Why Can’t the Assignment Operator (=) Be Used in Expressions in Python?

Why Can’t the Assignment Operator (=) Be Used in Expressions in Python?

In Python, the assignment operator (=) is not an expression but a statement. This means it does not return a value, so using it inside expressions (like if or while conditions) will cause a SyntaxError.

1. Incorrect Usage of = in Expressions (SyntaxError)

if (x = 10):  # ❌ SyntaxError
    print(x)

🔴 Error Output:

SyntaxError: invalid syntax

✅ Fix: You must assign x outside the if condition.

x = 10
if x > 5:
    print(f"x is {x}")  # ✅ Works correctly

2. Using the Walrus Operator (:=) Instead

Python 3.8 introduced the walrus operator (:=), which allows assignment within expressions.

✅ Correct Usage with :=

if (x := 10) > 5:
    print(f"x is {x}")  # Output: x is 10

💡 Why does this work?

• x := 10 assigns 10 to x and returns 10, which is then used in the if condition.

3. Using := in while Loops

Before Python 3.8, you had to assign values outside the loop.

data = input("Enter something: ")
while len(data) > 0:
    print(f"You entered: {data}")
    data = input("Enter something: ")

🔴 Problem: input() is called twice.

✅ Using Walrus Operator (:=) to Simplify

while (data := input("Enter something: ")):  # Stops when input is empty
    print(f"You entered: {data}")

✔ Cleaner and avoids calling input() twice.

4. Using := in List Comprehensions

Before :=, you had to write:

numbers = [10, 20, 30, 40]
squared = [num ** 2 for num in numbers if num ** 2 > 500]

🔴 Problem: num ** 2 is computed twice.

✅ With := (Avoids Recalculation)

numbers = [10, 20, 30, 40]
squared = [square for num in numbers if (square := num ** 2) > 500]
print(squared)  # Output: [900, 1600]

✔ Efficient because square is computed once.

For more details, see Walrus Operator (:=) in Python