Here is the code
class MyTest:
def __init__(self):
pass
def __getattr__(self, attr):
pass
t = MyTest()
print 'my test object: %r' %t
So print triggers a TypeError: 'NoneType' object is not callable while i only want to see if object exists.
Granted this code isn't very useful. But i've had a stub class like that in a big code base so i did
if module and module.class and module.class.propery:
# do something with that property
...
and got a Type Error: 'NoneType' object is not callable but the line doesn't call anything! I guess python is calling some functions implicitly behind the scenes.
Curiously this doesn't happen if the class inherits from Object
What's going on?
In an old-style class, __getattr__ is used for a greater variety of attribute access, including magic methods. The % operator is trying to call t.__repr__() in order to fill in the %r placeholder, but t.__repr__ is evaluated by t.__getattr__('__repr__'), which returns None.
In the if case, a different magic method is invoked, but the same problem occurs.
>>> class Foo:
... def __getattr__(self, attr):
... print(attr)
...
>>> f = Foo():
>>> if f:
... pass
__nonzero__
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
TypeError: 'NoneType' object is not callable
Use a new-style class, __getattr__ is only invoked if an attribute cannot be found via the normal method (checking the __dict__ attribute of the instance or of any of the class's in the instance's MRO).
>>> class Foo(object):
... def __init__(self):
... self.x = 3
... def __getattr__(self, attr):
... print(attr)
...
>>> f = Foo()
>>> if f:
... pass
...
>>> f.x
3
>>> f.y
y
In the if f case, f itself doesn't implement __nonzero__ or __len__, and neither does its parent object, but in that case, no attribute is used; the fact that f is, in fact, an object is used. In f.x, x is found in the instance's attribute dict, so its value is returned directly. Only y, which isn't otherwise defined by f, Foo, or object, invokes a call to __getattr__.