I love this type of discussion -- it really forces me to do researchRuud wrote:
So far for *how* it works. As to *why* it works like this, I don't know for sure. But my guess is that the reasoning was something as follows: if you define a function (regular or something special like a classmethod) only for an instance of a class, you obviously don't want to use it in a class context: it is -by definition- invisible to the class, or to other instances of the same class. One possible use case would be to store a callback function. And in that case you definitely don't want the class magic to happen when you reference the function.
Yep. Got it. Indeed the reason seems to be a valid optimization:
-in 99% of the cases you request something from an instance it is a plain old variable
-in 99% of the cases you request something from a class it's a
function.
So it would be a waste of time to check for the conversion when something exists in the __dict__ of the instance, indeed.
OTOH, I'm talking about the "concept of python" and not CPython implementation, and that's why I have these questions:)
Thanks,
jfj
on my assumptions. But in this case I don't believe that this is specific to CPython. From the Python Reference Manual, Section 3.2,
"The Standard Type Hierarchy", in the part about callable types, user-defined methods:
<quote>
Note that the transformation from function object to (unbound or bound) method object happens each time the attribute is retrieved from the class or instance. In some cases, a fruitful optimization is to assign the attribute to a local variable and call that local variable. Also notice that this transformation only happens for user-defined functions; other callable objects (and all non-callable objects) are retrieved without transformation. It is also important to note that user-defined functions which are attributes of a class instance are not converted to bound methods; this only happens when the function is an attribute of the class.
</quote>
So it seems that this mechanism is indeed fully part of the python concept, not an optimization specific for CPython implementation. When you define a function as an instance attribute (instead of a class attribute), then none of the class magic happens. When you access that attribute, you get the bare function, not a method.
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