[Python-Dev] Multiple expression eval in compound if statement?

[Guido van Rossum]

On 6/12/05, Skip Montanaro <skip at pobox.com> wrote:
> 
> I'm horsing around with recognizing switch-like if statements like:
> 
>     if x == 1:
>         print 1
>     elif x == 2:
>         print 2
>     else:
>         print "unknown"
> 
> in the compiler and generating O(1) code.  "x" can be any expression, but
> must be precisely the same in each elif clause, the comparison operator must
> be "==" and the RHS of the test must evaluate to a simple hashable constant
> (string, float, integer - still working on None, constant tuples may be
> allowed later).  I can currently recognize such constructs and am working on
> code generation.

Cool!

> This would represent a semantic change to the Python runtime, because x
> would only be evaluated once, whereas in existing usage it can potentially
> be evaluated many times.  If evaluating x has side-effects, code like the
> above that currently works would break.
> 
> In reading the language reference manual, it says:
> 
>     It selects exactly one of the suites by evaluating the expressions one
>     by one until one is found to be true (see section 5.10 for the
>     definition of true and false); then that suite is executed (and no other
>     part of the if statement is executed or evaluated). If all expressions
>     are false, the suite of the else clause, if present, is executed.
> 
> It says nothing about possibly caching values, which is what I'm doing
> effectively, but it seems pretty clear that each full test expression will
> be evaluated until one evaluates to true.
> 
> I can't imagine anyone relying on a side-effect when evaluating x in this
> context, and if someone did, their code would be truly fragile.  Still, any
> thought as to whether or not such a semantic change to the language might be
> acceptable?

I think it would be wiser if you only did this when x is a simple
local variable. For locals, we *know* that no side effect of any
expression can change the value.

-- 
--Guido van Rossum (home page: http://www.python.org/~guido/)