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"x == None" vs "x is None"

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  "x == None" vs "x is None" Ulli Horlacher <framstag@rus.uni-stuttgart.de> - 2016-01-17 09:51 +0000
    Re: "x == None" vs "x is None" Chris Angelico <rosuav@gmail.com> - 2016-01-17 21:15 +1100
      Re: "x == None" vs "x is None" Ulli Horlacher <framstag@rus.uni-stuttgart.de> - 2016-01-17 11:05 +0000
        Re: "x == None" vs "x is None" Peter Otten <__peter__@web.de> - 2016-01-17 12:26 +0100
        Re: "x == None" vs "x is None" Chris Angelico <rosuav@gmail.com> - 2016-01-17 22:29 +1100
        Re: "x == None" vs "x is None" <paul.hermeneutic@gmail.com> - 2016-01-17 11:01 -0700
        Re: "x == None" vs "x is None" Random832 <random832@fastmail.com> - 2016-01-17 16:33 -0500
        Re: "x == None" vs "x is None" Chris Angelico <rosuav@gmail.com> - 2016-01-18 08:38 +1100
        Re: "x == None" vs "x is None" Ben Finney <ben+python@benfinney.id.au> - 2016-01-18 13:46 +1100
    Re: "x == None" vs "x is None" fernando junior <fernandojr.ifcg@live.com> - 2016-01-19 14:20 -0800

#101838 — "x == None" vs "x is None"

I have seen at several places "x == None" and "x is None" within
if-statements.
What is the difference? 
Which term should I prefer and why?


-- 
Ullrich Horlacher              Server und Virtualisierung
Rechenzentrum IZUS/TIK         E-Mail: horlacher@tik.uni-stuttgart.de
Universitaet Stuttgart         Tel:    ++49-711-68565868
Allmandring 30a                Fax:    ++49-711-682357
70550 Stuttgart (Germany)      WWW:    http://www.tik.uni-stuttgart.de/

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#101839

On Sun, Jan 17, 2016 at 8:51 PM, Ulli Horlacher
<framstag@rus.uni-stuttgart.de> wrote:
> I have seen at several places "x == None" and "x is None" within
> if-statements.
> What is the difference?
> Which term should I prefer and why?

tl;dr: Prefer "x is None" as a check.

The two checks have slightly different meaning, and almost always you
want the identity check. "x is None" is asking the identity question
"does the name x refer to the exact same object as the singleton
None", but "x == None" is asking the value question "does the object
referred to by x want to consider itself equal to None". For instance:

class NoneLike(object):
    def __eq__(self, other):
        if other is None: return True
        return isinstance(other, NoneLike)

>>> x = NoneLike()
>>> x == None
True
>>> x is None
False

The object referred to as x is not None itself, but it has decided to
call itself equal to None. This is incredibly unusual, and will
generally not be what you want.

As an additional bonus, the identity check is faster. The equality
check has to ask the object if it's equal or not, but the identity
check just says "same object? Y/N", which is generally going to be a
cheap check (in CPython, the objects' pointers can be compared).

But the main reason is semantic - you generally do not _want_ the
possibility of objects comparing equal to None. And if you actually
do, you'd better have some kind of comment there, or someone will come
along and change your code to the more normal "is None".

ChrisA

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#101843

Chris Angelico <rosuav@gmail.com> wrote:
> On Sun, Jan 17, 2016 at 8:51 PM, Ulli Horlacher
> <framstag@rus.uni-stuttgart.de> wrote:
> > I have seen at several places "x == None" and "x is None" within
> > if-statements.
> > What is the difference?
> > Which term should I prefer and why?
> 
> tl;dr: Prefer "x is None" as a check.

And for the negation?
"if not x is None" or "if x is not None"

I have seen the last one several times, but i do not understand it, because:

>>> x=0
>>> x is not None
True
>>> not None
True
>>> x is True
False


-- 
Ullrich Horlacher              Server und Virtualisierung
Rechenzentrum IZUS/TIK         E-Mail: horlacher@tik.uni-stuttgart.de
Universitaet Stuttgart         Tel:    ++49-711-68565868
Allmandring 30a                Fax:    ++49-711-682357
70550 Stuttgart (Germany)      WWW:    http://www.tik.uni-stuttgart.de/

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#101846

Ulli Horlacher wrote:

> Chris Angelico <rosuav@gmail.com> wrote:
>> On Sun, Jan 17, 2016 at 8:51 PM, Ulli Horlacher
>> <framstag@rus.uni-stuttgart.de> wrote:
>> > I have seen at several places "x == None" and "x is None" within
>> > if-statements.
>> > What is the difference?
>> > Which term should I prefer and why?
>> 
>> tl;dr: Prefer "x is None" as a check.
> 
> And for the negation?
> "if not x is None" or "if x is not None"
> 
> I have seen the last one several times, but i do not understand it,
> because:
> 
>>>> x=0
>>>> x is not None
> True
>>>> not None
> True
>>>> x is True
> False

- "is" has higher precedence than "not"
- "is not" is an operator in its own right. 

