Question

Consider the following Python code: 

• A. Output of line Q is [10, 20]
• B. Output of line S is [10, 20, 40]
• C. Output of line R is [10, 20, 30]
• D. F1 & F2 share the same list

Hint:

Each function call to outer() creates a new closure with its own independent variable environment.

Answer 1

The Correct Option is A

Step 1: Understanding the Topic
This question tests the concept of closures in Python. A closure is a function object that remembers values in the enclosing scope even if they are not present in memory. When the `outer` function is called, it creates a new list `x` and defines an `inner` function that has access to this specific `x`. The `outer` function then returns the `inner` function.
Step 2: Key Approach - Tracing Execution with Independent Closures
The crucial point is that each call to `outer()` creates a new and independent execution environment. Therefore, the list `x` created when `F1 = outer()` is completely separate from the list `x` created when `F2 = outer()`. We need to trace the calls for `F1` and `F2` separately.
Step 3: Detailed Explanation
A. Trace calls involving F1:


`F1 = outer()`: A new list, let's call it `x1`, is created (`x1 = []`). The `F1` variable now holds an `inner` function that operates on `x1`.
`print(F1(10))`: The value `10` is appended to `x1`. `x1` becomes `[10]`. The function returns and prints `[10]`.
`print(F1(20))`: The value `20` is appended to the same list, `x1`. `x1` becomes `[10, 20]`. The function returns and prints `[10, 20]`. This means statement (A) is correct.
B. Trace calls involving F2:


`F2 = outer()`: Another, completely separate list, let's call it `x2`, is created (`x2 = []`). The `F2` variable now holds a different `inner` function that operates on `x2`.
`print(F2(30))`: The value `30` is appended to `x2`. `x2` becomes `[30]`. The function returns and prints `[30]`. This means statement (C) is incorrect.
`print(F2(40))`: The value `40` is appended to `x2`. `x2` becomes `[30, 40]`. The function returns and prints `[30, 40]`. This means statement (B) is incorrect.
C. Evaluate the final statement:
(D) F1 & F2 share the same list: As shown by our trace, `F1` operates on `x1` and `F2` operates on `x2`. They are independent lists. This means statement (D) is incorrect.
Step 4: Final Answer
The only correct statement is (A). The output of the second print statement, which calls `F1(20)`, is `[10, 20]`.