Chaining Comparisons with lt gt etc

Learn how to implement custom chained comparisons in Python using rich comparison methods like __lt__ and __gt__ for more expressive code.

Chaining Comparisons in Python with __lt__, __gt__, and Others

In Python, comparison operators like <, >, ==, and != can be customized for your own classes. This is possible thanks to Python’s rich comparison methods such as __lt__ (less than), __gt__ (greater than), __eq__ (equals), and others. They allow objects to behave intuitively when compared and enable complex chained comparisons like if a < b < c.

Why Customize Comparisons?

Custom comparisons make your objects integrate naturally with Python’s syntax. For instance, a Box class can compare instances by their volume, making conditions like if box1 > box2 meaningful. You can even enable chained comparisons for more readable and concise logic.

Implementing Rich Comparison Methods

To enable this, define the rich comparison dunder methods in your class:

• __lt__(self, other): less than
• __le__(self, other): less than or equal
• __gt__(self, other): greater than
• __ge__(self, other): greater than or equal
• __eq__(self, other): equals
• __ne__(self, other): not equals

Example with a Box class:

class Box:
    def __init__(self, length, width, height):
        self.volume = length * width * height

    def __lt__(self, other):
        return self.volume < other.volume

    def __gt__(self, other):
        return self.volume > other.volume

    def __eq__(self, other):
        return self.volume == other.volume

Testing Chained Comparisons

Let’s create some boxes and compare them:

box1 = Box(2, 3, 4)  # Volume = 24
box2 = Box(3, 3, 3)  # Volume = 27
box3 = Box(1, 6, 5)  # Volume = 30

print(box1 < box2 < box3)  # True
print((box3 > box1) == False)  # False

Note: Wrap the second comparison in parentheses to avoid unintended attribute errors.

Best Practices

Use functools.total_ordering to minimize boilerplate. Define __eq__ and one other comparison like __lt__, and Python will generate the others.

from functools import total_ordering

@total_ordering
class Box:
    def __init__(self, length, width, height):
        self.volume = length * width * height

    def __eq__(self, other):
        return self.volume == other.volume

    def __lt__(self, other):
        return self.volume < other.volume

Conclusion

By implementing rich comparison methods, your classes gain intuitive and clean comparison support. This is powerful when working with custom data types where ordering and equality have specific meaning.

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