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
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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