Explain the principle of the null coalescing operator and its application in programming.

The walrus operator (:=) in programming is an assignment operator with a null check or a conditional assignment operator. In Python, the "walrus operator" usually refers to the operator for assigning the result of an expression within another expression, introduced in version 3.8 and denoted as :=.

It allows assigning a value to a variable within an expression context, which reduces code and improves readability.

Example of usage:

# Without the walrus operator
line = input()
while line != '':
    print(line)
    line = input()

# With the walrus operator
while (line := input()) != '':
    print(line)

Here, the variable line gets its value inside the loop condition, eliminating the need to write a separate assignment line before the loop and at the end of the loop body.