Python Subtraction Assignment

x -= y

​

Underlying Mechanics

1. __isub__(self, other): Python first attempts to invoke the in-place subtraction method on the left operand. If implemented (typically by mutable types), this method modifies the object’s internal state directly and returns self. The identifier x is then reassigned to this same object, meaning the memory address remains unchanged.
2. __sub__(self, other): If the left operand does not implement __isub__ (which is standard for immutable types like int or float), Python falls back to the standard subtraction method. It evaluates x - y, allocates a new object in memory to store the result, and rebinds the identifier x to this new memory address.

​

Behavior with Immutable Types

x = 10
print(id(x))  # Example output: 140732827182152

x -= 3        # Falls back to x = x.__sub__(3)
print(x)      # Output: 7
print(id(x))  # Example output: 140732827182056 (Address changes due to rebinding)

​

Behavior with Mutable Types

s = {1, 2, 3}
print(id(s))  # Example output: 2345678901232

s -= {2}      # Invokes s.__isub__({2})
print(s)      # Output: {1, 3}
print(id(s))  # Example output: 2345678901232 (Address remains identical)