Pot: Class properties

As most programmers, I’ve been trained with the getter/setter idiom. It allows a programmer in any object-oriented programming language to create a class interface which makes sure that only valid data is written.

An example of a class with getter/setter in Python:

class Person1(object):
    def __init__(self):
        self._age = None
    def get_age(self):
        return self._age
    def set_age(self, age):
        self._age = age

We just wrote two methods - four lines - only to be able to modify and access some data. And we have to do that for every property. This is such a common idiom, that IDEs even have wizards for that.

Python takes a different road: properties. For starters, let’s write the simplest possible class with the behaviour from above:

class Person2(object):
    age = None

Usage of this class is a lot more natural - you just assign values - and it’s a lot shorter.

But the reason people started writing setters was access control. Maybe you want to make sure that no negative age is assigned? That’s where properties come in.

class Person3(object):
    _age = None
    def get_age(self):
        return self._age
    def set_age(self, age):
        if age < 0:
            raise Exception("Age can't be negative.")
        self._age = age
    age = property(get_age, set_age)

It’s almost the same as the first class - even a bit more code. But the usage of this class is exactly the same as before.

As you can see properties are extremely useful. They give you the flexibility of becoming more strict with the interface. But you don’t have to pass on this verbosity to the user. It also takes into account that most of the time you don’t care about the input values. So for the great majority of cases where the setter would really just be the one line, your class definition stays light and beautiful.

The full code including usage examples of this classes is available as a syntax-colored paste on Lodge It.

This post is part of the Python on the toilet series.