comparison with None

[Alan G Isaac]

>>None >= 0
False

>>None <= 0

True

Explanation appreciated.

Thanks,
Alan Isaac

[Terry Reedy]

Re: comparison with None


"Alan G Isaac" <aisaac@america n.eduwrote in message
news:3IqdnfRnCN WPC7vbnZ2dnUVZ_ jqdnZ2d@rcn.net ...
| >>None >= 0
| False
| >>None <= 0
| True
|
| Explanation appreciated.

Should be in the reference manual section on comparisons.

[Steven Howe]

Re: comparison with None

Alan G Isaac wrote:

>>None >= 0

False

>>None <= 0

True
>
Explanation appreciated.
>
Thanks,
Alan Isaac
>

I've read and found that 'None' comparisons is not always a good idea.
Better to:
from types import NoneType

x = None
if type( x ) == NoneType:
# true
< code >
else:
# false; do something else.
< more code >

Steven Howe

[Paul McGuire]

Re: comparison with None

On Apr 18, 5:19 pm, Steven Howe <howe.ste...@gm ail.comwrote:

Alan G Isaac wrote:

>>None >= 0

False

>>None <= 0

True

>

Explanation appreciated.

>

Thanks,
Alan Isaac

>
I've read and found that 'None' comparisons is not always a good idea.
Better to:
from types import NoneType
>
x = None
if type( x ) == NoneType:
# true
< code >
else:
# false; do something else.
< more code >
>
Steven Howe

None is a singleton - there is but one None and no other. The only
comparisons that make sense with None are "is" or "is not". type(x)
== NoneType is unnecessary, x is None is sufficient.

>>x = None
>>x is None

True

>>y = None
>>x is y

True

>>z = object()
>>z is None

False

>>z is not None

True

>>x is not None

False

>>y is not None

False

-- Paul

[brzrkr0@gmail.com]

Re: comparison with None

On Apr 18, 3:19 pm, Steven Howe <howe.ste...@gm ail.comwrote:

I've read and found that 'None' comparisons is not always a good idea.
Better to:
from types import NoneType
>
x = None
if type( x ) == NoneType:
# true
< code >
else:
# false; do something else.
< more code >
>
Steven Howe

Is that any better than this?

if x is None:
# do something
else:
# do something else

[Michael Hoffman]

Re: comparison with None

brzrkr0@gmail.c om wrote:

On Apr 18, 3:19 pm, Steven Howe <howe.ste...@gm ail.comwrote:

>I've read and found that 'None' comparisons is not always a good idea.
>Better to:
>from types import NoneType
>>
>x = None
>if type( x ) == NoneType:
> # true
> < code >
>else:
> # false; do something else.
> < more code >
>>
>Steven Howe

>
Is that any better than this?
>
if x is None:
# do something
else:
# do something else

To the contrary, it's not as good. "if x is None" is much clearer, and
probably faster.
--
Michael Hoffman

[Gary Herron]

Re: comparison with None

Alan G Isaac wrote:

>>None >= 0

False

>>None <= 0

True
>
Explanation appreciated.
>
Thanks,
Alan Isaac
>

So that we can sort lists of objects, even when the objects of are
different types, Python guarantees to supply a unique and consistent
ordering of any two objects. The definition of Python does not specify
what that ordering is -- that's implementation dependent -- but any two
objects of any two types *do* have an ordering and that ordering will
always be the same.

So in your implementation None is less than 0 (and probably less than
any integer). Given that, your two observations above are consistent.

Gary Herron

[Gary Herron]

Re: comparison with None

brzrkr0@gmail.c om wrote:

On Apr 18, 3:19 pm, Steven Howe <howe.ste...@gm ail.comwrote:
>

>I've read and found that 'None' comparisons is not always a good idea.
>Better to:
>from types import NoneType
>>
>x = None
>if type( x ) == NoneType:
> # true
> < code >
>else:
> # false; do something else.
> < more code >
>>
>Steven Howe
>>

>
Is that any better than this?
>
if x is None:
# do something
else:
# do something else
>

No. using
if x is None:
is the recommended way.

Gary Herron

[Robert Kern]

Re: comparison with None

Steven Howe wrote:

Alan G Isaac wrote:

> >>None >= 0

>False

> >>None <= 0

>True
>>
>Explanation appreciated.
>>
>Thanks,
>Alan Isaac
>>

I've read and found that 'None' comparisons is not always a good idea.
Better to:
from types import NoneType
>
x = None
if type( x ) == NoneType:
# true
< code >
else:
# false; do something else.
< more code >

The recommended idiom is to test for "x is None".

