Python has a built-in decorator @property. In this article, you will learn
why and how to use it.
The following super basic Location class is used:
def Location(object):
def __init__(self, longitude, latitude):
self.longitude = longitude
self.latitude = latitude
Getters and Setters
Let's say, you want to add a range check for the properties of the class. You could make something like:
class Location(object):
def __init__(self, longitude, latitude):
self.set_latitude(latitude)
self.set_longitude(longitude)
def set_latitude(self, latitude):
if not (-90 <= latitude <= 90):
raise ValueError('latitude was {}, but has to be in [-90, 90]'
.format(latitude))
self.latitude = latitude
def set_longitude(self, longitude):
if not (-180 <= longitude <= 180):
raise ValueError('longitude was {}, but has to be in [-180, 180]'
.format(longitude))
self.longitude = longitude
which would then be called like this:
my_position = Location(48.137222222222, 11.575555555556)
# Update after a while
my_position.set_latitude(48.2)
my_position.set_longitude(42.6)
This is what Java developers would do. Note that there is only the convention
that tells you that the latitude property has a set_latitude setter and
a get_latitude getter.
Property class
With Python 2.2, the property class was added to simplify it (source).
With that, you can change the class to
class Location(object):
"""Representation of a point on Earth."""
def __init__(self, longitude, latitude):
self.set_latitude(latitude)
self.set_longitude(longitude)
def set_latitude(self, latitude):
if not (-90 <= latitude <= 90):
raise ValueError('latitude was {}, but has to be in [-90, 90]'
.format(latitude))
self._latitude = latitude
def set_longitude(self, longitude):
if not (-180 <= longitude <= 180):
raise ValueError('longitude was {}, but has to be in [-180, 180]'
.format(longitude))
self._longitude = longitude
def get_longitude(self):
return self._latitude
def get_latitude(self):
return self._longitude
latitude = property(get_latitude, set_latitude)
longitude = property(get_latitude, set_latitude)
# Usage
my_position = Location(48.137222222222, 11.575555555556)
my_position.latitude = 48.2
my_position.longitude = 42.6
my_position.latitude = 123 # Fails
Here is how the help of this class looks like:
Help on class Location in module __main__:
class Location(builtins.object)
| Representation of a point on Earth.
|
| Methods defined here:
|
| __init__(self, longitude, latitude)
| Initialize self. See help(type(self)) for accurate signature.
|
| get_latitude(self)
|
| get_longitude(self)
|
| set_latitude(self, latitude)
|
| set_longitude(self, longitude)
|
| ----------------------------------------------------------------------
| Data descriptors defined here:
|
| __dict__
| dictionary for instance variables (if defined)
|
| __weakref__
| list of weak references to the object (if defined)
|
| latitude
|
| longitude
There is also a leading double underscore. The interpreter changes the name of the attribute to prevent naming colisions. Just have a look at `dir(some_example_class)`. See also: [What's the meaning of underscores (_ & __) in Python variable names?](https://www.youtube.com/watch?v=ALZmCy2u0jQ)
And then note how the objects attribte latitude now is not a float anymore, but a class!
This makes use of __setattr__ and __getattr__. So by using this, there is a strong relationship
between the getter/setter and the attribute!
So we simplified using the class, but the class itself now looks not so nice
anymore. Also, the help is polluted by the getters and setters.
Property decorator
The property decorator is what should be used in Python (source). It makes the behaviour less error-prone when you inherit from such a class.
Here is how it is used:
class Location(object):
"""Representation of a point on Earth."""
def __init__(self, longitude, latitude):
self.latitude = latitude
self.longitude = latitude
@property
def latitude(self):
"""Latitude of the location. North is positive, south is negative."""
return self._latitude
@property
def longitude(self):
"""Longitude of the location. East is positive, west is negative."""
return self._longitude
@latitude.setter
def latitude(self, latitude):
if not (-90 <= latitude <= 90):
raise ValueError('latitude was {}, but has to be in [-90, 90]'
.format(latitude))
self._latitude = latitude
@longitude.setter
def longitude(self, longitude):
if not (-180 <= longitude <= 180):
raise ValueError('longitude was {}, but has to be in [-180, 180]'
.format(longitude))
self._longitude = longitude
# Usage
my_position = Location(48.137222222222, 11.575555555556)
my_position.latitude = 48.2
my_position.longitude = 42.6
my_position.latitude = 123 # Fails
Here is how the help of this class looks like:
Help on class Location in module __main__:
class Location(builtins.object)
| Representation of a point on Earth.
|
| Methods defined here:
|
| __init__(self, longitude, latitude)
| Initialize self. See help(type(self)) for accurate signature.
|
| ----------------------------------------------------------------------
| Data descriptors defined here:
|
| __dict__
| dictionary for instance variables (if defined)
|
| __weakref__
| list of weak references to the object (if defined)
|
| latitude
| Latitude of the location. North is positive, south is negative.
|
| longitude
| Longitude of the location. East is positive, west is negative.