将列表的副本传递给python中的函数

Question

I'm trying to pass a copy of a list to a function in python so I can pop and append to a new list without losing the information in the old list. But I'm having trouble.

here's my code

def show_magicians(magicians):
    for magician in magicians:
        print (magician.title())
        return magicians

def make_great(magicians):
    for magician in magicians:
        new_magician = magicians.pop()
        new_magicians.append(new_magician)
        print (new_magician.title() + ", is a great magician!!")
        return magicians

new_magicians = []
magicians = ['merlin', 'blaine', 'agaybi', 'copperfield']   
show_magicians(magicians)
make_great(magicians[:])
print ('\n' , magicians)
print ('\n' , new_magicians)

The second function is supposed to move the elements from the old list to the new and print the simple statement for each one without emptying the old list. The problem is I get only 1 element printed and the same element is the only element moved.

What am I doing wrong?

Answer 1

You are returning after the first magician is processed

Change the make_great function as follows

def make_great(magicians):
    for magician in magicians:
        new_magician = magicians.pop()
        new_magicians.append(new_magician)
        print (new_magician.title() + ", is a great magician!!")
    return magicians

Answer 2

The existing answer solves your immediate problem, but your code is still unnecessarily confusing, which I think contributed to your getting stuck.

The "correct" way to do this is to avoid mutating magicians within make_great at all. A Python function taking a mutable argument like this should either mutate its argument and return None or create a new object and return that. It should certainly not (for such a trivial task at least) mutate both its argument and an object in the enclosing scope and return that second object.

For example, you could have done:

def make_great(magicians):
    """Make each magician in the input great."""
    new_magicians = []  # create a brand new list
    for name in magicians:  # iterate over old list
        new_magicians.append(name + ' is a great magician!')  # add to new list
    return new_magicians  # note this is outside the for loop

Then you don't need to create new_magicians outside the function, or pass a copy of the original magicians to it:

magicians = ['merlin', 'blaine', 'agaybi', 'copperfield']   
new_magicians = make_great(magicians)
print(magicians)  # still the same
print(new_magicians)  # brand new list

Answer 3

Ok so first of all thank you all so much. I finally figured out the problem and how to solve it. The exercise I'm working on specifically wants me to use the [:] to make a shallow copy of the list and pass it through a function to create a new list by popping.

The problem was I kept the for loop that prints the statement, inside the function instead of placing outside to be global.

Here's the new and correct code:

def show_magicians(magicians):
    for magician in magicians:
        print (magician.title())


def make_great(magicians):
    while magicians:
        new_magician = magicians.pop()
        new_magicians.append(new_magician)


new_magicians = []
magicians = ['merlin', 'blaine', 'agaybi', 'copperfield']
show_magicians(magicians)
make_great(magicians[:])

for magician in magicians:
    print ('\n' + magician.title() + " is a great magician!!")

print ('\n')
print (magicians)
print ('\n')
print (new_magicians)   

Thanks again and sorry for the confusion.