[Python-Dev] Dicts are broken Was: unicode hell/mixing str and unicode asdictionarykeys

[M.-A. Lemburg]

Martin v. Löwis wrote:
> M.-A. Lemburg schrieb:
>> Failure to decode a string doesn't imply inequality.
> 
> If the failure is "these bytes don't have a meaningful character
> interpretation", then the bytes are *clearly* not equal to
> some character string.
>
>> It implies
>> that the programmer needs to step in and correct the problem by
>> making an explicit and conscious decision.
> 
> There is no problem to correct. The strings *are* inequal.

If the programmer writes:

x = 'äöü'
y = u'äöü'
...
if x == y:
    do_something()

then he clearly has had the intention to compare two character
strings.

Now, if what you were saying were true, then the above would
simply continue to work without raising an exception, possibly
causing the application to return wrong results.

With the exception, the programmer will have a chance to correct
the problem (in this case, probably a forgotten u-prefix) and also
be safe in not having the application produce wrong data -
something that's usually hard to detect, debug and, more
importantly, can have effects which are a lot worse than
a failing application.

Note that we are not discussing changing the behavior of the
__eq__ comparison between strings and Unicode, since this has
always been to raise exceptions in case the automatic propagation
fails.

The discussion is about silencing exceptions in the dict lookup
mechanism - something which used to happen and now no longer
is done.

Since this behavior is an implementation detail of the
dictionary implementation, users perceive this change as random
exceptions occurring in their application.

While these exceptions do hint at programming errors (the main
reason for no longer silencing them), the particular case in
the dict implementation requires some extra thought.

I've suggested to go about this in a slightly more user-friendly
way, namely by giving a warning instead of raising an exception
in Python 2.5 and then going for the exception in Python 2.6.

-- 
Marc-Andre Lemburg
eGenix.com

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