code review

Terry Reedy tjreedy at udel.edu
Sat Jul 14 22:42:17 CEST 2012


On 7/14/2012 5:26 AM, Ian Kelly wrote:
> On Fri, Jul 13, 2012 at 5:09 PM, Terry Reedy <tjreedy at udel.edu> wrote:

>> I believe the following rule is true: if a op b is True or False raises,

Sorry, left out 'or' in 'or raises'
>
> I don't follow.  Raises what?

an Exception.

>> then op is a potentially chained comparison operation. They are (not) equal
>> (and (not) is), the 4 order comparisons, and (not) in. 'in' should be the
>> only surprise and most confusing.
>>>>> 1 < 3 in {3,4}
>> True
>>>>> 3 in {3,4} < {4}
>> False
>>
>> 'and' and 'or' are not included because they do not always return a bool,
>> and indeed, they are not binary operators in the usual sense because of
>> short-circuiting.
>
> The only one of those operators that can be said to always return a
> bool is "is (not)".  The others (apart from "and" and "or") all can be
> overloaded to return anything you want

OK, add 'by default, without over-rides ;-).
or 'for builtin classes'.

Python's flexibility makes it really hard to make any general statement. 
In my book-in-progress, I am currently resorting to saying "In Python*, 
..." whenever I know and remember that it is possibly to over-ride the 
customary behavior described in '...'.

-- 
Terry Jan Reedy






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