Question

I am trying to populate a list called images with files that have the extensions '.png', '.jpg' or 'jpeg' in a one line for loop. I tried this by using the logical or operator but that only resulted in an empty list.

images = [i for i in os.listdir('.') if i[-4:] == ('.png' or '.jpg' or 'jpeg')]

How do I check to see if '.png', '.jpg' or 'jpeg' are the last 4 characters of the file while keeping it all in a one line for loop?

Answer 1

Using or is a boolean expression which checks if any value in the expression evaluates to True.

i[-4:] == ('.png' or '.jpg' or 'jpeg')

evaluates if i[-4:] is equal to:

('.png' or '.jpg' or 'jpeg') 

which checks if each value is True and returns the first True value or the last value if there are no True values. In this case they all evaluate to True as only empty strings evaluate to False so '.png' is the result of the expression.

You can fix this by doing

[i for i in os.listdir('.') if i[-4:] in ('.png', '.jpg', 'jpeg')]

Or a better way

[i for i in os.listdir('.') if os.path.splitext(i)[1] in ('.png','.jpg','.jpeg')]