[Python-Dev] "and" and "or" operators in Py3.0
raymond.hettinger at verizon.net
Tue Sep 20 01:44:32 CEST 2005
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
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:
'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
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
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:
1 0 LOAD_NAME 0 (a)
3 JUMP_IF_FALSE 8 (to 14)
2 7 LOAD_NAME 1 (b)
11 JUMP_FORWARD 1 (to 15)
>> 14 POP_TOP
>> 15 LOAD_CONST 0 (None)
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)
>> 10 LOAD_CONST 0 (None)
Executive summary. Returning only Booleans reduces errors, makes the
code easier to review, follows other language norms, and
simplifies/speeds-up the generated code.
P.S. Simplifying "and" and "or" may create a need to introduce a
conditional operator but that is a discussion for another day.
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