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GO Classes DPP | GATE DA | Python & DSA | Extend Append

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3 Answers

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C. [3,4,5,9,10, None, None]

extend() : Adds all elements of another list to the current list, returns None
append() : Adds a single element to the list, returns None

s.extend([4,5]) leaves us with [3,4,5]

s.append() calls inside the next .extend are evaluated first, that add 9, 10 to s in the same order, while returning None each time.

This creates a list [None, None] to be extended to s. 

$\therefore$ Answer is C. [3,4,5,9,10, None, None]. 

 

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Initially:

s = [3]

After:

s.extend([4, 5])

The list becomes:

[3, 4, 5]

Now observe this line carefully:

s.extend([s.append(9), s.append(10)])

Python first evaluates the list inside $\texttt{extend}$.

s.append(9)

This changes $\texttt{s}$ to:

[3, 4, 5, 9]

But $\texttt{append}$ returns $\texttt{None}$.

Then:

s.append(10)

This changes s to:

[3, 4, 5, 9, 10]

Again, $\texttt{append}$ returns $\texttt{None}$.

So finally:

s.extend([None, None])

Therefore, the output is:

[3, 4, 5, 9, 10, None, None]
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