Problems with version control for dictionaries inside a python class

I'm doing something wrong in the code below. I have a method (update_dictonary) that changes a value or values in a dictionary based on what is specificed in a tuple (new_points).

Before I update the dictionary, I want to save that version in a list (history) in order to be able to access previous versions. However, my attempt below updates all dictionaries in history to be like the latest version.

I can't figure out what I'm doing wrong here.

test_dict = {'var0':{'var1':{'cond1':1,
                            'cond2':2,
                            'cond3':3}
                    }
            }

class version_control:
    
    def __init__ (self, dictionary):
        self.po = dictionary
        self.history = list()
        self.version = 0

    def update_dictionary(self, var0, var1, new_points):
            po_ = self.po
            self.history.append(po_)

            for i in new_points:
                 self.po[var0][var1][i[0]] = i[1]

            self.version += 1
    
    def get_history(self, ver):
        return self.history[ver]

a = version_control(test_dict)

new_points = [('cond1', 2),
             ('cond2', 0)]

a.update_dictionary('var0', 'var1', new_points)

new_points = [('cond3', -99),
             ('cond2', 1)]

a.update_dictionary('var0', 'var1', new_points)

print(a.get_history(0))
print(a.get_history(1))

Solution

Try this

from copy import deepcopy

...
def update_dictionary(self, var0, var1, new_points):
    po_ = deepcopy(self.po)
    self.history.append(po_)

    for i in new_points:
        self.po[var0][var1][i[0]] = i[1]

    self.version += 1
...

The problem here is that when you assign po_= self.po you expect po_ to a new variable with a new memory id but actually, you just make a shallow copy(same memory id) of your dictionary. This means if you update the self.po then op_ will automatically update.

To solve this problem by using deepcopy from the copy module(Built-in). It will create a new variable.

You can use this code to save the data into a JSON file.

import json

class version_control:

    def __init__(self, dictionary):
        self.po = dictionary
        self.version = 0

        self.ZEROth_version()

    def update_dictionary(self, var0, var1, new_points, version=None):
        self.version += 1

        for i in new_points:
            self.po[var0][var1][i[0]] = i[1]

        # set the ver to version if given else set to self.version
        ver = self.version if version is None else version

        with open("version.json", "r") as jsonFile:
            # loading the data from the file. 
            data = json.load(jsonFile)

            data[str(ver)] = self.po

        with open("version.json", "w") as jsonFile:
            # save the updated dictionary in json file
            json.dump(data, jsonFile, indent=4)

    def get_history(self, ver):
        try:
            with open("version.json", "r") as jsonFile:
                # I don't use .get here. I will catch key errors in except block. I don't want to add an if statement to check for None.  But string if you want you can add that.
                return json.load(jsonFile)[str(ver)]
        # Checking if the file not exists or is empty
        except (json.decoder.JSONDecodeError, FileNotFoundError, KeyError) as e:
            print("File or Version not found")

    def ZEROth_version(self):

        with open("version.json", "w") as f:
            data = {0: self.po}
            json.dump(data, f, indent=4)

I have explained some main points if you want more explanation then comment, I will reply as soon as possible.