How to update a pickle file in Python?

Python pickle module is used for serializing and de-serializing a Python object structure. Any object in Python can be pickled so that it can be saved on a disk. What pickle does is that it “serializes” the object first before writing it to file. Pickling is a way to convert a python object (list, dict, etc.) into a character stream. The idea is that this character stream contains all the information necessary to reconstruct the object in another python script

Pickle serializes a single object at a time, and reads back a single object – the pickled data is recorded in sequence on the file. If you simply do pickle.load you would be reading the first object serialized into the file (not the last one as you’ve written). After unserializing the first object, the file-pointer is at the beginning of the next object – if you simply call pickle.load again, it will read that next object – do that until the end of the file

Functions Used:

• load()– used to read a pickled object representation from the open file object file and return the reconstituted object hierarchy specified.

Syntax:

pickle.load(file, *, fix_imports = True, encoding = “ASCII”, errors = “strict”)

• seek(0)– Pickle records can be concatenated into a file, so yes, you can just pickle.load(f) multiple times, but the files themselves are not indexed in a way that would let you seek into a given record. What your f.seek(0) is doing is seeking into the third byte in the file, which is in the middle of a pickle record, and thus is unpickable. If you need random access, you might want to look into the built-in shelve module which builds a dictionary-like interface on top of a pickle using a database file module.
• dump()– used to write a pickled representation of obj to the open file object file

Syntax:

pickle.dump(obj, file, protocol = None, *, fix_imports = True)

• truncate()– changes the file size

To start with, we first have to insert it into a pickle file. The implementation is given below:

Approach

• Import module
• Open file in write mode
• Enter data
• Dump data to the file
• Continue until the choice is yes
• Close File

Program:

Python3

# code
import pickle
print("GFG")


def write_file():

    f = open("travel.txt", "wb")
    op = 'y'

    while op == 'y':

        Travelcode = int(input("enter the travel id"))
        Place = input("Enter the Place")
        Travellers = int(input("Enter the number of travellers"))
        buses = int(input("Enter the number of buses"))

        pickle.dump([Travelcode, Place, Travellers, buses], f)

        op = input("Dp you want to continue> (y or n)")

    f.close()


print("entering the details of passengers in the pickle file")
write_file()

Now since we have data entered into the file the approach to update data from it is given below along with implementation based on it:

Approach

• Import module
• Open file
• Enter some information regarding data to be deleted
• Update the appropriate data
• Close file

Program:

Python3

import pickle


def read_file():

    f = open("travel.txt", 'rb')
    while True:
        try:
            L = pickle.load(f)

            print("Place", L[1], "\t\t Travellers :",
                  L[2], "\t\t Buses :", L[3])

        except EOFError:
            print("Completed reading details")
            break
    f.close()


def update_details():

    f1 = open("travel.txt", "rb+")
    travelList = []
    print("For a example i will be updating only Buses details")
    t_code = int(input("Enter the travel code for the updation: "))

    while True:
        try:
            L = pickle.load(f1)
            if L[0] == t_code:
                buses = int(input("Enter the number Buses ..."))
                L[3] = buses
            travelList.append(L)
        except EOFError:
            print("Completed Updating details")
            break

    f1.seek(0)
    f1.truncate()

    for i in range(len(travelList)):
        pickle.dump(travelList[i], f1)
    else:
        f1.close()


print("update the file")
update_details()
read_file()

Output: