bytecode JUMP_IF_* always followed by POP_TOP?

[Robert Brewer]

Playing around with bytecodes some more:

>>> def g(x):
... 	if x:
... 		y
... 	else:
... 		z
... 		
>>> dis.dis(g)
  2           0 LOAD_FAST                0 (x)
              3 JUMP_IF_FALSE            8 (to 14)
              6 POP_TOP             

  3           7 LOAD_GLOBAL              1 (y)
             10 POP_TOP             
             11 JUMP_FORWARD             5 (to 19)
        >>   14 POP_TOP             

  5          15 LOAD_GLOBAL              2 (z)
             18 POP_TOP             
        >>   19 LOAD_CONST               0 (None)
             22 RETURN_VALUE        

I notice that, whether JUMP_IF_FALSE jumps or not, the next instruction
it executes is POP_TOP (in the above, instruction numbers 6 and 14). Are
there any constructions whereby this does not happen for JUMP_IF_FALSE
and JUMP_IF_TRUE? If there aren't, is it just explicitness that kept
JUMP_IF_* from doing the POP on its own, before the jump (which would
thereby save two instructions)?


Robert Brewer
MIS
Amor Ministries
fumanchu at amor.org