Operator Precedence/Boolean Logic

Andreas Röhler andreas.roehler at online.de
Thu Jun 23 02:34:41 EDT 2016



On 23.06.2016 06:47, Lawrence D’Oliveiro wrote:
> On Thursday, June 23, 2016 at 3:12:52 PM UTC+12, Larry Hudson wrote:
>> On 06/22/2016 12:42 AM, Lawrence D’Oliveiro wrote:
>>> * boolean operators don’t have to operate on boolean values. The
>>>    language spec
>>>    <https://docs.python.org/3/reference/expressions.html#boolean-operations>
>>>    says:
>>>
>>>      “...the following values are interpreted as false: False, None, numeric
>>>      zero of all types, and empty strings and containers (including strings,
>>>      tuples, lists, dictionaries, sets and frozensets). All other values are
>>>      interpreted as true.”
>>>
>>> I feel that’s a needlessly complicated rule. It would have been simpler if
>>> boolean operators (and conditional expressions like in if-statements and
>>> while-statements) only allowed values of boolean types. But that’s one of
>>> the few warts in the design of Python...
>> Wart??  I *strongly* disagree.  I find it one of the strengths of Python,
>> it enhances Python's expressiveness.
> Tightening it up would rule out a whole class of common errors, from misunderstanding (or forgetting) the rule about what exactly gets interpreted as true and what as false <https://bugs.python.org/issue13936> <http://stackoverflow.com/questions/28116931/datetime-time0-0-evaluates-as-false-in-boolean-context>.

Indeed, why should the result of 4 - 4 have a different truth-value than 
4 - 3 ?
This implementation seems to be a legacy from languages without boolean 
types.


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