Why are certain elements of this python list ignored?

Question

Hi,

I'm new to Python, and i'm struggling to understand the output of this simple program:

list = os.listdir(os.getcwd())
print(list)
print()
for element in list:
    print(element)
    if 'txt' not in element: list.remove(element)

Which gives me this output :

['examples_list.txt', 'generate_patterns.py', 'lehoczky_example_3.txt', 'patterns', 'test.py', 'test2.py']

examples_list.txt
generate_patterns.py
patterns
test2.py

why are certain elements (such as 'lehoczky_example_3.txt'), ignored by the "for" loop?

Answer 1

You cannot remove elements from the list while looping through it. Try and loop over a copy of the list, or create a new one with only the elements you want to keep.

See also Python strange behavior in for loop or lists

Answer 2

Try list comprehension. This will keep your list of files and create a new list noTxtFiles without touching the files.

files = os.listdir(os.getcwd())
noTxtFiles = [ element for element in files if 'txt' not in element ]

And don't use the reserved word list for a variable name.