"x not in y" or "not x in y"

Question

When testing for membership, we can use:

x not in y 

Or alternatively:

not x in y 

There can be many possible contexts for this expression depending on x and 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?

Answer 1

They always give the same result.

In fact, 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.