On Tue, 2010-12-14 at 08:08 +0100, Ulrich Eckhardt wrote: > Steven D'Aprano wrote: > > Replacing "while True" with "while 1" may save a tiny bit of overhead. > > Whether it is significant or not is another thing. > > Is this the price for an intentional complexity or just a well-known > optimizer deficiency?
At least on older pythons, you can assign to the name "True" so it's not
possible to optimize that loop - you must look up the name "True" on
each iteration. For example, in python 2.6 this loop will exit after
one iteration:
>>> while True:
... True = False
...
>>>
To see the difference, take a look at the bytecode python generators for
the type types of loop:
>>> import dis
>>> def while1():
... while 1:
... pass
...
>>> def whileTrue():
... while True:
... pass
...
>>> dis.dis(while1)
2 0 SETUP_LOOP 3 (to 6)
3 >> 3 JUMP_ABSOLUTE 3
>> 6 LOAD_CONST 0 (None)
9 RETURN_VALUE
>>>
>>> dis.dis(whileTrue)
2 0 SETUP_LOOP 12 (to 15)
>> 3 LOAD_GLOBAL 0 (True)
6 JUMP_IF_FALSE 4 (to 13)
9 POP_TOP
3 10 JUMP_ABSOLUTE 3
>> 13 POP_TOP
14 POP_BLOCK
>> 15 LOAD_CONST 0 (None)
18 RETURN_VALUE
>>>
Still, I just can't bring myself to write "while 1" in favour of "while
True" in code.
Python 3 does away with this madness entirely:
>>> while True:
... True = False
...
File "<stdin>", line 2
SyntaxError: assignment to keyword
>>>
Looking at the bytecode shows that in Python 3, "while 1" and "while
True" are indeed identical.
Cheers,
Ryan
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Ryan Kelly
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