Why does `None is None is None` return True? [duplicate]

Question

Today, in an interview, the CTO asked me what looks like an easy question,

What does this statement return ? :

None is None is None 

I thought Python executed the first operation None is None and would return True. After that, it would compare True is None which would return False. But, to my surprise, the right answer is True. I am trying to find answer to this question, but after a couple of days searching I didn't find anything. Can someone explain why this happens?

Answer 1

The bytecode shows that two comparisons are being performed here with the middle being duplicated:

>>> import dis >>> def a(): ...     return None is None is None ...  >>> dis.dis(a)   2           0 LOAD_CONST               0 (None)               3 LOAD_CONST               0 (None)               6 DUP_TOP               7 ROT_THREE               8 COMPARE_OP               8 (is)              11 JUMP_IF_FALSE_OR_POP    21              14 LOAD_CONST               0 (None)              17 COMPARE_OP               8 (is)              20 RETURN_VALUE         >>   21 ROT_TWO              22 POP_TOP              23 RETURN_VALUE 

As stated in the docs for comparisons this is because these operators chain together.

a op b op c will be translated to a op b and b op c (note b is duplicated in the bytecode as shown above)