Binary Files with pickle

Binary Files with pickle

The pickle module turns a Python object (a list, a dictionary, a tuple...) into bytes and back. Turning it into bytes is pickling (serialisation); restoring it is unpickling. Pickled data goes into a file opened in a binary mode.

• pickle.dump(obj, file): write one object.
• pickle.load(file): read the next object; raises EOFError when there are no more.

import pickle

with open("students.dat", "wb") as f:          # write records
    pickle.dump({"roll": 1, "name": "Asha", "marks": 88}, f)
    pickle.dump({"roll": 2, "name": "Ravi", "marks": 72}, f)

with open("students.dat", "rb") as f:          # search for roll 2
    try:
        while True:
            rec = pickle.load(f)
            if rec["roll"] == 2:
                print(rec["name"])
    except EOFError:
        pass

To append records open the file with "ab". To update a record, read all the records into a list, change the one you want, and write the whole list back with "wb" (or, with "rb+", seek() back to the record's position, which works only if its pickled size does not change).

import io
import pickle

f = io.BytesIO()   # stands in for open("students.dat", "wb+")
for _ in range(int(input())):
    roll, name, marks = input().split()
    pickle.dump([int(roll), name, int(marks)], f)
f.seek(0)
# load records until EOFError and print the ones with marks >= 75