Question

I realized that return breaks in a function thus I need to find an alternate way to return a modified list.

def multiply(lst):
    new = ''
    for i in range(len(lst)):
        var = lst[i] * 3
        new = new + str(var)
    return new
list1 = [1,2,3]
received = multiply(list1)
print [received]

I’m trying to multiply all elements by 3 and return (I have to use return). This just gives returns

[‘369’]

The list I’m trying to return is

[3,6,9]

Answer 1

That’s because new isn’t a list. In the first line of multiply(), you do new = ''. This means that new is a string.

For strings, the addition operator, +, performs concatenation. That is to say:

'a' + 'b' => 'ab'

and similarly:

'1' + '2' => '12'

or

new = ''
new + str(var) => '' + '3' => '3'

Clearly, this is not what you want. In order to do what you want, you should construct a new list and return it:

def multiplyBy3(lst):
    newList = [] #create an empty list
    for i in range(len(lst)):
        newList.append(lst[i] * 3)
    return newList

Of course, accessing the list elements by index is not very “pythonic”. The rule of thumb is: “unless you need to know the indices of the list elements (which you don’t, in this case), iterate over the values of the list”. We can modify the above function accordingly:

def multiplyBy3(lst):
    newList = [] 
    for item in lst: #iterate over the elements of the list
        newList.append(item * 3)
    return newList

Finally, python has something called a list comprehension, which does the same thing as above, but in a much more concise (and faster) way. This is the preferred method:

def multiplyBy3(lst):
    return [item * 3 for item in lst] #construct a new list of item * 3.

For more on list comprehensions, see the following tutorial: http://www.blog.pythonlibrary.org/2012/07/28/python-201-list-comprehensions/