Exploring List Properties 2

Start Quiz:

# What is the difference between these two pieces of code?
list1 = [1,2,3,4,5]
list2 = [1,2,3,4,5]

def proc(mylist):
    mylist = mylist + [6, 7]

def proc2(mylist):
    mylist.append(6)
    mylist.append(7)

# Can you explain the results given by the print statements below?

print "demonstrating proc"
print list1
proc(list1)
print list1
print
print "demonstrating proc2"
print list2
proc2(list2)
print list2

# Python has a special assignment syntax: +=.  Here is an example:

list3 = [1,2,3,4,5]
list3 += [6, 7]

# Does this behave like list1 = list1 + [6,7] or list2.append([6,7])? Write a
# procedure, proc3 similar to proc and proc2, but for +=.

Solution:

Here's one possible solution to this problem:

def proc3(mylist):
    mylist += [6,7]

list4 = [1,2,3,4,5]
proc3(list4)

>>> list4
>>> [1, 2, 3, 4, 5, 6, 7]

Notice that after proc3(list4) is called, list4 now has the elements [6,7] appended to it, just like proc1 and proc2. Thus we can see that using += is the same as adding or appending a value to a variable.