Yoda condition with "not x is None"

Question

The previous dev has left a really strange not x is None yoda condition in the code:

>>> x = None
>>> not x is None
False
>>> x = 1
>>> not x is None
True

After some testing, I seem the same output as x is not None.

>>> x = None
>>> not x is None
False
>>> x is not None
False
>>> x = 1
>>> not x is None
True
>>> x is not None
True

Is not x is None always equivalent to x is not None?

To break down the condition, is it not (x is None) or (not x) is None? Or will the former always be equivalent to the latter?

It's all about the operator precedence. Read this answer http://stackoverflow.com/questions/31421379/why-does-nottrue-in-false-true-return-false/31458009#31458009 — Mazdak, Apr 17 '17 at 08:50
Yup according to the [document](https://docs.python.org/3/reference/expressions.html#operator-precedence), **not** has lower precedence than comparison **is**. So your former assumption should be correct. — umutto, Apr 17 '17 at 08:53

Elisha · Accepted Answer · 2017-04-17T09:25:33.687

Since is has higher precendence than not, then the expressions are equivalent:

In case x = None: x is None evaluates to True, and not x is None evaluates to False In case x = 1: x is None evaluates to False, and not x is None evaluates to True

In case x = None: x is not None evaluates to False In case x = 1: x is not None evaluates to True.

Therefore, even though the actions are not syntactically equivalent, the results are equivalent.

This is the AST of not x is None:

This is the AST of x is not None:

As can be seen in the second diagram, the inner node is the compare node, and therefore x is None is evaluated before the not.

Regarding actual evaluation of the expression, it seems that python creates the same bytecode for both. It can be seen in this example:

def foo(x):
    x is not None


def bar(x):
    not x is None

import dis
dis.dis(foo)
dis.dis(bar)

As both generate:

      0 LOAD_FAST                0 (x)
      3 LOAD_CONST               0 (None)
      6 COMPARE_OP               9 (is not)
      9 POP_TOP             
     10 LOAD_CONST               0 (None)
     13 RETURN_VALUE

score 1 · Answer 2 · answered Apr 17 '17 at 09:09

1

is not is (I believe, the only) two-keyword operator in Python, and x is not y is entirely equivalent to not x is y. The same result will also be returned by id(x) != id(y). The canonical spelling for best reader comprehension would be x is not y.

answered Apr 17 '17 at 09:09

holdenweb

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`not in` is another. – Ry- Apr 17 '17 at 09:20
This is why I try to avoid being too bombastic about my beliefs - there's usually someone better-informed or smarter to correct me. Thanks! – holdenweb Apr 17 '17 at 09:30

Yoda condition with "not x is None"

2 Answers2