Metaclass' __init__ Does not Initialize

Alex Martelli aleax at
Thu Nov 13 15:11:08 CET 2003

achan wrote:

> Thanks for all who responded!
> I did figure out the solution while thinking about how to get the values
> of
> __slots__ iteratively.  That is, by using getattrib(obj, name, value).
> Therefore, the opposite way should be setattrib(obj, name).  :-)

Wrong way 'round, as well as wrong spellings for the built-in functions:
getattr takes 2 arguments (the third one is optional and only used as the
default value when an attribute is not present), setattr takes 3.

> And for those who are puzzled why I insist on using __slots__, I'm using
> it to reduce errors and not for saving space, wink!

Right: you're using this feature for the wrong motivation, for a purpose
it's not meant nor designed to support, and in a way that's most likely
to reduce your coding productivity rather than enhancing it.  Like the
vast majority of users of __slots__, btw; you're all looking for the kind
of "security blanket" that languages with declarations have made you used
to, and you're not gonna let a little detail such as the fact that you're
just not going to get it stand in your way.

If you're truly terrified that somebody, one day, might make a typo
and assign to an attribute that wasn't part of the class as you
designed it, you should:

1. relax.  It's not any worse than the typo which some day will make
   someone code "net=income+tax" where they MEANT "net=income-tax",
   and no B&D language feture will let you catch THAT one anyway.  So:

2. code unit-tests (ideally BEFORE the "production" code, feature by
   feature).  If your unit-tests are even halfways semidecent they'll
   catch all the typos AND many more silly errors besides.

3. for occasional debug runs with super-thorough checks, if you still
   feel the need for them, code a __setattr__ that vets each setting
   for attributenames being in whatever set you want (you may then even
   make that set into a dict, making each name optionally correspond
   with a checkingpredicate that can be run on the value)

4. better than 3 (more general and far more useful), use DBC and turn
   it on for those "super-nitpicky checks", that will catch MANY more
   potential errors, not just typos on attributenames &c... and also
   serve as useful documentation re invariants, preconditions, and
   postconditions of your functions and methods.

For an implementation of [4], see, eg:


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