Since Python strives for there to be one right way, I'm wondering what the purpose of property.getter is. In this example WhyMe defines a getter but Other doesn't so I'm wondering what the point of property.getter is over just using property.
class WhyMe(object):
def __init__(self):
self._val = 44
@property
def val(self):
print 'I am not called'
return self._val
@val.getter # What advantage do I bring?
def val(self):
print 'getter called'
return self._val
class Other(object):
def __init__(self):
self._val = 44
@property
def val(self):
print 'I AM called'
return self._val
And using them:
>>> why = WhyMe()
>>> why.val
getter called
44
>>> other = Other()
>>> other.val
I AM called
44
I'm no stranger to properties, I'm just wondering if there is some advantage to making a getter or if was just put there for symmetry?
@property
let's you define a whole new property, by defining a getter for that property. The @original.getter
syntax lets you override just the existing getter. There are also .setter
and .deleter
decorators, to the other two methods available to a property.
Imagine you subclassing a custom class that is using a property, with both a getter and a setter defined. If you only wanted to override the getter of that property but leaving the setter in place (or the deleter), using @BaseClass.original.getter
lets you do that:
>>> class Foo(object):
... @property
... def spam(self):
... print 'Foo.spam called'
... return 'spam'
... @spam.setter
... def spam(self, value):
... print 'Foo.spam.setter called'
... @property
... def ham(self):
... print 'Foo.ham called'
... return 'ham'
... @ham.setter
... def ham(self, value):
... print 'Foo.ham.setter called'
...
>>> class Bar(Foo):
... @Foo.spam.getter
... def spam(self):
... print 'Bar.spam override'
... return 'eggs and spam'
... @property
... def ham(self):
... print 'Bar.ham override'
... return 'eggs and ham'
...
>>> Bar().spam
Bar.spam override
'eggs and spam'
>>> Bar().spam = 'foo'
Foo.spam.setter called
>>> Bar().ham
Bar.ham override
'eggs and ham'
>>> Bar().ham = 'foo'
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
AttributeError: can't set attribute
Note that I only replaced the spam
getter, while it's setter was preserved. The Bar.ham
setter does not exist, I replaced the Foo.ham
property wholesale.