On Tue, 16 Apr 2013 23:19:25 +0100, Bruce McGoveran
<bruce.mcgove...@gmail.com> wrote:
Hello. I am new to this group. I've done a search for the topic about
which I'm posting, and while I have found some threads that are
relevant, I haven't found anything exactly on point that I can
understand. So, I'm taking the liberty of asking about something that
may be obvious to many readers of this group.
The relevant Python documentation reference is:
http://docs.python.org/2/reference/expressions.html#boolean-operations.
I'm trying to make sense of the rules of or_test, and_test, and not_test
that appear in this section. While I understand the substance of the
text in this section, it is the grammar definitions themselves that
confuse me. For example, I am not clear how an or_test can be an
and_test. Moreover, if I follow the chain of non-terminal references, I
move from or_test, to and_test, to not_test, to comparison. And when I
look at the definition for comparison, I seem to be into bitwise
comparisons. I cannot explain this.
Perhaps an example will help put my confusion into more concrete terms.
Suppose I write the expression if x or y in my code. I presume this is
an example of an or_test. Beyond that, though, I'm not sure whether
this maps to an and_test (the first option on the right-hand side of the
rule) or to the or_test "or" and_test option (the second on the
right-hand side of the rule).
If people can offer some thoughts to put me in the right direction (or
out of my misery), I would appreciate it.
What the grammar rules are giving you is operator precedence -- "not" has
higher precedence than "and", which is higher than "or". As you follow
the non-terminals back you go further through precedence chain:
comparisons, then bitwise operators (*not* bitwise comparisons!), then
shifts, then arithmetic operators, the unary operators, the power
operator, and finally primaries.
--
Rhodri James *-* Wildebeest Herder to the Masses
--
http://mail.python.org/mailman/listinfo/python-list
