Understanding Boolean Expressions

[Rhodri James]

On Tue, 16 Apr 2013 23:19:25 +0100, Bruce McGoveran
<bruce.mcgoveran at 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