[Python-Dev] Cleaning-up the new unittest API (original) (raw)
Michael Foord fuzzyman at voidspace.org.uk
Tue Nov 2 18:23:45 CET 2010
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On 02/11/2010 17:17, exarkun at twistedmatrix.com wrote:
On 04:29 pm, fuzzyman at voidspace.org.uk wrote:
On 02/11/2010 16:23, Terry Reedy wrote:
On 11/2/2010 10:05 AM, C. Titus Brown wrote:
...but, as someone who has to figure out how to teach stuff to CSE undergrads (and biology grads) I hate the statement "...any programmer should expect this..."
And indeed I (intentionally) did not say that. People who are ignorant and inexperienced about something should avoid making expectations in any direction until they have read the doc and experimented a bit. Expectations come from consistent behaviour. sorted behaves consistently for most of the built-in types and will also work for custom types that provide a 'standard' (total ordering) implementation of lt. It is very easy to not realise that a consequence of sets (and frozensets) providing partial ordering through operator overloading is that sorting is undefined for them. Perhaps. The documentation for sets says this, though: Since sets only define partial ordering (subset relationships), the output of the list.sort() method is undefined for lists of sets.
Right, I did quote that exact text earlier in the thread. False expectations come when there are exceptions to otherwise-consistent behaviour.
Particularly as it still works for other mutable collections. Worth being aware that custom implementations of standard operators will break expectations of users who aren't intimately aware of the problem domains that the specific type may be created for. I can't help thinking that most of this confusion is caused by using < for determining subsets. If < were not defined for sets and people had to use "set.issubset" (which exists already), then sorting a list with sets would raise an exception, a much more understandable failure mode than getting back a list in arbitrary order. I agree. This is a cost of overloading operators with domain specific meanings.
All the best,
Michael Foord
Jean-Paul
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