don't append to lists like this please

lists.py

def append_10(the_list=None):
    if the_list is None:
        the_list = []
    the_list.append(10)
    return the_list

original_list = [4, 5]
print('unused list in top level scope')
print('\toriginal_list: ', original_list, sep='\t')

updated_list = append_10(the_list=original_list)
print('appending 10 to list argument in a function')
print('\tupdated_list: ', updated_list, sep='\t')
print('\toriginal_list:', original_list, sep='\t')

original_list.append(15)
print('appending 15 to the orginal in top level scope')
print('\toriginal_list:', original_list, sep='\t')
print('\tupdated_list:', updated_list, sep='\t')

updated_list.append(20)
print('appending 20 to the updated in top level scope')
print('\toriginal_list:', original_list, sep='\t')
print('\tupdated_list:', updated_list, sep='\t')

print('\nThe updated list is a reference to the orgiginal list.')
print('Changing values of the orgiginal_list means you change the value of the updated_list.')
print('Changing values of the updated_list **also** means you change the value of the original_list.')
print('Generally don\'t append to the list you enter as an argument to a function!' )
print('append_10() default behavior', append_10(), sep='\n\t')
print('As you can see this is different!')
print('append_10() again, default behavior', append_10(), sep='\n\t')
print('A new list is initialized each time in the scope of the function')
print('NOT modifying the original_list in the outer scope.')