--
Robert Kern

"I have come to believe that the whole world is an enigma, a harmless enigma
that is made terrible by our own mad attempt to interpret it as though it had
an underlying truth."
-- Umberto Eco

[Alan Isaac]

Re: comparison with None

"Terry Reedy" <tjreedy@udel.e duwrote in message
news:mailman.66 86.1176934558.3 2031.python-list@python.org ...

Should be in the reference manual section on comparisons.

Only to this extent:

http://www.python.org/doc/2.4/ref/comparisons.html

objects of different types always compare unequal, and are ordered
consistently but arbitrarily.

(This unusual definition of comparison was used to simplify the
definition of operations like sorting and the in and not in
operators.
In the future, the comparison rules for objects of different types
are
likely to change.)

... Most other types compare unequal unless they are the same
object;
the choice whether one object is considered smaller or larger than
another one is made arbitrarily but consistently within one
execution
of a program.

This does not provide a direct answer to "why" None comparisons.
(As far as I can tell, None is less than any object.)

However, Gary Herron's explanation makes sense: this provides a stable
sort when None is involved, and meets the criterion that objects of
different types must always compare unequal. However this would also
be true if None always compared greater than any object, and the current
behavior does not seem to be guaranteed.

Is that about right?

Cheers,
Alan Isaac

[Steven D'Aprano]

Re: comparison with None

On Wed, 18 Apr 2007 15:19:26 -0700, Steven Howe wrote:

I've read and found that 'None' comparisons is not always a good idea.

You're probably thinking of testing against None with equality:

if x == None: do_something()

That can go wrong if x is a class that has an overly-broad concept of
equality, e.g.:

class FalseKlass:
def __eq__(self, other):
# equal to anything that is False
return not other

Better to:
from types import NoneType
>
x = None
if type( x ) == NoneType:
# true
< code >
else:
# false; do something else.
< more code >

Not necessary. Since None is a guaranteed singleton, the only test you
need to make is "if x is None: ...".

But if you wanted to do extra work unnecessarily, a less unnecessary
amount of extra work would be:

if type(x) == type(None): ...

You don't need to look up the type of None in the types module when you
can easily get it from the type() function.



--
Steven.

[Bruno Desthuilliers]

Re: comparison with None

Steven Howe a écrit :
(snip)

I've read and found that 'None' comparisons is not always a good idea.
Better to:
from types import NoneType
>
x = None
if type( x ) == NoneType:
# true
< code >
else:
# false; do something else.
< more code >

Actually, None is garanteed to be a singleton, so the idiomatic way is
to use an identity test:

if x is None:
# code here

But this doesn't answer the OP...

HTH

[Alex Martelli]

Re: comparison with None

Alan Isaac <aisaac@america n.eduwrote:

currently documented behavior:
"objects of different types always compare unequal".

Where is that documented? URL please?

>>1.0 == 1

True

>>type(1.0), type(1)

(<type 'float'>, <type 'int'>)

here, just as an example, are two objects of different types that
compare equal; therefore that "documented behavior" is flat wrong and
needs to be fixed.


Alex

[Alex Martelli]

Re: comparison with None

Steven D'Aprano <steve@REMOVE.T HIS.cybersource .com.auwrote:

But if you wanted to do extra work unnecessarily, a less unnecessary
amount of extra work would be:
>
if type(x) == type(None): ...

Of course, like the original poster's proposal, this CAN be faked out
(while the 'is' test cannot, one more weird reason why it's better):

>>class metaWeird(type) :

.... def __eq__(self, other): return True
....

>>class Weird: __metaclass__ = metaWeird

....

>>x = Weird()
>>type(x) == type(None)

True


(warning to all innocent bystanders: don't try this at home, kids!-)


Alex

[Tommy Grav]

Re: comparison with None


On Apr 19, 2007, at 11:00 PM, Alex Martelli wrote:

Alan Isaac <aisaac@america n.eduwrote:
>

>currently documented behavior:
>"objects of different types always compare unequal".

>
Where is that documented? URL please?
>

>>>1.0 == 1

True

>>>type(1.0), type(1)

(<type 'float'>, <type 'int'>)
>

Isn't this an example of numerical comparison (= or !=) versus
object comparison (is or is not). I think the documentation needs
to state that there is a difference between the two types.

Cheers
Tommy