters.. not sure if i am clear enough..

class BougieBuffer:
    def __init__(self):
        self.bougiebuffer=deque(maxlen = 10000)
        self.maximum = Maximum()

    def update(self,bougie):
        self.maximum.value = random.randint(-1,1)
        bougie.maximum.value = self.maximum.value
        self.bougiebuffer.append(bougie)
        print len(self.bougiebuffer)
        for i in range (len(self.bougiebuffer),0,-1):
            print self.bougiebuffer[i-1].prixFermeture,   self.bougiebuffer[i-1].maximum.value

I would have wrote naturally something like below but obviously this is not working and it returns the same value for all

bougie.maximum = self.maximum

The value of of an object? An instance is an object, it's class is an object, the instance attributes are all objects. What are you trying to achieve here? — Martijn Pieters
– Martijn Pieters, Commented Apr 18, 2013 at 10:11
Hi Martin, I am trying to save in bougie.maximum the self.maximum, however the class maximum contains several attributes (ie value, time, etc) i dont want to have to write bougie.maximum.value = self.maximum.value and repeat this for all different attributes of the class so i need to write bougie.maximum = self.maximum to "save" everything in one line, however it didnt work and it "saved" only the last instance for all. — matel
– matel, Commented Apr 18, 2013 at 10:24
I found out though at the meantime if i am "resetting" the instance and use self.maximum = Maximum() at the end of the method update it does the expected results : every items in bougiebuffer carries their own "maximum" value. Could you please explain with your words why do we need to write self.maximum = Maximum() at the end of update? — matel
– matel, Commented Apr 18, 2013 at 10:24
So you want to create a copy of the Maximum() instance to assign to the other class. — Martijn Pieters
– Martijn Pieters, Commented Apr 18, 2013 at 10:25
thats right, but i couldn't find a way to have this copy "independant" and not getting changed anymore if it makes sense... — matel
– matel, Commented Apr 18, 2013 at 10:27

Martijn Pieters · Accepted Answer · 2013-04-18 10:28:06Z

1

You want to create a copy of the Maximum() instance to assign to the bougie.maximum attribute; use either copy.copy or copy.deepcopy:

from copy import deepcopy

bougie.maximum = deepcopy(self.maximum)

You'll need deepcopy if there are any attributes of Maximum that are mutable; a list, dict, set or another custom class instance are all mutable, but things like integers and strings are not.

answered Apr 18, 2013 at 10:28

Martijn Pieters

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