'Using is not None, may not always work'
Doug Fort
Since I installed 2.4a2 I've been getting a warning from pychecker: Using
is not None, may not always work'. I thought 'is not None' was the right
thing to do. I've had problems with 'if not x:', because some objects
return False in this context.
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Doug Fort, Consulting Programmer
http://www.dougfort.com
Heiko Wundram
> Since I installed 2.4a2 I've been getting a warning from pychecker: Using
> is not None, may not always work'. I thought 'is not None' was the right
> thing to do. I've had problems with 'if not x:', because some objects
> return False in this context.
That's probably what pychecker warns you about: that you might get an object
in the respective context which evaluates to boolean false, but is not
None... Although I'd find this strange...
Heiko.
Michael Hudson
Does seem an odd warning, seeing as when I write "x is not None" I'm
generally avoiding the case of, e.g., x being the empty list.
Cheers,
mwh
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Grant Edwards
> Since I installed 2.4a2 I've been getting a warning from
> pychecker: Using is not None, may not always work'.
Huh?!
That implies that there are cases where "None is not None" will
evaluate as True. I find that extremely hard to believe.
> I thought 'is not None' was the right thing to do.
It's certainly what I want to do if I want to know if a name is
bound to None or not.
> I've had problems with 'if not x:', because some objects
> return False in this context.
Exactly.
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Peter Otten
> Since I installed 2.4a2 I've been getting a warning from pychecker: Using
> is not None, may not always work'. I thought 'is not None' was the right
> thing to do. I've had problems with 'if not x:', because some objects
> return False in this context.
This is harmless. Starting with 2.4a2 None is a constant:
Python 2.3.3 (#1, Jan 3 2004, 13:57:08)
[GCC 3.2] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> import dis
>>> def f():
... if x is None: pass
...
>>> dis.dis(f)
2 0 LOAD_GLOBAL 0 (x)
3 LOAD_GLOBAL 1 (None)
6 COMPARE_OP 8 (is)
9 JUMP_IF_FALSE 4 (to 16)
12 POP_TOP
13 JUMP_FORWARD 1 (to 17)
>> 16 POP_TOP
>> 17 LOAD_CONST 0 (None)
20 RETURN_VALUE
>>>
Python 2.4a2 (#1, Aug 6 2004, 16:38:38)
[GCC 3.2] on linux2
Type "help", "copyright", "credits" or "license" for more information.
[python 2.4a2]
>>> import dis
>>> def f():
... if x is None: pass
...
>>> dis.dis(f)
2 0 LOAD_GLOBAL 0 (x)
3 LOAD_CONST 0 (None)
6 COMPARE_OP 8 (is)
9 JUMP_IF_FALSE 4 (to 16)
12 POP_TOP
13 JUMP_FORWARD 1 (to 17)
>> 16 POP_TOP
>> 17 LOAD_CONST 0 (None)
20 RETURN_VALUE
When PyChecker sees the constant it supposes you are doing something like
if x is "some string": pass
which always had the LOAD_CONST op-code and is indeed dangerous. PyChecker
needs to be fixed to special-case None for 2.4.
Peter
Doug Fort
> Doug Fort wrote:
>
>> Since I installed 2.4a2 I've been getting a warning from pychecker: Using
>> is not None, may not always work'. I thought 'is not None' was the right
>> thing to do. I've had problems with 'if not x:', because some objects
>> return False in this context.
>
> This is harmless. Starting with 2.4a2 None is a constant:
Thanks, that it.
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