Re: [Python-ideas] accurate errors for "magic" methods

Actually, I'm wrong: it's perfectly clear as long as the programmer
is able to follow all the necessary reflexion path; then probably
also able to solve the problem without any help from python.

The issue here is that a very specific (and meaningful) case
(dict-like behaviour missing) is adressed using a very generic (and
thus helpless) message (attributeError).

I think error cases about "magic" methods, that implement
conceptually meaningful behaviours, should have appropriate
messages. In the case above, maybe something like: "Values instance
is not an item container (no __getitem__ method found)."

The time machine strikes again:
class A(object): pass

...

A()['a']

Traceback (most recent call last):
File "<stdin>", line 1, in <module>
TypeError: 'A' object is unsubscriptable


(the difference being that A is new-style, while Values is
old-style.)


Except that the error "object is unsubscriptable" might as well be in
Klingon to most people, particularly newbies.

(1) It's easy to misread it as "unscriptable", which is even more
mysterious.

(2) As far as I know, there's no tradition of describing key or index
lookup as "subscripting" in Python. I've never noticed it in doc
strings or the online docs, and after hanging around comp.lang.python
extensively for years, I feel safe to say it's not a common term among
even experienced Python developers. I suppose that there's a weak
connection between index lookup and subscripts in mathematics.

(3) The classic error message tells the newbie exactly what the error
is: the object has no __getitem__ method. The new error message tells
the newbie nothing useful. Given that obj is unsubscriptable, what
needs to be done to make it subscriptable?

Wait a minute!

(1) __getitem__ can easily be confused with __gotitem__
(2) __gotitem__ is not far from __gotitom__
(3) which an unsuspecting newbie might interpret as __got it, Tom!__
(4a) in case the newbie is called Tom, he will only assume 'it' refers
to his mind, i.e. will naturally conclude the error message is the
evil way of the python interpreter that it managed to take control of
his mind because what else can these underscores mean than a scary
loud muhahahahhahahha accompanied by a 5 seconds tune by a classic
orchestra? He will either switch off his computer or cut his own veins
with a nearby USB plug.
(4b) in case the newbie is not called Tom, well, it's better not even
thinking about this option

So you just be careful!

Cheers,
Daniel


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