First, let's understand the difference between expressions and statements.

An expression resolves to a value, while a statement does not. For example, 4-2 is an expression. if clauses, for and while loops are all statements. Function and class definitions are statements as well.

An assignment expression is a new syntax introduced in Python 3.8. It is commonly known as the walrus operator (:= a colon followed by an equal sign).

The walrus operator assigns a value to a variable but acts as an expression instead of an assignment. The best way to understand the usage of the walrus operator is through an example:

revenue = 1000000

opex = 900000

profit = revenue - opex

if profit == 100000:
    bonus = profit * .1
    print(bonus)
else:
    print("No bonus this year.")

# output
# 10000.0

In the code example above, on line 5, we assigned the value returned from the expression revenue - opex to the profit variable. Then, we used the profit variable as part of a conditional expression in the if statement on line 7.

The most common use case of the walrus operator is the scenario above - when a value needs to be evaluated and then immediately used in the statements that follow.

We can rewrite the program above using the walrus operator as follows:

revenue = 1000000

opex = 900000

if profit := revenue - opex:
    bonus = profit * .1
    print(bonus)
else:
    print("No bonus this year.")

# output
# 10000.0

In the code example above, on line 5, we have computed the result of an expression (revenue - opex) and assigned the value to the profit variable, all in a single statement.

As we can see, using the walrus operator helps us reduce code repetition.