Python is, == operator precedence

Question

In Python3,

a = b = 3
a is None == b is None

returns False, but

(a is None) == (b is None)

returns True.

Answer 1

What you see here is operator chaining and there is no precedence involved at all!

Python supports expressions like

1 < a < 3

To test that a number is in between 1 and 3; it's equal to (1 < a) and (a < 3) except that a is only evaluated once.

Unfortunately that also means that e.g.

None is None == None

actually means

(None is None) and (None == None)

which is of course True, and the longer example you started with

a = b = 3
a is None == b is None

means

(a is None) and (None == b) and (b is None)

which can only be True if both a and b are None.

Documentation here, see the bit about chaining.

Very useful sometimes but it also pops up when you least expect it!