I think you've found an unintended and undocumented backdoor. I admit I don't understand how this works in CPython. Overloaded operators like __add__ or __call__ should be methods in the class, and we don't look for them in the instance. But somehow defining them with @property works (I guess because @property is in the class). What's different for __call__ is that callable() exists. And this is probably why I exorcised it Python 3.0 -- but apparently it's back. :-( In the end callable() doesn't always produce a correct answer; but maybe we can make it work in this case by first testing the class and then the instance? Something like (untested): def callable(x): return hasattr(x.__class__, '__call__') and hasattr(x, '__call__') On Fri, Apr 17, 2015 at 2:19 PM, Ionel Cristian Mărieș <contact@ionelmc.ro> wrote:
__add__ as a property/descriptor seems to work fine, eg:
class C: ... @property ... def __add__(self): ... return lambda other: [self, other] ...
C() + C() [<__main__.C object at 0x0000000003652AC8>, <__main__.C object at 0x0000000003652CC0>]
Am I missing something?
Thanks, -- Ionel Cristian Mărieș, http://blog.ionelmc.ro
On Sat, Apr 18, 2015 at 12:15 AM, Guido van Rossum <guido@python.org> wrote:
You won't have any more luck defining __add__ as a property -- just don't do that.
On how to implement a proxy, I'll let other explain. But this is not it.
On Fri, Apr 17, 2015 at 2:04 PM, Ionel Cristian Mărieș < contact@ionelmc.ro> wrote:
Well yes, from that example it look right, because the call operator uses the __call__ attribute from the type of the object. However, when the call operator gets the __call__ method it will actually use it as a descriptor. From that perspective it's inconsistent.
Also there's the issue about not being able to implement a true proxy (as outlined before).
What actually prevents this being fixed?
Thanks, -- Ionel Cristian Mărieș, http://blog.ionelmc.ro
On Fri, Apr 17, 2015 at 11:55 PM, Guido van Rossum <guido@python.org> wrote:
I think you're fighting windmills. Like most special operations (e.g. __add__), a __call__ attribute on the object does not work, i.e. it does not make the object callable. E.g.
$ python3 Python 3.5.0a2 (v3.5.0a2:0337bd7ebcb6, Mar 8 2015, 01:12:06) [GCC 4.2.1 (Apple Inc. build 5666) (dot 3)] on darwin Type "help", "copyright", "credits" or "license" for more information.
> class C: pass ... > c = C() > c.__call__ = lambda *a: a > c() Traceback (most recent call last): File "<stdin>", line 1, in <module> TypeError: 'C' object is not callable > callable(c) False > hasattr(c, '__call__') True >
On Fri, Apr 17, 2015 at 1:45 PM, Ionel Cristian Mărieș < contact@ionelmc.ro> wrote:
Hello,
I had an issue today with the `callable` builtin because it doesn't correctly check that the object has the __call__ attribute.
Effectively what `callable(a)` does is `hasattr(type(a), '__call__')` but that's not very straightforward. A more straightforward implementation would do something like `hasattr(a, '__call__')`.
For example:
Python 3.4.3 (v3.4.3:9b73f1c3e601, Feb 24 2015, 22:44:40) [MSC v.1600
64 bit (AMD64)] on win32 Type "help", "copyright", "credits" or "license" for more information. >>> callable <built-in function callable> >>> class A: ... @property ... def __call__(self): ... raise AttributeError('go away') ... >>> a = A() >>> a <__main__.A object at 0x000000000365B5C0> >>> a.__call__ Traceback (most recent call last): File "<stdin>", line 1, in <module> File "<stdin>", line 4, in __call__ AttributeError: go away >>> callable(a) True >>> # it should be False :(
So it boils down to this:
>>> hasattr(a, "__call__") False >>> hasattr(type(a), "__call__") True
My issue is that I didn't call `callable(type(a))` but just `callable(a)`. Clearly mismatching what happens when you do hasattr(a, "__call__").
To put in contrast, this is legal and clearly indicates the descriptors are being used as expected:
>> class B: ... @property ... def __call__(self): ... return lambda: 1 ... >>> b = B() >>> b() 1
There 's some more discussing in issue 23990 <http://bugs.python.org/issue23990> where I get slightly angry, sorry.
So were is this change actually useful? Proxies! Since new-style objects in Python you cannot really proxy the callable aspect of objects, because `callable` just checks that a field is set in a C struct. This is fairly inconvenient because you have to know upfront if your target is going to be callable or not.
Thanks, -- Ionel Cristian Mărieș, http://blog.ionelmc.ro
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-- --Guido van Rossum (python.org/~guido)
-- --Guido van Rossum (python.org/~guido)
-- --Guido van Rossum (python.org/~guido)