Overriding properties in python
The Python docs on the property
decorator suggest the following idiom:
class C(object): def __init__(self): self._x = None @property def x(self): return self._x @x.setter def x(self, value): self._x = value @x.deleter def x(self): del self._x
And then subclasses can override a single setter/getter like this:
class C2(C): @C.x.getter def x(self): return self._x * -1
This is a little warty because overriding multiple methods seems to require you to do something like:
class C3(C): @C.x.getter def x(self): return self._x * -1 # C3 now has an x property with a modified getter # so modify its setter rather than C.x's setter. @x.setter def x(self, value): self._x = value * 2
Of course at the point that you're overriding getter, setter, and deleter you can probably just redefine the property for C3.
I'm sure you've heard this before, but apply
has been deprecated for eight years, since Python 2.3. Don't use it. Your use of locals()
is also contrary to the Zen of Python -- explicit is better than implicit. If you really like the increased indentation, there is no need to create a throwaway object, just do
if True: @property def foo(self): return self._foo @foo.setter def foo(self, val): self._foo = val
Which doesn't abuse locals
, use apply
, require creation of an extra object, or need a line afterwards with foo = foo()
making it harder to see the end of the block. It works just as well for your old-fashioned way of using property
-- just do foo = property(fget, fset)
as normal.
If you want to override a property in an arbitrary subclass, you can use a recipe like this.
If the subclass knows where the property was defined, just do:
class ATimesTwo(A): @A.foo.setter def foo(self, val): self._foo = val * 2
The answer of stderr satisfies most use cases.
I'd like to add a solution for the case where you want to extend a getter
, setter
and/or deleter
. Two ways to do this are:
1. Subclass property
First way to do this is by subclassing the builtin property
and adding decorators that are versions of getter
, setter
and/or deleter
that extend the current get, set and delete callbacks
Example for a property that supports appending methods to the set-functions:
class ExtendableProperty(property): def append_setter(self, fset): # Create a wrapper around the new fset that also calls the current fset _old_fset = self.fset def _appended_setter(obj, value): _old_fset(obj, value) fset(obj, value) # Use that wrapper as setter instead of only the new fset return self.setter(_appended_setter)
Usage is the same as for normal properties, only now it is possible to add methods to the property setters:
class A(object): @ExtendableProperty def prop(self): return self._prop @prop.setter def prop(self, v): self._prop = vclass B(A): @A.prop.append_setter def prop(self, v): print('Set', v)
>>> a = A()>>> a.prop = 1>>> a.prop1>>> b = B()>>> b.prop = 1Set 1>>> b.prop1
2. Overwrite getter, setter and/or deleter
Use a normal property, overwrite the getter, setter or deleter and then add calls to the fget
, fset
or fdel
in the property of the parent class.
Example for the type of property as in example 1:
class A(object): @property def prop(self): return self._prop @prop.setter def prop(self, v): self._prop = vclass B(A): @A.prop.setter def prop(self, v): A.prop.fset(self, v) # This is the call to the original set method print('Set {}'.format(v))
I think the first option looks nicer because the call to the super property's fset is not necessary