I can't understand this problem

[muhammedkhairy]

So sorry, I'm a beginner with programming and I face something in lists confused me.
I know that when I concatenate 2 lists with (+) I create a new list.
I'm using Python 2.7

list1 = [1,2,3]
list1 = list1 + [4,5]
print list1

here I get list1 = [1,2,3,4,5]
but where I put this in a procedure I don't get the same result

list2 = [3,4,5]
def mylist(my_list):
    my_list = my_list + [6,7]
mylist(list2)
print list2

here I get list2 = [3,4,5]
Why that?
another question why I get "none" message when I try to print

print mylist(list2)

. What is the difference between calling the procedure then print it

mylist(list2)
print list2

and print directly the procedure

print mylist(list2)