[Python-Dev] "and" and "or" operators in Py3.0

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
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.
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.
Raymond
P.S. Simplifying "and" and "or" may create a need to introduce a
conditional operator but that is a discussion for another day.