Exploring the Role of .__lt__() and .__gt__() in min() and max()

00:00 Exploring the Role of .__lt__() and .__gt__() in min() and max(). As you’ve learned so far, the built-in min() and max() functions are versatile enough to work with values of various data types, such as numbers and strings.

00:15 The secret behind this flexibility is that min() and max() embrace Python’s duck typing philosophy by relying on the .__lt__() and .__gt__() special methods.

00:26 These methods are part of what Python calls rich comparison methods. Specifically, .__lt__() and .__gt__() support the less than (<) and greater than (>) operators, respectively.

00:36 But what’s the meaning of support here? When Python encounters a line such as seen on-screen, internally it does this. The takeaway is that you can use min() and max() with values of any data type that implement .__lt__() and .__gt__().

00:53 That’s why these functions work with values of all Python’s built-in data types.

01:12 As you can see, Python’s built-in data types implement the .__lt__() and .__gt__() special methods. So, you can feed any of these data types into min() and max(), with the only condition being that the involved data types are comparable.

01:30 You can also make instances of custom classes compatible with min() and max(). To achieve this, you need to provide your own implementations of .__lt__() and .__gt__().

01:42 Consider the Person class seen on-screen as an example of this compatibility.

02:15 Note that the implementation of both dunder methods requires an argument that’s typically named other. This argument represents the second operand in the underlying comparison operations.

02:26 For example, an expression such as x < y, x will be self and y will be other. Here, the dunder methods return the result of comparing two people’s .birth_date attributes.

02:40 You can see this working in practice on-screen.

03:01 You can process Person objects with min() and max() because the class provides implementations of .__lt__() and .__gt__().

03:09 The call to min() returns the youngest person, and the call to max() returns the oldest. The .__lt__() and .__gt__() methods provides support for only two comparison operators, < and >.

03:25 If you ever want a class that provides all the comparison operators, but you only want to write a few special methods, then you can use the @functools.total_ordering decorator.

03:36 If you have a class that defines .__eq__() and other rich comparison methods, then this decorator will automatically supply the rest of the comparison methods.

03:44 Note that if a given custom class doesn’t provide these methods, then its instances won’t support min() and max() operations.

04:13 Because the Number class doesn’t provide suitable implementations of .__lt__() and .__gt__(), min() and max() respond with a TypeError.

04:21 The error message tells you that the comparison operations aren’t supported in the current class.

04:29 In the next section of the course, you’ll see how to emulate min() and max() with pure Python code.