On 2021-03-04 03:39, Avi Gross via Python-list wrote:
As a guess, Rob, precedence rules for not may not bind as strongly as you think.
1 + (not 1)
With parentheses, "not 1" is a subexpression that should be performed first and might
return the value "False"
1 + False
treats False in a numeric context as a zero so evaluates to 1.
But
1 + not 1
Looks a bit ambiguous as "+ " expects something it considers a number or that
can be converted to a number. Without parentheses, it may try to do
1 + not
Which gets the same Syntax error.
I looked and operator "not" is evaluated much later than "+" and just before "and" and
"or" so you need parentheses to force the interpretation you may intend. Similarly, some used of "and"
require parentheses as do other operators.
If you look through the grammar, the relevant bits are:
sum: sum '+' term
term: factor
factor: power
power: await_primary
await_primary: primary
primary: atom
and an 'atom' can be a number, etc, but can never start with "not".
A 'primary' can be an 'atom' and can have a subscript.
-----Original Message-----
From: Python-list <python-list-bounces+avigross=verizon....@python.org> On
Behalf Of Rob Cliffe via Python-list
Sent: Wednesday, March 3, 2021 10:19 PM
To: Python <python-list@python.org>
Subject: a + not b
I can't work out why
1 + - 1
1 + (not 1)
are legal syntax, but
1 + not 1
isn't.
Is there a good reason for this?
Thanks
Rob Cliffe
--
https://mail.python.org/mailman/listinfo/python-list
