Skip to main content

Save to and Load From Files, Auto-Save, Preventing Data Loss

 


Save to and Load From Files

There are some things that primary storage in the RAM does better.
For example, it's easy to access, amend, or remove a piece of data held in a list (in the RAM).
Holding data in secondary storage in a file makes this more difficult. Or does it?
With Python, there's more than meets the eye


👉 The program below lets me add & remove events and dates into a diary system. It adds the name & date of an event to the 2D list. Or it searches for an existing name & date and removes it.

myEvents = []
def prettyPrint():
  print()
  for row in myEvents:
    print(f"{row[0] :^15} {row[1] :^15}")
  print()
while True:
  menu = input("1: Add, 2: Remove\n")
  if menu == "1":
    event = input("What event?: ").capitalize()
    date = input("What date?: ")
    row = [event,date]
    myEvents.append(row)
    prettyPrint()
  else:
    criteria = input("What event do you want to remove?: ").title()
    for row in myEvents:
      if criteria in row:
        myEvents.remove(row)

Manually saving and loading from this program to a file each time would be a massive faff. Instead, we can set up an auto-save by writing the save code at the end of our infinite loop.

Auto-Save

👉 At the bottom of the code, we are going to add an autosave just before the loop repeats.
Make sure this new code matches the indent for the while loop, so it is part of the loop.
myEvents = []
def prettyPrint():
  print()
  for row in myEvents:
    print(f"{row[0] :^15} {row[1] :^15}")
  print()
while True:
  menu = input("1: Add, 2: Remove\n")
  if menu == "1":
    event = input("What event?: ").capitalize()
    date = input("What date?: ")
    row = [event,date]
    myEvents.append(row)
    prettyPrint()
  else:
    criteria = input("What event do you want to remove?: ").title()
    for row in myEvents:
      if criteria in row:
        myEvents.remove(row)
  ########### THIS IS THE NEW BIT ########
  f = open("calendar.txt", "w") # Permissions set to 'w' because we are deleting the file and replacing it with the whole 2D list every time.
  f.write(str(myEvents)) # Need to cast the list to a single string
  f.close()
  ######################################### 


Preventing Data Loss 

Did you find the problem?

Yep. Every time we run the program, it creates a new, blank myEvents[] list which gets written to the file.

This overwrites any events in the file that we saved when we ran the program previously.

To solve this, we set up the program to load any pre-existing data from the file into the myEvents list at the very start of the code.

Pay close attention to the eval() function. It's the special sauce here

👉 eval() takes the text from the file, converts it into running code, and assigns it to myEvents[] as a 2D list. Good, eh?

myEvents = []
####### THIS IS THE NEW BIT ################
f=open("calendar.txt","r") # Only need read permissions here
myEvents = eval(f.read())
f.close()
########################################
def prettyPrint():
  print()
  for row in myEvents:
    print(f"{row[0] :^15} {row[1] :^15}")
  print()
while True:
  menu = input("1: Add, 2: Remove\n")
  if menu == "1":
    event = input("What event?: ").capitalize()
    date = input("What date?: ")
    row = [event,date]
    myEvents.append(row)
    prettyPrint()
  else:
    criteria = input("What event do you want to remove?: ").title()
    for row in myEvents:
      if criteria in row:
        myEvents.remove(row)
  
  f = open("calendar.txt", "w") 
  f.write(str(myEvents)) 
  f.close()




Comments

Post a Comment

Popular posts from this blog

Web Scraping

 Web Scraping Some websites don't have lovely APIs for us to interface with. If we want data from these pages, we have to use a tecnique called scraping. This means downloading the whole webpage and poking at it until we can find the information we want. You're going to use scraping to get the top ten restaurants near you. Get started 👉 Go to a website like Yelp and search for the top 10 reastaurants in your location. Copy the URL.   url = "https://www.yelp.co.uk/search?find_desc=Restaurants&find_loc=San+Francisco%2C+CA%2C+United+States"   Import libraries 👉 Import your libraries. Beautiful soup is a specialist library for extracting the contents of HTML and helping us parse them. Run the Repl once your imports are sorted because we want the Beautiful Soup library to be installed (it'll run quicker this way). import requests from bs4 import BeautifulSoup url = "https://www.yelp.co.uk/search?find_desc=Restaurants&find_loc=San+Francisco%2C+CA%2C+Unite...
Post a Comment
Read more

String Manipulation

   String Manipulation Does this code look familiar from the insult generator project? name = input("What's your name? ") if name == "David" or name == "david":   print("Hello Baldy!") else:    print("What a beautiful head of hair!") Right now, if the user writes "DAVID" or "david", the if statement works correctly. However, "DaVID" does not give the correct output. To the computer, " david", "dAviD", and "david" are completely different. To simplify what the user typed in, we can add these functions to the end of the name of the variable: .lower() = all letters are lower case .upper() = all letters are upper case .title() = capital letter for the first letter of every word .capitalize() = capital letter for the first letter of only the first word    
Post a Comment
Read more

Authentication

 Authentication Efficient code often has the drawback of being very hard to understand at first. It's often very dense, with lots of things happening on a single line of code. That's why teachers often teach what could be described as 'the long way round' when designing lessons on new topics. With all that in mind, don't be mad at us when we say that there is an easier way to create a login system than by using sessions. You've spent the past few days getting a really thorough grounding in what's going on behind the scenes, which was the whole point. No, really. We promise. Replit Authentication Here at Replit, we know that you will probably be using authentication a lot. So we've baked in the feature for you. 👉 Run your code, then head over to your left hand files pane and scroll until you see authentication. Then, erm, turn it on. That's it. Now you will see that your repl uses the default Replit login page. I can also access a bunch of informatio...
Post a Comment
Read more