Look at this small snippet of Python code:
x = TooMuchAlcohol()
x.value = 10
print x.value
x.value = 'foo'
print x.value
x.value = [1,2,3]
print x.value
Seems like nothing special, but look what it spits out:
20
foofoo
[1, 2, 3, 1, 2, 3]
Oh my god, I see everything double!
While it looks like we just write and read the attribute value of the object x, there is actually some additional processing taking place behind the scenes.
The trick is the Python built-in property. With this function (or is it a class?) you can hide getter and setter functions behind what looks like standard attribute access.
As an illustration, the implementation of the TooMuchAlcohol class used above is as follows:
class TooMuchAlcohol(object):
def _get_value(self):
return self._value * 2
def _set_value(self, value):
self._value = value
value = property(_get_value, _set_value)
The setter function _set_value() just stores the value in a "private" attribute _value. The getter function _get_value() returns this value multiplied by two. And finally, the magic is in the last line, where the getter and setter are tied to the public (class) attribute value.
For more info:
- The Python documentation on "property"
- This blog post by Adam Gomaa gives a nice, more in-depth discussion