Q.Predict the output of the following code: def callon(b=20,a=10): b=b+a a=b-a print(b,"#",a) return b x=100 y=200 x=callon(x,y) print(x,"@",y) y=callon(y) print(x,"@",y)

This tests positional arguments, default arguments, and how a returned value updates a global variable.

1. x = callon(x, y) calls callon(100, 200), so b = 100 and a = 200. Inside: b = b + a = 300, then a = b - a = 300 - 200 = 100. It prints 300 # 100 and returns 300, so global x becomes 300.
2. print(x, "@", y) prints 300 @ 200 (y is still 200). …

Concept and Intuition

This problem tests your understanding of fundamental Python concepts: function definition, parameter passing, default arguments, local versus global scope, and the flow of execution.

When a function is called, new local variables are created for its parameters. These local variables are distinct from any global variables with the same name. Changes made to parameters inside the function do not directly affect the original global variables unless the function explicitly returns a value that is then assigned back to a global variable.

Python uses "pass by assignment" for arguments. This means that when you pass a variable to a function, the function's parameter variable gets a copy of the reference to the object. For immutable types like integers (which we have here), modifying the parameter inside the function effectively creates a new local integer object for that parameter, leaving the original global variable untouched.

Default arguments come into play when a function is called with fewer arguments than parameters defined. If a parameter has a default value, and no argument is provided for it during the call, the default value is used.

The key to solving this problem is to meticulously trace the values of x and y in the global scope and a and b within each callon function execution, noting when values are printed and when global variables are updated by return values.

Step-by-Step Execution

Let's trace the execution of the code line by line.

1. Initial Global State:

   The script begins by initializing two global variables:

   x = 100

   y = 200

2. First Function Call: x = callon(x,y)

   The callon function is called with x (value 100) and y (value 200) as arguments.

   Inside callon(b=20, a=10):

   • b is assigned the value of the first argument, which is x's value: b = 100.
   • a is assigned the value of the second argument, which is y's value: a = 200.
   • The default values for b and a (20 and 10) are ignored because explicit arguments were provided.

   Now, execute the lines inside the function:

   • b = b + a b = 100 + 200 = 300
   • a = b - a a = 300 - 200 = 100
   Tip

   This sequence of operations (b = b + a, a = b - a) effectively assigns the original value of b to a, and the sum of the original a and b to b.

   If b_old and a_old were the initial values:

   b becomes b_old + a_old

   a becomes (b_old + a_old) - a_old = b_old

   • print(b, "#", a)

     This prints the current local values of b and a.

     Output 1: 300 # 100

   • return b

     The function returns the current local value of b, which is 300.

   Back in the global scope:

   • x = callon(x,y) The global variable x is updated with the returned value. …