[melbourne-pug] foo is None bar is 2 so is foo < bar
Mike Dewhirst
miked at dewhirst.com.au
Fri Feb 7 09:04:09 CET 2014
On 7/02/2014 6:36pm, Chris Maclachlan wrote:
> Hi Mike
>
> None is not the same as int(0) - it's a bit like javascript's NaN
> (not-a-number); it's not 0, it's a complete absence of value; an
> indeterminate value. Because of this, it can't actually be successfully
> cast to a numeric:
>
> >>> a = None
> >>> int(a)
> Traceback (most recent call last):
> File "<stdin>", line 1, in <module>
> TypeError: int() argument must be a string or a number, not 'NoneType'
>
> Python 3's a bit stricter about trying to compare incomparable types
> (See e.g.
> http://stackoverflow.com/questions/8961005/comparing-none-with-built-in-types-using-arithmetic-operators
> for more).
Thanks Chris (and Ben).
I guess that's what they mean by breaking backwards compatibility. I
suppose its for my own good. I'll just rewrite all my code. At least it
will still work in Python 2
But it doesn't all have to be changed because ...
>>> bar = 'x'
>>> foo = None
>>> if not foo:
... bar = 'y'
...
>>> bar
'y'
Mike
The way to correctly compare to None is to use "is", ie:
>
> >>> a = None
> >>> b = 1
> >>> if a is not b:
> ... print("woo")
> ...
> woo
>
> Cheers
>
> Chris
>
>
> On Fri, Feb 7, 2014 at 6:21 PM, Mike Dewhirst <miked at dewhirst.com.au
> <mailto:miked at dewhirst.com.au>> wrote:
>
> It is in Python 2.7 ...
>
> Python 2.7.6 (default, Nov 10 2013, 19:24:18) [MSC v.1500 32 bit
> (Intel)] on win32
> Type "help", "copyright", "credits" or "license" for more information.
> >>> foo = None
> >>> bar = 2
> >>> foo < bar
> True
> >>>
>
> But it seems not in Python 3.3 ...
>
> Python 3.3.3 (v3.3.3:c3896275c0f6, Nov 18 2013, 21:18:40) [MSC
> v.1600 32 bit (Intel)] on win32
> Type "help", "copyright", "credits" or "license" for more information.
> >>> foo = None
> >>> bar = 2
> >>> foo < bar
> Traceback (most recent call last):
> File "<stdin>", line 1, in <module>
> TypeError: unorderable types: NoneType() < int()
> >>>
>
> Should I rtfm?
>
> Maybe there is a trick to it?
>
> Thanks
>
> Mike
>
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