Why does `False is False is False` evaluate to `Tr

问题:

Why in Python it is evaluated this way:

>>> False is False is False
True

but when tried with parenthesis is behaving as expected:

>>> (False is False) is False
False

回答1:

Chaining operators like a is b is c is equivalent to a is b and b is c.

So the first example is False is False and False is False, which evaluates to True and True which evaluates to True

Having parenthesis leads to the result of one evaluation being compared with the next variable (as you say you expect), so (a is b) is c compares the result of a is b with c.

回答2:

Quoting Python official documentation,

Formally, if a, b, c, ..., y, z are expressions and op1, op2, ..., opN are comparison operators, then a op1 b op2 c ... y opN z is equivalent to a op1 b and b op2 c and ... y opN z, except that each expression is evaluated at most once.

So, False is False is False is evaluated as

(False is False) and (False is False)

The second False is False expression uses the second False in the original expression, which effectively translates to

True and True

That is why the first expression evalutes to be True.

But in the second expression, the evaluation sequence is as follows.

(False is False) is False

Which is actually

True is False

That is why the result is False.

回答3:

Your expression

False is False is False

is treated as

(False is False) and (False is False)

So you get

True and True

and that evaluates to True.

You can use this kind of chaining with other operators too.

1 < x < 10

回答4:

I think that False is False is False means (False is False) and (False is False), but (False is False) is False means :

>>> (False is False) is False
False
>>> a_true = (False is False)
>>> a_true
True
>>> a_true is False
False

So, you get the result.

回答5:

>>> False is False is False
True

In this case, each of False pair is evaluated. The first two False is evaluated, if it's True, then the second and the third False is evaluated and return the result.

In this case, False is False is False is equivalent to and of results of 2 commands False is False