Python != giving unexpected results

Question

I came across a very confusing problem on a website for speed learning Python. It's about the operand != and how it works. The result/outcome of using != between 3 operands baffles me and my classmates. We have tried to search around and couldn't find any discussion/explanation on this. Please help. Specifically, please see below:

True != False != 1

output: True

True != False != True

output: True

True is not False is not True

output: True

Breaking down the first statement step by step, True!=False should be evaluated first to yield a True. This True is then compared against 1 using != again. True != 1 should yield a False. Therefore the entire statement should evaluate to False! But instead a True is produced.

Breaking down the second statement step by step, True!=False should yield True, then this True != True should yield a False. The statement should evaluate to False !But instead a True is produced.

Likewise for the third statement.

Trying the exact same statements in Javascript and JAVA both gave the expected result - False, which makes complete sense. Python however, is the exception.

Note: We know that explicitly forcing the order of the operation by putting parentheses around two of operands gives the "right" result (which is False).We just can't understand why it's the way it is without parentheses.

Any help would be much appreciated.

You are chaining comparison operators. `True != False != 1` is executed as `(True != False) and (False != 1)`, `True != False != True` is executed as `(True != False) and (False != True)`, etc. — Martijn Pieters, Jun 27 '17 at 14:09
In other words, your breakdown is incorrect. That's not how chained comparisons work. — Martijn Pieters, Jun 27 '17 at 14:10
@AdamHughes: `True == 1` is true, so `True != 1` is indeed false. So a better question is: why do you think it shouldn't? — Martijn Pieters, Jun 27 '17 at 14:10

score 0 · Accepted Answer · answered Jun 27 '17 at 14:13

0

It is because a != b != c is equivalent to a != b and b != c.

It is called chained comparison and you can find more info here: https://docs.python.org/3/reference/expressions.html

answered Jun 27 '17 at 14:13

Felipe Andrés Ramírez Darvich

86
4

I got it ,thanks a lot!! – mini_way Jun 27 '17 at 14:59

Python != giving unexpected results

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