[Python-Dev] NotHashableError? (Re: dict containment annoyance) (original) (raw)

Scott Dial scott+python-dev at scottdial.com
Mon Aug 14 05:49:09 CEST 2006

Greg Ewing wrote:

Guido van Rossum wrote:

try: hash(x) except TypeError: # apparently x is not hashable

then you're also swallowing any type errors in the computation of a legitimate hash function. Maybe it would help if there were a specific exception, such as NotHashableError, that hash functions were expected to raise in this situation instead of a generic TypeError.

I was going to suggest this, but then I realized that what is really desirable is a check for hashability. But since hashability is a deep property, it is difficult to check for.. seems like an very specific exception type would be a simple solution. FYI, I appreciate that swallowing all TypeErrors is bad form, so it would be nice if it was possible to differentiate ;-)

Some brief googling on the subject yield a discussion on py3k about this very subject.

http://mail.python.org/pipermail/python-3000/2006-July/002697.html

Guido wrote:

Personally, I'm not sure this problem needs solving; I don't recall ever needing to know whether something is hashable. So perhaps it's of purely theoretical importance? That would suit me fine given the above dilemma...

--Guido

-- Scott Dial scott at scottdial.com scodial at indiana.edu

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