Python sub Method

Last modified April 8, 2025

This comprehensive guide explores Python's __sub__ method, the special method that implements subtraction operation (-). We'll cover basic usage, vector math, custom behavior, and practical examples.

Basic Definitions

The __sub__ method is called to implement the subtraction operator (-). When you write x - y, Python attempts to call x.__sub__(y).

Key characteristics: it must accept two parameters (self and other), returns the result of subtraction, and can be overridden to customize behavior. It works with both built-in and custom types.

Basic sub Implementation

Here's a simple class implementing __sub__ to demonstrate basic subtraction behavior between objects of the same type.

basic_sub.py

class Number:
    def __init__(self, value):
        self.value = value
    
    def __sub__(self, other):
        if isinstance(other, Number):
            return Number(self.value - other.value)
        return NotImplemented

a = Number(10)
b = Number(3)
result = a - b
print(result.value)  # Output: 7

This example shows how to implement basic subtraction between Number objects. The __sub__ method checks if the other operand is also a Number.

Returning NotImplemented tells Python to try the reverse operation (__rsub__) if available. This maintains operator flexibility.

Vector Subtraction

The __sub__ method is commonly used for mathematical operations like vector subtraction, where we subtract corresponding components.

vector_sub.py

class Vector:
    def __init__(self, x, y):
        self.x = x
        self.y = y
    
    def __sub__(self, other):
        if isinstance(other, Vector):
            return Vector(self.x - other.x, self.y - other.y)
        return NotImplemented
    
    def __repr__(self):
        return f"Vector({self.x}, {self.y})"

v1 = Vector(5, 7)
v2 = Vector(3, 2)
result = v1 - v2
print(result)  # Output: Vector(2, 5)

This Vector class implements component-wise subtraction. The __sub__ method creates and returns a new Vector with subtracted components.

The __repr__ method provides a readable string representation of the vector, making debugging and output clearer.

Subtracting Different Types

__sub__ can handle operations between different types by checking the type of the other operand and implementing appropriate behavior.

mixed_sub.py

class Currency:
    def __init__(self, amount, currency_code):
        self.amount = amount
        self.currency_code = currency_code
    
    def __sub__(self, other):
        if isinstance(other, Currency):
            if self.currency_code != other.currency_code:
                raise ValueError("Cannot subtract different currencies")
            return Currency(self.amount - other.amount, self.currency_code)
        elif isinstance(other, (int, float)):
            return Currency(self.amount - other, self.currency_code)
        return NotImplemented
    
    def __repr__(self):
        return f"{self.amount} {self.currency_code}"

usd1 = Currency(100, "USD")
usd2 = Currency(30, "USD")
print(usd1 - usd2)  # 70 USD
print(usd1 - 15)    # 85 USD

This Currency class allows subtraction between Currency objects of the same type and also supports subtracting numbers directly from the amount.

The method includes type checking and currency validation to ensure only meaningful operations are performed. This makes the class more robust.

Reverse Subtraction with rsub

When the left operand doesn't support subtraction with the right operand, Python tries the reverse operation using __rsub__.

reverse_sub.py

class Temperature:
    def __init__(self, value):
        self.value = value
    
    def __sub__(self, other):
        if isinstance(other, Temperature):
            return Temperature(self.value - other.value)
        return NotImplemented
    
    def __rsub__(self, other):
        if isinstance(other, (int, float)):
            return Temperature(other - self.value)
        return NotImplemented
    
    def __repr__(self):
        return f"{self.value}°C"

temp = Temperature(10)
result1 = temp - Temperature(3)  # Uses __sub__
result2 = 20 - temp              # Uses __rsub__
print(result1)  # 7°C
print(result2)  # 10°C

This example shows both normal and reverse subtraction. When Python encounters 20 - temp, it first tries int.__sub__ which fails.

Then it tries temp.__rsub__(20) which succeeds. This makes the class more flexible when used with built-in types.

In-Place Subtraction with isub

The __isub__ method implements in-place subtraction (-=) which modifies the object instead of creating a new one.

inplace_sub.py

class Account:
    def __init__(self, balance):
        self.balance = balance
    
    def __sub__(self, other):
        if isinstance(other, (int, float)):
            return Account(self.balance - other)
        return NotImplemented
    
    def __isub__(self, other):
        if isinstance(other, (int, float)):
            self.balance -= other
            return self
        return NotImplemented
    
    def __repr__(self):
        return f"Account(balance={self.balance})"

acc = Account(100)
acc -= 30  # Uses __isub__
print(acc)  # Account(balance=70)

The __isub__ method modifies the object's state directly and returns self. This is more efficient than __sub__ when you don't need a new object.

For mutable objects, implementing __isub__ can provide better performance by avoiding unnecessary object creation.

Best Practices

Type checking: Always validate operand types in __sub__
Return NotImplemented: For unsupported operations
Implement __rsub__: For reverse operation support
Consider __isub__: For efficient in-place operations
Maintain consistency: Ensure subtraction follows mathematical rules

Source References

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Python __sub__ Method