To illustrate the question check the following code:
class MyDescriptor(object):
def __get__(self, obj, type=None):
print "get", self, obj, type
return self._v
def __set__(self, obj, value):
self._v = value
print "set", self, obj, value
return None
class SomeClass1(object):
m = MyDescriptor()
class SomeClass2(object):
def __init__(self):
self.m = MyDescriptor()
x1 = SomeClass1()
x2 = SomeClass2()
x1.m = 1000
# -> set <__main__.MyDescriptor object at 0xb787c7ec> <__main__.SomeClass1 object at 0xb787cc8c> 10000
x2.m = 1000 # I guess that this overwrites the function. But why?
# ->
print x1.m
# -> get <__main__.MyDescriptor object at 0xb787c7ec> <__main__.SomeClass1 object at 0xb787cc8c> <class '__main__.SomeClass1'> 10000
print x2.m
# -> 10000
- Why doesn't x2.m = 1000 not call the __set__-function? It seems that this overwrites the function. But why?
- Where is _v in x1? It is not in x1._v
-
To answer your second question, where is
_v
?Your version of the descriptor keeps
_v
in the descriptor itself. Each instance of the descriptor (the class-level instanceSomeClass1
, and all of the object-level instances in objects of classSomeClass2
will have distinct values of_v
.Look at this version. This version updates the object associated with the descriptor. This means the object (
SomeClass1
orx2
) will contain the attribute_v
.class MyDescriptor(object): def __get__(self, obj, type=None): print "get", self, obj, type return obj._v def __set__(self, obj, value): obj._v = value print "set", self, obj, value
Philipp der Rautenberg : Thanks, that is a good idea to implement. One question to _v: I also couldn't access it via "MyDescriptor._v" - which is clear as it not a class attribute of MyDescriptor. But what is the explicit name of the instance? It should be accessible somehow, right?Sebastjan Trepča : You cannot access it, because it gets overwritten. In S.Lott case, you can access it via x._vSebastjan Trepča : ehh, sorry, x1._vS.Lott : Descriptor instances are HARD to access -- the point is that the containing class transparently calls their __set__ and __get__ functions for you. It never refers to the descriptor -- it calls the __set__ or __get__.S.Lott : @Sebastjan Trepča: it's better to completely fix a comment by posting a new one and deleting the old comment. A sequence of comments is hard to read. But a new, complete comment is better. -
You should read this and this.
It overwrites the function because you didn't overload the
__set__
and__get__
functions of SomeClass but of MyDescriptor class. Maybe you wanted for SomeClass to inherit MyDescriptor? SomeClass1 prints the "get" and "set" output because it's a static method AFAIK. For details read the upper links. -
I found _v of x1: It is in SomeClass1.__dict__['m']._v
For the version suggested by S.Lott within the other answer: _v is in x1._v
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