CS50 P-Shirt

CS50 shirt

After finishing CS50 itself, students on campus at Harvard traditionally receive their very own I took CS50 t-shirt. No need to buy one online, but like to try one on virtually?

In a file called shirt.py, implement a program that expects exactly two command-line arguments:

  • in sys.argv[1], the name (or path) of a JPEG or PNG to read (i.e., open) as input
  • in sys.argv[2], the name (or path) of a JPEG or PNG to write (i.e., save) as output

The program should then overlay shirt.png (which has a transparent background) on the input after resizing and cropping the input to be the same size, saving the result as its output.

Open the input with Image.open, per pillow.readthedocs.io/en/stable/reference/Image.html#PIL.Image.open, resize and crop the input with ImageOps.fit, per pillow.readthedocs.io/en/stable/reference/ImageOps.html#PIL.ImageOps.fit, using default values for method, bleed, and centering, overlay the shirt with Image.paste, per pillow.readthedocs.io/en/stable/reference/Image.html#PIL.Image.Image.paste, and save the result with Image.save, per pillow.readthedocs.io/en/stable/reference/Image.html#PIL.Image.Image.save.

The program should instead exit via sys.exit:

  • if the user does not specify exactly two command-line arguments,
  • if the input’s and output’s names do not end in .jpg, .jpeg, or .png, case-insensitively,
  • if the input’s name does not have the same extension as the output’s name, or
  • if the specified input does not exist.

Assume that the input will be a photo of someone posing in just the right way, like these demos, so that, when they’re resized and cropped, the shirt appears to fit perfectly.

If you’d like to run your program on a photo of yourself, first drag the photo over to VS Code’s file explorer, into the same folder as shirt.py. No need to submit any photos with your code. But, if you would like, you’re welcome (but not expected) to share a photo of yourself wearing your virtual shirt in any of CS50’s communities!

  • Note that you can determine a file’s extension with os.path.splitext, per docs.python.org/3/library/os.path.html#os.path.splitext.
  • Note that open can raise a FileNotFoundError, per docs.python.org/3/library/exceptions.html#FileNotFoundError.
  • Note that the Pillow package comes with quite a few classes and methods, per pypi.org/project/Pillow. You might find its handbook and reference helpful to skim. You can install the package with:
    pip install Pillow

    You can open an image (e.g., shirt.png) with code like:

    shirt = Image.open("shirt.png")

    You can get the width and height, respectively, of that image as a tuple with code like:

    size = shirt.size

    And you can overlay that image on top of another (e.g., photo) with code like

    photo.paste(shirt, shirt)

    wherein the first shirt represents the image to overlay and the second shirt represents a “mask” indicating which pixels in photo to update.

  • Note that you can open an image (e.g., shirt.png) in VS Code by running
    code shirt.png

    or by double-clicking its icon in VS Code’s file explorer.



before before before


after after after

Before You Begin

Log into cs50.dev, 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 shirt

to make a folder called shirt in your codespace.

Then execute

cd shirt

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

code shirt.py

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

wget https://cs50.harvard.edu/python/2022/psets/6/shirt/shirt.png

to download shirt.png. Also be sure to run

wget https://cs50.harvard.edu/python/2022/psets/6/shirt/muppets.zip

to download muppets.zip into your folder. You can run

unzip muppets.zip

to extract a collection of muppet photos!

How to Test

Here’s how to test your code manually:

  • Run your program with python shirt.py. Your program should exit using sys.exit and provide an error message:
    Too few command-line arguments   
  • Be sure to download muppets.zip and extract a collection of muppet photos using unzip muppets.zip. Run your program with python shirt.py before1.jpg before2.jpg before3.jpg. Your program should output:
    Too many command-line arguments
  • Run your program with python shirt.py before1.jpg invalid_format.bmp. Your program should exit using sys.exit and provide an error message:
    Invalid output
  • Run your program with python shirt.py before1.jpg after1.png. Your program should exit using sys.exit and provide an error message:
    Input and output have different extensions
  • Run your program with python shirt.py non_existent_image.jpg after1.jpg. Your program should exit using sys.exit and provide an error message:
    Input does not exist
  • Run your program with python shirt.py before1.jpg after1.jpg. Assuming you’ve downloaded and unzipped muppets.zip, your program should create an image like the below:

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/shirt

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/shirt