permanently store a value to be used in a dict

Question

I have a dictionary self.total = defaultdict(int) in my Order class that is used to input and print totals of every item that is ordered in my ordering system. Object are stored as

coffee_available=[Coffee(1, "1: Flat White", 3.50), 
            Coffee(2, "2: Long Black", 3.50), 
            Coffee(3, "3: Cappuccino", 4.00), 
            Coffee(4, "4: Espresso", 3.25), 
            Coffee(5, "5: Latte", 3.50)]
`

and the way I "order" these objects is by calling a function in my order class

    def order(self, order_amount, coffee_id):

        self.total[coffee_id] += order_amount

and i print each item type and their respective order amounts by using another function in my order class

def print_order(self):
    print(" ")
    print("Coffee Type\t\tAmount of Times ordered")                       
    print("-----------\t\t-----------------------")
    for coffee in coffee_available:

        print("{}\t- - -\t {} ".format(coffee.coffee_type,   self.total[coffee.coffee_id]))

this works well as each time my code is run in the same session it stores the order_amount value each time and displays it correctly, the only problem is that if i terminate the program it doesn't store the data for when i open it next. If I were to permanently store data how would I do it and what should i permanently store?

python `pickle` library is the key: https://wiki.python.org/moin/UsingPickle — Harsh Trivedi, Jul 16 '16 at 05:55

Harsh Trivedi · Accepted Answer · 2016-07-16T07:10:42.550

1

So you can make any python object to persist in a file using pickle. So lets say you want to persist the total

import pickle
# total is a dictionary
# to store it in a file
with open('total_dictionary.pickle', 'wb') as handle:
  pickle.dump(total, handle)

# lets say program terminated here

Now,

# to load it, if it was previously stored
import pickle
with open('total_dictionary.pickle', 'rb') as handle:
  total = pickle.load(handle)

# total dictionary is exactly same even if the program terminated.

So, your order method updates your self.total and print_order just views it. So basically you need to store the self.total after every update (order call). And, in the initializer of the class where you first declare self.total, you need to initialze self.total with dictionary loaded from pickled file (if it exists)

Just to be little more specific:

import pickle
..
def order(self, order_amount, coffee_id):
    self.total[coffee_id] += order_amount
    with open('total_dictionary.pickle', 'w') as handle:
        pickle.dump(total, handle)

and in initializer of the Order class

import os
..
def __init__(self):
    ...
    if os.path.exists("total_dictionary.pickle"):
        with open('total_dictionary.pickle', 'r') as handle:
            total = pickle.load(handle)
    else:
        total = defaultdict(int)
    ...

You can remove/delete total_dictionary.pickle to reset.

Hope it helps :)

edited Jul 16 '16 at 07:10

answered Jul 16 '16 at 06:04

Harsh Trivedi

1,594
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so the way I use pickle is to store the values in the self.total dict and call it when I need to call the order function later – killerstick1 Jul 16 '16 at 06:18
the `order` function updates `self.total` and `print_order` accesses it. So all you need to do is: after each update call ie `order` call store the `total` dictionary in file using pickle. And in initializer of the class, make sure that if the pickled file is present then initialize total with that dictionary. Cool ? – Harsh Trivedi Jul 16 '16 at 06:26
thanks for the help, it works fine now. It doesn't store the very first order but thats fine – killerstick1 Jul 16 '16 at 06:48
your update throws this error 'AttributeError: module 'ntpath' has no attribute 'exist' – killerstick1 Jul 16 '16 at 07:07
updated. Mistake was that function is `os.path.exists` and not `os.path.exist`. plural 's'. Also make sure you have imported os module. – Harsh Trivedi Jul 16 '16 at 07:11
works perfect now, thank you – killerstick1 Jul 16 '16 at 07:15
Am glad that it helped :) – Harsh Trivedi Jul 16 '16 at 07:17

permanently store a value to be used in a dict

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