On 28/06/2011 14:20, Steven D'Aprano wrote:
Michael Foord wrote:
What do you mean by "instances can have methods as instance
attributes"? Once you attach a bound method directly to an instance
it becomes a slightly different beast I think. (On top of which that
is pretty rare behaviour.)
>>> class C:
... def method(self, x):
... return x+1
...
>>> c = C()
>>> c.method = types.MethodType(lambda self, x: x+101, c)
>>> c.method(1)
102
I don't know how rare it is, but it's a useful trick for customising
the behaviour of instances.
Right - that method is an instance attribute.
As I see it, there are three dichotomies we sometimes need to make:
(1) Instance attributes vs class (shared) attributes.
Broadly speaking, whether the attribute is in instance.__dict__ or
type(instance).__dict__.
(2) Computed vs non-computed attributes.
Attributes which are computed by __getattr__ or via the descriptor
protocol (which includes properties) are all computed attributes;
everything else is non-computed.
Technically also via __getattribute__ when overridden.
(3) Method attributes (methods) vs non-method/data attributes.
Broadly speaking, methods are callable, non-method (data) attributes
are not.
The three are orthogonal: e.g. a staticmethod is a method by virtue of
being callable, computed by virtue of being generated by a descriptor,
and a class attribute by virtue of existing in the type __dict__
rather than the instance __dict__.
Strictly speaking, (3) is not truly a dichotomy, since functions and
methods are first class-objects in Python. E.g. one may store a
function as an attribute with the intention of using it as data rather
than as a method. But that's a moderately obscure corner case, and in
my opinion it's not worth obscuring the practical distinction between
"methods are things you call, data are not" for the sake of it. Leave
the functions-as-data case for a footnote.
Yep, useful summary.
Michael
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