Simplest method to edit password in text file Python

Question

I have the following code, that goes through the stages of reading a file in line by line, converting it into a list, editing a field in that list, but the last stage is rewriting the original file, with this edit included.

I would be interested in answers that suggest the easiest/simplest fix (without the use of pandas/numpy etc) and using the original code provided.

My current algorithm includes having to create a new file with all the records, except the one linked to this username, and then writing in this list to it. This seems arduous and unnecessary. Any solutions gratefully received!

The task is explained more clearly in the comments:

Code

 """ ==============TASK
    ALLOW THE USER TO CHANGE OR EDIT THEIR PASSWORD
    1. Search for any given username
    2. Edit the password field for that given username
    3. Save the new updated username and password to file / updated
    """

    import csv
    def main():
        #1. This code snippet asks the user for a username and then allows them to change password for that record
        updatedlist=[]
        with open("fakefacebook.txt",newline="") as f:
          reader=csv.reader(f)
          print("CHANGE PASSWORD?!")
          username=input("Enter the username for the required user:")
          for row in reader: #for every row in the file
              for field in row:

                    if field==username: #if a field is == to the required username
                        updatedlist.append(row) #add each row, line by line, into a list called 'udpatedlist'
                        newpassword=input("Enter new password")
                        #print(updatedlist[0][1]) (this is how we get at the password field, noting it is a nested list)
                        updatedlist[0][1] = newpassword #set the field for password to the new password


          updatepassword(updatedlist)

    def updatepassword(updatedlist):
        with open("fakefacebook.txt","w",newline="") as f:
            Writer=csv.writer(f)
            Writer.writerows(updatedlist)
            print("File has been updated")


    main()

Note: at the moment the code just allows the user to change password (and this is changed in the list). The list is comprised of just that user's record. It overwrites this single record (with the changed password) to the text file, instead of what is required (original file contents + just this edit)

File Contents

username,password,email,no_of_likes
marvR,pass123,marv@gmail.com,400
smithC,open123,cart@gmail.com,200
blogsJ,2bg123,blog@gmail.com,99

Required output

If tested with: marvR change password to: boo123

new file should contain:

username,password,email,no_of_likes
marvR,**boo123**,marv@gmail.com,400
smithC,open123,cart@gmail.com,200
blogsJ,2bg123,blog@gmail.com,99

Any comments/explanations about the best way to approach this for teaching beginners would also be appreciated. It seems odd that Python hasn't developed a module of some sort to make editing a field in a file easier than this 3 step algorithm which is really quite arduous for beginners (i'm talking 13-14 year olds) to work out

Answer 1

One of the related issues was not addressed so far. You have mentioned "teaching beginners", so please consider this answer as a supplement to other answers.

When a file like password file is edited, the original file should not be overwritten like in the code presented here. A data loss is possible if a write fails for any reason including disk full or power outage. Even more important, the file should be always in a consistent state, i.e. nobody should ever see a partially written file.

To achieve that:

1. read the data from file
2. modify it in memory process the data as needed
3. save the result to a new temporary file, close that file (with does it automatically), you might even want to call os.fsync()
4. if there was any error, remove the temporary file and stop here
5. only if everything is fine, atomically rename the temporary file to the original file preferrably with os.replace() (Python 3.3+). Atomic operation are done in one single step.

---

UPDATE: modified updatepassword code with some comments:

FILE = "fakefacebook.txt"

def updatepassword(updatedlist):
    # create the tempfile in the same directory (os.replace requires the same filesystem)
    tempfile = FILE + ".tmp"
    try:
        # "x" = fail with the FileExistsError if the file exists already
        with open(tempfile, "x", newline="") as f:
            writer = csv.writer(f)
            writer.writerows(updatedlist)
            f.flush()               # flush the internal buffers
            os.fsync(f.fileno())    # write to disk
        os.replace(tempfile, FILE)
        print("File has been updated")
        tempfile = None     # tempfile renamed
    except FileExistsError:
        print("Another copy is running or stale tempfile exists")
        tempfile = None     # tempfile does not belong to this process
    except OSError as err:
        print("Error: {}".format(err))
    finally:
        if tempfile:
            try:
                os.unlink(tempfile)
            except OSError:
                print("Could not delete the tempfile")

Answer 2

@VPfB's answer should be taken into consideration as well.

This is just the answer to updating the records without replacing your existing records in the file. But there are better ways to do this.

import csv

def main():
    #1. This code snippet asks the user for a username and then allows them to change password for that record
    updated_list = []
    cached_list = []

    with open("fakefacebook.txt", newline="") as f:
        reader = list(csv.reader(f)) # Convert iterable to list to make things easier.
        print("CHANGE PASSWORD?!")
        username=input("Enter the username for the required user: ")
        cached_list = reader # store copy of data.

        for row in reader: #for every row in the file
            for field in row:  
                if field == username: #if a field is == to the required username
                    updated_list.append(row) #add each row, line by line, into a list called 'udpated_list'
                    newpassword = input("Enter new password: ")
                    #print(updatedlist[0][1]) (this is how we get at the password field, noting it is a nested list)
                    updated_list[0][1] = newpassword #set the field for password to the new password

        update_password(updated_list, cached_list)

def update_password(updated_list, cached_list):
    for index, row in enumerate(cached_list):
        for field in row:
            if field == updated_list[0]:
                cached_list[index] = updated_list # Replace old record with updated record.

    with open("fakefacebook.txt","w", newline="") as f:
        Writer=csv.writer(f)
        Writer.writerows(cached_list)
        print("File has been updated")


main()

Answer 3

You can use a dictionary instead of a list data structure here. Dictionaries map values to user-defined keys rather than integers. So instead of saying iterating through a list to find the correct username, you could simply store the entire file in a data structure and use random access to change the desired username:

from collections import defaultdict
def main():
        inputDict=defaultdict(string)
        with open("fakefacebook.txt",newline="") as f:
          reader=csv.reader(f)
          for row in reader: #for every row in the file
              line = row.split(',') #splits the CSV row into an array

              #sets the key to be the username, and everything else 
              #to be value, and then adds it to inputDict
              inputDict[line[0]] = line[1:] 

          print("CHANGE PASSWORD?!")
          username=input("Enter the username for the required user:")
          newpassword=input("Enter new password")
          if (inputDict[username]):
              inputDict[username] = newpassword
          else: print("No such username!")


          updatepassword(inputDict)

You would have to modify your updatepassword() to work with a dictionary.