GATE DA 2026 | Question: 6

Question

Consider the given Python program.

def append_to_lst(val, lst=[]):
    lst.append(val)
    return lst
print(append_to_lst(1))
print(append_to_lst(2))
print(append_to_lst(3, []))

Which of the following is the correct output of this program?

• A. $\begin{array}{l}{[1]} \\ {[2]} \\ {[3]}\end{array}$
• B. $\begin{array}{l}{[1]} \\ {[1,2]} \\ {[3]}\end{array}$
• C. $\begin{array}{l}{[1]} \\ {[2]} \\ {[1,2,3]}\end{array}$
• D. $\begin{array}{l}{[1]} \\ {[1,2]} \\ {[1,3]}\end{array}$

Answer 1

Remember:

 

In Python, default arguments are evaluated only once at the time of function definition.  

If the default argument is mutable (like a list), the same object is reused across function calls.

 

 

First call:
\[
\texttt{append_to_lst(1)} \rightarrow [1]
\]

Second call (same default list reused):
\[
\texttt{append_to_lst(2)} \rightarrow [1, 2]
\]

Third call (new empty list passed explicitly):
\[
\texttt{append_to_lst(3, [])} \rightarrow [3]
\]

 

Final Output:
\[
[1]
\]
\[
[1, 2]
\]
\[
[3]
\]

 

Hence, the correct option is:
\[
{(B)}
\]