Hi Drew,
I understand the points you make but I seem to draw the opposite
conclusion ;(
If you want to test if a variable contains the value None then
if variable is None:
seems a lot safer to me than
if not variable:
which would also be true if variable contains the value False,
which might not be what you wanted?
As Keith said, it's all about the context of the operation.
- dermot
Drew Fisher wrote:
Dermot:
Take a look here:
http://docs.python.org/release/2.5.2/lib/truth.html
None evaluates to false (the truth value) the same as 0, False (the
boolean), [] , (), {}, etc.
Using 'is' can be very dangerous. 'is' is used to test object
identity. It checks to make sure the two varaibles are the same
object and not just equal.
Example:
>>> a = 256
>>> b = 256
>>> a is b
True
>>> a = 257
>>> b = 257
>>> a is b
False
>>> 257 is 257
True
You CAN use 'is' with None because None is actually a class (run
dir(None) to see) but if we're simply testing for true/false (truth
values), it's easier to be consistent and simply use "if not
<variable>". This way if <variable1> is None/Not None and <variable2>
is True/False and <variable3> is 0/!0, a simple truth value test can
all be written the same way.
I hope this makes sense.
-Drew
Drew,
Just picking up on one item:
On 07/19/10 18:40, Drew Fisher wrote:
- tests for None can be done with
if not variable:
instead of
if variable == None:
and likewise for not None.
I have always heard that the recommended way to do this is:
if variable is None:
(or if variable is not None:)
Is your suggestion:
if not variable:
the preferred PEP8 way? Wouldn't that also evaluate to true if variable
had a value of False, rather than None?
- Dermot
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