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Re: "x == None" vs "x is None"

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From Peter Otten <__peter__@web.de>
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Subject Re: "x == None" vs "x is None"
Date Sun, 17 Jan 2016 12:26:45 +0100
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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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Thread

"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

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