ActiveState Code

Recipe 52238: __getattr__: handle with care

__getattr__ is a very nice and powerful method that must be handled with care, otherwise you can discover that it has more power than you expected...

Python 
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class SuperClass:
    def supermethod(self):
        return 'Output of "SuperClass.supermethod(self)".'

class SubClass(SuperClass):
    def __getattr__(self, name):
        if name == 'special':
            return 'Value of attribute "special".'
        else:  raise AttributeError, name  # <<< DON'T FORGET THIS LINE !!

Discussion

If you forget the last line of __getattr__ above, __getattr__ implicitly returns "None" for every attribute name other than "special" and that includes the (otherwise inherited) "supermethod" attribute, so you finish with an unexpected overload of "supermethod"!

Comments

  1. 1. At 8:12 a.m. on 13 mar 2001, willem broekema said:

    __getattr__ only comes into play after looking in superclasses. When trying to get attribute/method of an object, the object's superclasses are looked up before the __getattr__:

    class SuperClass:
    special = 'super special'

class SubClass(SuperClass):
    def __getattr__(self, name):
        if name == 'special':
            return 'sub special'

if __name__ == '__main__':
    S = SubClass()
    print S.special

    This will print 'super special'.

    (But it IS correct that the value returned by __getattr__ is 'None' if no other is given!) willem broekema

  2. 2. At 3:44 p.m. on 4 may 2001, Michael Chermside said:

    Please Fix. William is right... this recepie is incorrect, and should be fixed (or removed). I'd prefer a fix. (If this were a wiki, I'd just MAKE the fix, but there's no mechanism for that in the cookbook. Yet.)

    -- Michael Chermside

  3. 3. At 6:17 a.m. on 15 jan 2003, Martin Schmettow said:

    another problem: loosing callability. There's another problem if you forget the else-branch: your objects loose callability.

    class Test:
    def __init__(self):
        pass
    def __getattr__(self,name):
        if name == 'myattr':
            return 'value of myattr'

test = Test()
test
Traceback (most recent call last):
  File "", line 1, in ?
TypeError: 'NoneType' object is not callable
  4. 4. At 11:33 p.m. on 21 may 2006, dum dee said:

    The "NoneType" is not callable is because it is trying to look up "__repr__" in order to display it to you, and so it gets back a None from __getattr__ and promptly tries to call it (because it assumes it's a __repr__). You can see this by adding "print 'getting', attr" to your __getattr__()s

    I ran into this very problem which is what led me here. Adding the exception line as in the recepie solved my problem; I believe python checks for AttributeErrors and keeps falling back until it can't fall back anymore.

    __repr__ is a special case; it should be one of the only (the only?) one that will trigger the strange "NoneType is not callable" error because, if I understand correctly, it's only when you type in an object at the interactive prompt that the follow pattern is run: try: print getattr(object, '__repr__')() except AttributeError: print generic_repr(object) In all other cases it will happily return None to you which would instantly signal to you "oops"

  5. 5. At 11:39 p.m. on 21 may 2006, dum dee said: 
    And in fact, according to the language reference:
"This method should return the (computed) attribute value or raise an AttributeError exception."

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