This question is related to iteration and mutability in python; The relevant lessons are

• loops
• lists, mutability

For this question, the following code will be executed by python (2.7):

for datum in data:
  datum *= 2 #equivalent to datum = datum * 2
print data

The code will be run sequentially on the following data:

i.    data = [1,2,3,4]
ii.   data = [[1,2],[3,4]]
iii.  data = [(1,2),(3,4)]

What is the output in each case?

a.
  [2,4,6,8]
  [[2,4],[6,8]]
  [(2,4),(6,8)]
b.
  [1,2,3,4]
  [[1,2],[3,4]]
  [(1,2),(3,4)]
c.
  [2,4,6,8]
  [[1,2,1,2],[3,4,3,4]]
  [(1,2,1,2),(3,4,3,4)]
d.
  [1,2,3,4]
  [[1,2,1,2],[3,4,3,4]]
  [(1,2),(3,4)]

Correct answer is d.

• a.
  • Incorrect understanding of mutation: ints and tuples are immutable, lists are mutable
  • Incorrect understanding of * operator OR misunderstanding of iteration:
    • data is not unpacked twice in the for loop
    • * on a list or tuple returns something like (L,L), and does not do any arithmetic.
• b. Incorrect understanding of mutation: ints and tuples are immutable, lists are mutable
  • The list will be changed in-place
• c. Incorrect understanding of mutation: ints and tuples are immutable, lists are mutable
  • The tuple will not be changed in-place
  • The ints will not be changed in-place

What is output in the following code (under the same conditions):

computed = [datum*2 for datum in data]
print data

a.
  [1,2,3,4]
  [[1,2],[3,4]]
  [(1,2),(3,4)]
b.
  [1,2,3,4]
  [[1,2],[3,4]]
  [(1,2),(3,4)]
c.
  [2,4,6,8]
  [[1,2,1,2],[3,4,3,4]]
  [(1,2,1,2),(3,4,3,4)]
d.
  [1,2,3,4]
  [[1,2,1,2],[3,4,3,4]]
  [(1,2),(3,4)]