Day 51: Saving and Loading Data

• “Auto-save” can be done by adding an open, .write, and .close function at the end of the inside of a loop, e.g.

  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()
  

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

  

  • eval() has to be used on a file that exists already**

Day 52: Avoiding Crashes

• The new construct to get around errors is called try.... except

  

  

  myStuff = []
  
  try:
    f.open("Stuff.mine","r")
    myStuff = eval(f.read())
    f.close()
  # Try to find a file called 'Stuff.mine' and open it
  
  except:
    print("ERROR: Unable to load")
  # If the file can't be found, show the error instead of crashing the whole program
    
  
  for row in myStuff:
    print(row)
  

• there are problems with just putting the whole code  in a 'try except'.

  • to know what sort of error has occurred, you can tell except what type of erros to look for

  • Exception  (capital 'E') means 'every type'. I've captured the error type in the 'err' variable and printed it out to tell me what the error is.

  • Here's a list  of some built in except  error codes.

    

    except Exception as err:
      print("ERROR: Unable to load")
      print(err)
    

• We could even get rid of the 'err' variable entirely and print a traceback, which will show you the red error tracing you see when python crashes.

  debugMode = True
  myStuff = []
  
  try:
    f.open("Stuff.mine","r")
    myStuff = eval(f.read())
    f.close()
  # Try to find a file called 'Stuff.mine' and open it
  
  except Exception:
    print("ERROR: Unable to load")
  
    if debugMode:
      print(traceback)
  
  for row in myStuff:
    print(row)
  

• try is not finished without an except.

• Replit code for pizza program (the solution walkthrough was really helpful too)

Day 53: project day video game inventory

• skipping as well. replit’s code:

new function learned:

also: except, pass (”silences any and all exceptional conditions that come up while the code covered in the try: block is being run”) - apparently it’s bad coding practice though, but essentially just means skip past doing anything with the error that is found

Day 54: Comma-separated values (CSVs)

• Python has built-in libraries for csvs

  

• 👉  join allows us combine lists in a more interesting way

  import csv 
  
  with open("January.csv") as file: 
    reader = csv.reader(file) 
    line = 0
  
    for row in reader: 
      print (", ".join(row)) # adds a comma and space and then joins data, you could try joining with tabs too with `\\t`
  

• 👉 The trick is to treat the CSV like a dictionary, using the csv.DictReader() function. In the code below, I've filtered it so that it only shows the net total from each day

  import csv # Imports the csv library
  
  with open("January.csv") as file: 
    reader = csv.DictReader(file) # Treats the file as a dictionary 
    line = 0
    for row in reader: 
      print (row["Net Total"])
  

• adding net totals from each day to create a total - notice the casting data as a float because library treats it as text