Python class @property: use setter but evade getter?
Solution 1:
Don't use a property
in this case. A property
object is a data descriptor, which means that any access to instance.var
will invoke that descriptor and Python will never look for an attribute on the instance itself.
You have two options: use the .__setattr__()
hook or build a descriptor that only implements .__set__
.
Using the .__setattr__()
hook
class MyClass(object):
var = 'foo'
def __setattr__(self, name, value):
if name == 'var':
print "Setting var!"
# do something with `value` here, like you would in a
# setter.
value = 'Set to ' + value
super(MyClass, self).__setattr__(name, value)
Now normal attribute lookups are used when reading .var
but when assigning to .var
the __setattr__
method is invoked instead, letting you intercept value
and adjust it as needed.
Demo:
>>> mc = MyClass()
>>> mc.var
'foo'
>>> mc.var = 'bar'
Setting var!
>>> mc.var
'Set to bar'
A setter descriptor
A setter descriptor would only intercept variable assignment:
class SetterProperty(object):
def __init__(self, func, doc=None):
self.func = func
self.__doc__ = doc if doc is not None else func.__doc__
def __set__(self, obj, value):
return self.func(obj, value)
class Foo(object):
@SetterProperty
def var(self, value):
print 'Setting var!'
self.__dict__['var'] = value
Note how we need to assign to the instance .__dict__
attribute to prevent invoking the setter again.
Demo:
>>> f = Foo()
>>> f.var = 'spam'
Setting var!
>>> f.var = 'ham'
Setting var!
>>> f.var
'ham'
>>> f.var = 'biggles'
Setting var!
>>> f.var
'biggles'
Solution 2:
property
python docs: https://docs.python.org/2/howto/descriptor.html#properties
class MyClass(object):
def __init__(self):
self._var = None
# only setter
def var(self, newValue):
self._var = newValue
var = property(None, var)
c = MyClass()
c.var = 3
print ('ok')
print (c.var)
output:
ok
Traceback (most recent call last):
File "Untitled.py", line 15, in <module>
print c.var
AttributeError: unreadable attribute
Solution 3:
The @WeizhongTu answer
class MyClass(object):
def __init__(self):
self._var = None
# only setter
def var(self, newValue):
self._var = newValue
var = property(None, var)
c = MyClass()
c.var = 3
print ('ok')
print (c.var)
Is fine, except from the fact that is making the variable ungettable...
A similar solution but preserving getter is with
var = property(lambda self: self._var, var)
instead of
var = property(None, var)