“Ah, well,” said Tonks, slamming the trunk’s lid shut, “at least it’s all in. That could do with a bit of cleaning, too.” She pointed her wand at Hedwig’s cage. “Scourgify.” A few feathers and droppings vanished.

Harry Potter and the Order of the Phoenix

Data, too, often needs to be “cleaned,” as by reformatting it, so that values are in a consistent, if not more convenient, format. Consider, for instance, this CSV file of students, before.csv, below:

"Abbott, Hannah",Hufflepuff
"Bell, Katie",Gryffindor
"Bones, Susan",Hufflepuff
"Boot, Terry",Ravenclaw
"Brown, Lavender",Gryffindor
"Bulstrode, Millicent",Slytherin
"Chang, Cho",Ravenclaw
"Clearwater, Penelope",Ravenclaw
"Crabbe, Vincent",Slytherin
"Creevey, Colin",Gryffindor
"Creevey, Dennis",Gryffindor
"Diggory, Cedric",Hufflepuff
"Edgecombe, Marietta",Ravenclaw
"Finch-Fletchley, Justin",Hufflepuff
"Finnigan, Seamus",Gryffindor
"Goldstein, Anthony",Ravenclaw
"Goyle, Gregory",Slytherin
"Granger, Hermione",Gryffindor
"Johnson, Angelina",Gryffindor
"Jordan, Lee",Gryffindor
"Longbottom, Neville",Gryffindor
"Lovegood, Luna",Ravenclaw
"Lupin, Remus",Gryffindor
"Malfoy, Draco",Slytherin
"Malfoy, Scorpius",Slytherin
"Macmillan, Ernie",Hufflepuff
"McGonagall, Minerva",Gryffindor
"Midgen, Eloise",Gryffindor
"McLaggen, Cormac",Gryffindor
"Montague, Graham",Slytherin
"Nott, Theodore",Slytherin
"Parkinson, Pansy",Slytherin
"Patil, Padma",Gryffindor
"Patil, Parvati",Gryffindor
"Potter, Harry",Gryffindor
"Riddle, Tom",Slytherin
"Robins, Demelza",Gryffindor
"Scamander, Newt",Hufflepuff
"Slughorn, Horace",Slytherin
"Smith, Zacharias",Hufflepuff
"Snape, Severus",Slytherin
"Spinnet, Alicia",Gryffindor
"Sprout, Pomona",Hufflepuff
"Thomas, Dean",Gryffindor
"Vane, Romilda",Gryffindor
"Warren, Myrtle",Ravenclaw
"Weasley, Fred",Gryffindor
"Weasley, George",Gryffindor
"Weasley, Ginny",Gryffindor
"Weasley, Percy",Gryffindor
"Weasley, Ron",Gryffindor
"Wood, Oliver",Gryffindor
"Zabini, Blaise",Slytherin


Even though each “row” in the file has three values (last name, first name, and house), the first two are combined into one “column” (name), escaped with double quotes, with last name and first name separated by a comma and space. Not ideal if Hogwarts wants to send a form letter to each student, as via mail merge, since it’d be strange to start a letter with:

Dear Potter, Harry,

Rather than with, for instance:

Dear Harry,

In a file called, implement a program that:

  • Expects the user to provide two command-line arguments:
    • the name of an existing CSV file to read as input, whose columns are assumed to be, in order, name and house, and
    • the name of a new CSV to write as output, whose columns should be, in order, first, last, and house.
  • Converts that input to that output, splitting each name into a first name and last name. Assume that each student will have both a first name and last name.

If the user does not provide exactly two command-line arguments, or if the first cannot be read, the program should exit via sys.exit with an error message.



Before You Begin

Log into, click on your terminal window, and execute cd by itself. You should find that your terminal window’s prompt resembles the below:


Next execute

mkdir scourgify

to make a folder called scourgify in your codespace.

Then execute

cd scourgify

to change directories into that folder. You should now see your terminal prompt as scourgify/ $. You can now execute


to make a file called where you’ll write your program. Be sure to run


to download before.csv into your folder.

How to Test

Here’s how to test your code manually:

  • Run your program with python Your program should exit using sys.exit and provide an error message:
    Too few command-line arguments
  • Create empty files 1.csv, 2.csv, and 3.csv. Run your program with python 1.csv 2.csv 3.csv. Your program should output:
    Too many command-line arguments
  • Run your program with python invalid_file.csv output.csv. Assuming invalid_file.csv doesn’t exist, your program should exit using sys.exit and provide an error message:
    Could not read invalid_file.csv
  • Run your program with python before.csv after.csv. Assuming before.csv exists, your program should create a new file, after.csv, whose columns should be, in order, first, last, and house.

You can execute the below to check your code using check50, a program that CS50 will use to test your code when you submit. But be sure to test it yourself as well!

check50 cs50/problems/2022/python/scourgify

Green smilies mean your program has passed a test! Red frownies will indicate your program output something unexpected. Visit the URL that check50 outputs to see the input check50 handed to your program, what output it expected, and what output your program actually gave.

How to Submit

In your terminal, execute the below to submit your work.

submit50 cs50/problems/2022/python/scourgify