deleting a specific record from a binary file in python

Question

each record in binary files is stored like this

import pickle
student={}
def add():
    
    n=int(input("enter number of entries"))
    for i in range(n):
        name=input("enter names")
        marks=eval(input("enter marks"))
        student[name]=marks
    f=open("sample.dat","wb")
    pickle.dump(student,f)
    f.close()

if want to delete a specific record with name=<"some name"> how to do it

i tried like this but its not correct

def delete():
    x=input("enter name to be deleted")
    for i in student:
        if x==i:
            del student[x]

        else:
            print("name not found")

Asked By: joel.t.mathew

||

Source

Answer 1

First of, a couple of improvements

In your code you del an element of student while you iterate over it.
This is an error, while iterating over a dictionary you should not modify its keys.
For more information, read this:
https://www.geeksforgeeks.org/python-delete-items-from-dictionary-while-iterating/
You don’t need to iterate over the dictionary at all. Dictionary keys are unique, so checking if x is in the dictionary is sufficient. You can do that with if x in students:.
Don’t explicitely close file descriptors. Use context managers instead.
Like here: https://cmdlinetips.com/2016/01/opening-a-file-in-python-using-with-statement/

For everything else, I have to guess, because your question is not specific enough.

I guess that you expect that when you call the delete() function that the file content changes. But the delete() function does not write to any file, so maybe you forgot saving the new student dictionary to the file?

Just a wild guess:

def delete():
    x=input("enter name to be deleted")
    if x in student:
        del student[x]
    else:
        print("name not found")

    with open("sample.dat","wb") as f:
        pickle.dump(student,f)

Answered By: Finomnis

Answer 2

You can not modify the contents of dictionary while iterating over it.

What you can do though is iterate over an array of keys of dictionary and delete items from dictionary if it matches.

i.e change your delete function to:

def delete():
x=input("enter name to be deleted")
for key in list(dict.keys(student)):
    if key==x:
        print('matched')
        del student[key]

Answered By: Pawan Sharma

Answer 3

import pickle
def delete():
    f = open('bin.dat', 'rb+')
    n=int(input('Enter roll no. which you want to delete:'))
    i=[]
    v=[]
    flag=0
    try:
        while True:
            i = pickle.load(f)
            v.append(i)
            if i[0]==n:
                v.remove(i)
                print("successfully deleted!!!!!!")
                flag=1
    except Exception:
        if flag==0:
            print("record not found")
        f.close()
    with open("bin.dat",'wb') as g:
        for i in range(len(v)):
            pickle.dump(v[i],g)

Answered By: Dev Varshney

deleting a specific record from a binary file in python

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