So the evaluation is

not (x is None)
x (is not) None

https://docs.python.org/dev/library/stdtypes.html#comparisons
https://docs.python.org/dev/reference/expressions.html#operator-precedence

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#101847

On Sun, Jan 17, 2016 at 10:05 PM, Ulli Horlacher
<framstag@rus.uni-stuttgart.de> wrote:
> Chris Angelico <rosuav@gmail.com> wrote:
>> On Sun, Jan 17, 2016 at 8:51 PM, Ulli Horlacher
>> <framstag@rus.uni-stuttgart.de> wrote:
>> > I have seen at several places "x == None" and "x is None" within
>> > if-statements.
>> > What is the difference?
>> > Which term should I prefer and why?
>>
>> tl;dr: Prefer "x is None" as a check.
>
> And for the negation?
> "if not x is None" or "if x is not None"
>
> I have seen the last one several times, but i do not understand it, because:
>
>>>> x=0
>>>> x is not None
> True
>>>> not None
> True
>>>> x is True
> False

There's no actual difference:

>>> dis.dis(lambda x: x is not None)
  1           0 LOAD_FAST                0 (x)
              3 LOAD_CONST               0 (None)
              6 COMPARE_OP               9 (is not)
              9 RETURN_VALUE
>>> dis.dis(lambda x: not x is None)
  1           0 LOAD_FAST                0 (x)
              3 LOAD_CONST               0 (None)
              6 COMPARE_OP               9 (is not)
              9 RETURN_VALUE

The compiler can tell that "not x is None" is exactly the same thing
as "x is not None". Personally, I'd rather spell it "is not None",
partly because it reads more like English that way, and partly because
one operator is better than two; but you're welcome to spell it "not x
is None" if that has other benefits (eg consistency).

ChrisA

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#101859

I prefer (x is None) and (x is not None).

This matches the SQL concept of NULL.

(X = NULL) is not valid since NULL is not a value and cannot be compared
with anything.

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#101860

<paul.hermeneutic@gmail.com> writes:

> I prefer (x is None) and (x is not None).
>
> This matches the SQL concept of NULL.
>
> (X = NULL) is not valid since NULL is not a value and cannot be compared
> with anything.

The suitably generic SQL operator is "is (not) distinct from", in some
dialects of SQL [certainly if you're using NULL directly you can simply
use is/is not, but the "distinct from" operators can be used to compare
two expressions that may either or both be NULL.

"X is not distinct from Y" == "X = Y or X is NULL and Y is NULL"

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#101861

On Mon, Jan 18, 2016 at 8:33 AM, Random832 <random832@fastmail.com> wrote:
> <paul.hermeneutic@gmail.com> writes:
>
>> I prefer (x is None) and (x is not None).
>>
>> This matches the SQL concept of NULL.
>>
>> (X = NULL) is not valid since NULL is not a value and cannot be compared
>> with anything.
>
> The suitably generic SQL operator is "is (not) distinct from", in some
> dialects of SQL [certainly if you're using NULL directly you can simply
> use is/is not, but the "distinct from" operators can be used to compare
> two expressions that may either or both be NULL.
>
> "X is not distinct from Y" == "X = Y or X is NULL and Y is NULL"

It's worth noting that Python's None is not really the same as SQL's
NULL... although that's partly because nothing is. NULL isn't quite
the same as IEEE NaN, nor C's null pointer, nor Python's None, all of
which are quite different from each other. All of them can be used to
represent some of the same concepts, but they behave very differently.
Trying to explain one in terms of another is likely to trip people up
somewhere.

ChrisA

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#101868

<paul.hermeneutic@gmail.com> writes:

> I prefer (x is None) and (x is not None).

There are good reasons to prefer this.

But this is not a good reason:

> This matches the SQL concept of NULL.

That's not really helpful, because it *doesn't* match.

> (X = NULL) is not valid since NULL is not a value and cannot be
> compared with anything.

SQL Null fails comparison with any value. Python ``None`` is a value and
can be compared like any other value.

SQL Null is neither truthy nor falsy; three-value logic is required when
Null can occur. Python ``None`` is falsy by definition, and two-value
(Boolean) logic continues to obtain.

SQL Null is not a value and has no data type. Python ``None`` is an
object and, like any other object, has a type defining the operations
that it can perform.

And so on. While some libraries do conflate SQL Null with Python
``None``, the two concepts really behave quite differently. It is
needlessly misleading to say they “match” in any sense.

I'm in the camp that says, while SQL is quite useful, its NULL is a wart
<URL:https://dba.stackexchange.com/questions/5222/why-shouldnt-we-allow-nulls/6049>.

-- 
 \        “… it's best to confuse only one issue at a time.” —Brian W. |
  `\    Kernighan and Dennis M. Ritchie, _The C programming language_, |
_o__)                                                             1988 |
Ben Finney

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#101911

> I have seen at several places "x == None" and "x is None" within
> if-statements.
> What is the difference? 
> Which term should I prefer and why?
> 
> 
> -- 
> Ullrich Horlacher              Server und Virtualisierung
> Rechenzentrum IZUS/TIK         E-Mail: horlacher@tik.uni-stuttgart.de
> Universitaet Stuttgart         Tel:    ++49-711-68565868
> Allmandring 30a                Fax:    ++49-711-682357
> 70550 Stuttgart (Germany)      WWW:    http://www.tik.uni-stuttgart.de/

In this case, the Style Guide for Python Code [1] recommends use "is or is not, never the equality operators" [2].

If you use the pep8 tool [3] to check your code, the error code that will be raised is E711 [4].

[1] https://www.python.org/dev/peps/pep-0008/
[2] https://www.python.org/dev/peps/pep-0008/#programming-recommendations
[3] http://pep8.readthedocs.org/en/latest/intro.html
[4] http://pep8.readthedocs.org/en/latest/intro.html#error-codes

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