George Adams Blog - Python Properties

28 April 2010, 00:35

Python Properties
When just getting started with Python, it may seem odd that everything is public in Python. Well, yes, everything is public, but there are conventions to denote that something is internal only and should not be accessed outside the scope of the class or module. Anything with a leading underscore is considered internal and it cannot be relied upon.

Say I have a member variable that I can only allow to be an integer, but I also need it to be accessible outside of my class. I can supply traditional getters and setters, but Python has a better way. They are called properties, and there are two ways of implementing them. Each method makes use of the global property function. The signature of this function is: property([fget[, fset[, fdel[, doc]]]]).

The first method, in my opinion, clutters the namespace a bit. However, this is localized to a single class, and it is really no more clutter than a Java class with getters and setters.
```
class Foo(object):
    def __init__(self, x=0):
        self._x = x

    def _get_x(self):
        print 'get'
        return self._x

    def _set_x(self, new_x):
        print 'set'
        if type(new_x) is int:
            self._x = new_x
        else:
            raise AttributeError('x must be an int')

    def _del_x(self):
        print 'del'
        self._x = 0

    x = property(_get_x, _set_x, _del_x)
```
In this Foo class, the getter, setter, and deleter are denoted as internal using the leading underscore. However, a new member is added, x, that uses the supplied functions to perform the retrieval, updating, and deletion of x.

That method is fine, but I much prefer the second method. It just feels cleaner.
```
class Foo(object):
    def __init__(self, x=0):
        self._x = x

    @property
    def x(self):
        print 'get'
        return self._x

    @x.setter
    def x(self, new_x):
        print 'set'
        if type(new_x) is int:
            self._x = new_x
        else:
            raise AttributeError('x must be an int')

    @x.deleter
    def x(self):
        print 'del'
        self._x = 0
```
This second method is identical to the outside world. Obviously, there is no _get_x method any more, but the outside shouldn't be calling that anyway. In 2.6, the getter, setter, and deleter methods were added. They create a copy of the property with the decorated function. In the first example, if you want to create a getter and deleter, but no setter, you would need to type x = property(fget, None, fdel). In the second example, however, you would only use the deleter method of the existing property.

Basically, stop writing getters and setters in Python. Please.
- python
- tutorial
—geowa4

George Adams IV

Software Engineer

28 April 2010, 00:35

Python Properties