Question

Write a single Python statement that calls silly2 and results in the output: 2 2 We're dead, Fred

def silly2(nums, indices, extra=None):
print(len(nums), len(indices))

new_indices = []
for i in indices:
if i >= 0 and i < len(nums):
new_indices.append(i)

if extra:
new_indices.append(extra)

try:
for index in new_indices:
if index < len(nums):
print(nums[index])
except IndexError:
print("We're dead, Fred")
  
silly2([1, 2], [0, -1])

Answer 1

Answer:

silly2( [1,2] , [-2, -1] , -3 )

Explanation:

Since we need 2 2 at the output and we can find a statement print(len(nums), len(indices)), it is obvious that the size/length of both list we pass to the function should be 2 such that len(nums) = 2 and len(indices) = 2 .

Now the question is what should be the nums list and indices list.

First consider nums list. Since we are not using any values in nums list, it doesn't matter which values we give to that list. (Note that there is a print statement which prints the values of nums, but we don't want that statement to execute as per given output. That print statement is controlled by other aspects which we will consider below)

Now what should be the indices list.

For this, we should have a greater understanding of what the program does.

The program basically selects some indices from the indices list such that these indices are less than the size/length of nums list and greater than 0. (Alternatively we are selecting some valid indices for nums list).

These indices are then added to a new list( new_indices ). An extra is also added if present. The program then prints the values present in nums list at valid positions specified by new_indices. (ie.., for each index value in new_indices , the program print value present index position of nums list if index is less than size of nums list)

Now, from output specification, we can find that we don't want to print any values of nums list, but wants an exception Indexerror which happens when a invalid index is tried with a list. ie.., the new_indices must not contain any valid indices for nums list and must contain at least one invalid index such that it is less that size/length of nums. Now since, when adding elements from from indices to new_indices, we are checking both conditions (i>=0 and i< len(nums) ), only valid indices will get added to new_indices. But we don't want that (we don't want to print any values in numslist). Therefore indices must contains two elements such that the are not valid (either less than 0 or greater than or equal to length of nums) for nums list. Therefore -2,-1 which are less than 0.

Now the new_indices must contain an invalid index which is less than length of nums list. The only way of adding this is through extra. Instead of taking default value for extra, we need to give some value that satisfies our condition. So which value for extra.

We can give any invalid index value less than length of nums list. Since python allows to index using negative number (eg -1 for last element, -2 for second last element ) we need to give to give a negative number such that nums list gets out of bound. ie..., a negative value less than negative of number of elements in nums list ( < - len(nums) ) and hence choose -3.

Now -3 will be added to new_indices and since it is less than size/length of nums list, it will try to print nums[-3] which results in a index error and it will output: We're dead, Fred. Therefore after combining previous output, it will print 2 2 We're dead, Fred.