On 1/12/21 10:53 AM, Mark Shannon wrote: > Hi everyone, > > Should the optimizer eliminate tests that it can prove have no effect > on the control flow of the program, even if that may eliminate some > side effects in __bool__()? > > For several years we have converted > > if a and b: > ... > > to > > if a: > if b: > ... > > which are equivalent, unless bool(a) has side effects the second time > it is called. > > In master we convert `if x: pass` to `pass` which is equivalent, > unless bool(x) has side effects the first time it is called. This is a > recent change. > > This is one of those "easy to fix, if we can decide on the semantics" > bugs. > > > Submit your thoughts to https://bugs.python.org/issue42899, please. > > Cheers, > Mark.
One key point about 'and' and 'or' is that those operators are defined to be 'short circuiting', i.e. that with a and b, that if a is false, then b is not evaluated at all. This can be important if a is a condition that test if the expression b is 'safe' to evaluate. In fact, isn't it true that 'a and b' is defined to be the equivalent to: 'a if not a else b' so b doesn't need to be evaluated unless a is truthy. I know that I would be very surpised if a statement like if foo(): pass ended up optimizing itself and not calling foo(), which is only one step away from the quoted case of if b: pass which is the equivalent to if b.__bool__(): pass Yes, perhaps there is more of an expectation that __bool__() is innocuous, but is that a an assumption that should be baked into the language. -- Richard Damon _______________________________________________ Python-Dev mailing list -- [email protected] To unsubscribe send an email to [email protected] https://mail.python.org/mailman3/lists/python-dev.python.org/ Message archived at https://mail.python.org/archives/list/[email protected]/message/HODNO42SFFRFX4IJY5K562YHT2MTIHIQ/ Code of Conduct: http://python.org/psf/codeofconduct/