The problem Link to heading

For an object to respond to <, <=, >, >=, == and !=, Python expects you to implement __lt__, __le__, __gt__, __ge__, __eq__ and __ne__. That is six nearly identical methods, tedious to write and easy to leave inconsistent (one says a < b and another disagrees).

The tip Link to heading

Two pieces cover everything:

  • The dataclass already generates __eq__, comparing all the object’s fields.
  • The functools.total_ordering decorator derives __le__, __gt__ and __ge__ from __eq__ and one ordering method.

In other words, you write only __lt__ and get the complete set.

Code Link to heading

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from dataclasses import dataclass
from decimal import Decimal
from functools import total_ordering


@dataclass(frozen=True)
@total_ordering
class Money:
    amount: Decimal
    currency: str = "BRL"

    def __lt__(self, other):
        if not isinstance(other, Money):
            return NotImplemented
        return self.amount < other.amount

The isinstance avoids comparing Money with unrelated types: returning NotImplemented lets the other side try. With that, sorting a list of prices with sorted() starts working for free.

Where the example came from Link to heading

The Money class is the same one from the article “The Money Object”, taken from capital_gains/money.py, a refactoring of a technical assignment where I explored more advanced Python techniques.

That’s it, folks!

See you next time!

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