issue with callable, detects more than call

Question

I am testing the simpleTAL templating library which makes use of callable to test if an object passed into the template is a function or not. The definition of callable says that an object is callable if it contains the magic method __call__. See also What is a "callable" in Python?.

However, an object created with the following class definition (python 2.7.4)

class H:
    def __init__(self, val):
        self.a = val
    def __getattr__(self, name):
        return 'blah'

h = H(1)
callable(h)

will return True. If, however, __getattr__ raises AttributeError, which does not make sense to do systematically, it will return False!

Can someone shed some light on this issue and possibly a solution? (I don't want h to be callable).

Martijn Pieters · Accepted Answer · 2013-09-05T20:38:28.720

6

Your __getattr__ hook returns a value for all attributes:

>>> h.__call__
'blah'

and thus a test for hasattr(h, '__call__') returns True:

>>> hasattr(h, '__call__')
True

Make sure you raise a AttributeError for attributes the object doesn't support instead.

Note: this applies only to old-style instances; instances for new-style classes (the default in Python 3, and any class inheriting from object in Python 2) are not tested for the __call__ attribute; instead the class itself is consulted.

You could thus switch to new-style classes as well to mitigate this, but your __getattr__ should still not be too broad.

edited Sep 05 '13 at 20:38

answered Sep 05 '13 at 20:16

Martijn Pieters

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2

It's worth mentioning that new-style classes don't behave this way. – user2357112 Sep 05 '13 at 20:18
2

@user2357112 how do they behave? Could you provide your own answer perhaps? I'm very interested. – Mike Vella Sep 05 '13 at 20:21
doh!! so a simple solution is to raise AttributeError when name='__call__' !!! Let's hope there are not other traps with implementing __getattr__!! many thanks! – koriander Sep 05 '13 at 20:21
@koriander: test for leading underscores perhaps? – Martijn Pieters Sep 05 '13 at 20:23
1

Specifically, `callable` does not call `hasattr` when testing new-style classes. – Marcin Sep 05 '13 at 20:25
OK - so this code will this code return `False` or `True` with a new-style-class? – Mike Vella Sep 05 '13 at 20:27
1

@MikeVella: Because `hasattr()` is not used, `callable(h)` will return `False` even though `h.__getattr__('__call__')` returns a value. – Martijn Pieters Sep 05 '13 at 20:29
1

@MikeVella: Instead, dunder methods (double underscore) are looked up directly on the class. – Martijn Pieters Sep 05 '13 at 20:30
Martijn, thus a better solution is to use new-style classes? – koriander Sep 05 '13 at 20:32
1

@koriander: that could also be a solution, yes. There are many other benefits to using new-style classes, making it the better choice. – Martijn Pieters Sep 05 '13 at 20:34
great! perhaps you could add it also to your post. Many thanks again, I struggled with this issue for too long. – koriander Sep 05 '13 at 20:36

issue with callable, detects more than __call__

1 Answers1

issue with callable, detects more than call