[stringtemplate-interest] Problem in multivalued attribute handling?

[Colin Bean]

Okay, I've got the beginnings of a fix.  The problem is indeed in
checking for the existence of an attribute.  As Praki mentioned
earlier, using a conditional like "if x:" looks like it should simply
check for the existence of x, but in fact evaluates to false not only
if x is None, but also if x is an empty container or a numeric zero.
It's a weird little python feature that has bitten me a couple of
times; the solution is to explicitly check if a value is None.
I'm sure that there are many other places in the code that could use a
review for this, the following changes made PyST correctly evaluate a
(single or multi-valued) attribute with the integer value of zero.  I
changed one line in StringTemplate.py, where it checks for the
attribute's existence before setting it, and one in
language/ASTExpr.py, where it again checks existence before writing an
attribute out.  I ran the code through the unit tests and it was fine,
but other than that and my "zero" test I haven't run this code
anywhere else -- ymmv.  Here's a diff of the changes:

--- StringTemplateOrig.py       2006-07-10 15:53:44.000000000 -0700
+++ StringTemplate.py   2006-07-10 15:59:49.000000000 -0700
@@ -677,7 +677,7 @@
             # a normal call to setAttribute with unknown attribute
             raise KeyError("no such attribute: " + name +
                " in template context " +
self.getEnclosingInstanceStackString())
-        if value:
+        if value is not None:
             attributes[name] = value
         elif isinstance(value, list) or \
              isinstance(value, dict) or \
--- language/ASTExprOrig.py     2006-07-10 15:57:35.000000000 -0700
+++ language/ASTExpr.py 2006-07-10 15:11:35.000000000 -0700
@@ -390,7 +390,7 @@
         return self._write(this, o, out, separator)

     def _write(self, this, o, out, separator):
-        if not o:
+        if o is None:
             return 0
         n = 0
         try:


If I find anything else relevant to this I'll post it.
Regards,
-Colin