List capital letters

[DeaD_EyE]

This should work.

lista =[]
def capital_letters(arg):
   for i in arg:
       if i.isupper():
           lista.append(i) # append the i to the list
   return lista # return the list after the for-loop is done
 
print(capital_letters('tZtTtT'))

arg, i and lista is a bad name.

You could write the function as generator:

def filter_capital_letters(text):
    for char in text:
        if char.isupper():
            yield char
            # the yield statement makes a generator
            # the generator must be consumed

list(filter_capital_letters('tZtTtT'))

Or you could write it as list comprehension or generator expression.

text = 'Hello World'
upper_chars = [char for char in text if char.isupper()]

upper_chars_genetator = (char for char in text if char.isupper())
# upper_chars_generator does nothing until it's consumed by
# a list/tuple or in a for-loop
# this interesting for data, which does not fit in memory.