Trying to implement a cache on load of this file

Question

The cache would have an initial size of 20 elements and upon reaching its limit, to add any new element it would remove the least recently accessed element. On shutdown it should store the cached data back to the file. The data should be stored in the cache according to a caching strategy. Provide options for cache CRUD. Testing Data set : records of student.

import json
from collections import OrderedDict
import time
import os

if os.path.exists("qwerty.json"):
    record = json.load(open("qwerty.json", "r"), object_pairs_hook=OrderedDict)
else:
    record = OrderedDict({})

fo = open("foo.txt", "wb")

x = list(record.items())[:20]; x2 = sorted(x, key=lambda k: k[1]['time'], reverse=True)
print(x2)

command = ""
while command != 'exit':
    command = input('Enter a command(options: create,read,save): ')
    if command == "create":
        name = input('Enter name of the Student:')
        p = input('Student ID: ')
        a = input('Class: ')
        n = input('Marks: ')
        time = time.time()

        record[name] = {'Student ID:': p, 'Class:': a, 'Marks': n, 'time': time }

    elif command == 'read':
        z = json.load(open("qwerty.json", "r"), object_pairs_hook=OrderedDict)
        print(z)

    elif command == 'save':
        json.dump(record, open('qwerty.json', "w"))

fo.close()

Answer 1

You can actually maintain order with a single file, using a combination of json and collections.OrderedDict.

Your initial setup is like so:

from collections import OrderedDict
phone_book = OrderedDict({})

When creating, add elements into an ordered dict and then dump it as JSON. The order of keys is preserved. After you declared phone_book like above, the rest of the code for create remains the same. Note that when you write to the file, you don't close it, so you can't read the contents later. This should be replaced with something like:

import os

if os.path.exists("qwerty.json")
    phone_book = json.load(open("qwerty.json", "r"), object_pairs_hook=OrderedDict)
else:
    phone_book = OrderedDict({})

command = ""
while command != 'exit':
    command = input('Enter a command(options: create,read,save): ')
    if command == "create":
        ...

    elif command == 'read':
        ...

    elif command == 'save':
        json.dump(phone_book, open('qwerty.json', "w"))

For reading, you'll have to make some changes:

elif command == 'read':
    z = json.load(open("C:\\Users\\qwerty.txt", "r"), object_pairs_hook=OrderedDict)
    ...

This loads the dict in the order the keys were stored. You can now call list(z.items())[-20:] to get only the last 20 items. Also, when reading a particular key, you update its "last-read-time" by deleting and recreating it:

import copy
key = ...
temp = copy.copy(z[key])
del z[key]
z[key] = temp

This will update the position of key in the dict. This should be enough for you to implement the rest yourself.