When testing for membership, we can use:
x not in y
not x in y
There can be many possible contexts for this expression depending on
y. It could be for a substring check, list membership, dict key existence, for example.
- Are the two forms always equivalent?
- Is there a preferred syntax?
They always give the same result.
not 'ham' in 'spam and eggs' appears to be special cased to perform a single “not in” operation, rather than an “in” operation and then negating the result:
import dis def notin(): 'ham' not in 'spam and eggs' dis.dis(notin) 2 0 LOAD_CONST 1 ('ham') 3 LOAD_CONST 2 ('spam and eggs') 6 COMPARE_OP 7 (not in) 9 POP_TOP 10 LOAD_CONST 0 (None) 13 RETURN_VALUE def not_in(): not 'ham' in 'spam and eggs' dis.dis(not_in) 2 0 LOAD_CONST 1 ('ham') 3 LOAD_CONST 2 ('spam and eggs') 6 COMPARE_OP 7 (not in) 9 POP_TOP 10 LOAD_CONST 0 (None) 13 RETURN_VALUE def not__in(): not ('ham' in 'spam and eggs') dis.dis(not__in) 2 0 LOAD_CONST 1 ('ham') 3 LOAD_CONST 2 ('spam and eggs') 6 COMPARE_OP 7 (not in) 9 POP_TOP 10 LOAD_CONST 0 (None) 13 RETURN_VALUE def noteq(): not 'ham' == 'spam and eggs' dis.dis(noteq) 2 0 LOAD_CONST 1 ('ham') 3 LOAD_CONST 2 ('spam and eggs') 6 COMPARE_OP 2 (==) 9 UNARY_NOT 10 POP_TOP 11 LOAD_CONST 0 (None) 14 RETURN_VALUE
I had thought at first that they always gave the same result, but that
not on its own was simply a low precedence logical negation operator, which could be applied to
a in b just as easily as any other boolean expression, whereas
not in was a separate operator for convenience and clarity.
The disassembly above was revealing! It seems that while
not obviously is a logical negation operator, the form
not a in b is special cased so that it’s not actually using the general operator. This makes
not a in b literally the same expression as
a not in b, rather than merely an expression that results in the same value.
- No, there is no difference.
not inis defined to have the inverse true value of
- I would assume
not inis preferred because it is more obvious and they added a special case for it.
They are identical in meaning, but the pycodestyle Python style guide checker (formerly called pep8) prefers the
not in operator in rule E713:
E713: test for membership should be
See also “Python
if x is not None or
if not x is None?” for a very similar choice of style.
Others have already made it very clear that the two statements are, down to a quite low level, equivalent.
However, I don’t think that anyone yet has stressed enough that since this leaves the choice up to you, you should
choose the form that makes your code as readable as possible.
And not necessarily as readable as possible to anyone, even if that’s of course a nice thing to aim for. No, make sure the code is as readable as possible to you, since you are the one who is the most likely to come back to this code later and try to read it.
In Python, there is no difference. And there is no preference.
Syntactically they’re the same statement. I would be quick to state that
'ham' not in 'spam and eggs' conveys clearer intent, but I’ve seen code and scenarios in which
not 'ham' in 'spam and eggs' conveys a clearer meaning than the other.