On 9/19/05, Raymond Hettinger <[EMAIL PROTECTED]> wrote: > I propose that in Py3.0, the "and" and "or" operators be simplified to > always return a Boolean value instead of returning the last evaluated > argument. > > 1) The construct can be error-prone. When an error occurs it can be > invisible to the person who wrote it. I got bitten in published code > that had survived testing and code review: > > def real(self): > 'Return a vector with the real part of each input element' > # do not convert integer inputs to floats > return self.map(lambda z: type(z)==types.ComplexType and z.real or > z) > > The code fails silently when z is (0+4i). It took a good while to trace > down a user reported error (when Matlab results disagreed with my matrix > module results) and determine that the real() method contained an error. > Even when traced down, I found it hard to see the error in the code. > Now that I know what to look for, it has not happened again, but I do > always have to stare hard at any "and/or" group to mentally verify each > case. >
I agree. While I have used the short-circuiting and the return value myself, it has often been for just fun one-liners or when it is extemely simple and I was just too lazy to write out an 'if' statement. A conditional operator is much more proper for this (and, as Guido has said in another email, should be considered as well). > > 2) When going back and forth between languages, it is easy to forget > that only Python returns something other than a boolean. > > > 3) Even when it isn't being used, the possibility of non-boolean return > value complicates the bytecode and parser. To allow for "and/or", the > conditional opcodes leave the tested value on the stack. In most cases > both branches go directly to a POP_TOP instruction. Since the POP_TOP > shouldn't be executed twice, the body of the positive branch has to > close with a jump over the other branch even when it is empty. For > instance, the simplest case: > > if a: > b > > compiles to: > > 1 0 LOAD_NAME 0 (a) > 3 JUMP_IF_FALSE 8 (to 14) > 6 POP_TOP > > 2 7 LOAD_NAME 1 (b) > 10 POP_TOP > 11 JUMP_FORWARD 1 (to 15) > >> 14 POP_TOP > >> 15 LOAD_CONST 0 (None) > 18 RETURN_VALUE > > this could be simpler and faster: > > 1 0 LOAD_NAME 0 (a) > 3 JUMP_IF_FALSE 8 (to 10) > 2 6 LOAD_NAME 1 (b) > 9 POP_TOP > >> 10 LOAD_CONST 0 (None) > 13 RETURN_VALUE > > > Executive summary. Returning only Booleans reduces errors, makes the > code easier to review, follows other language norms, and > simplifies/speeds-up the generated code. Glad the simplification of the bytecode is not the only part of your proposal. =) -Brett _______________________________________________ Python-Dev mailing list Python-Dev@python.org http://mail.python.org/mailman/listinfo/python-dev Unsubscribe: http://mail.python.org/mailman/options/python-dev/archive%40mail-archive.com
