issue with string.Template

[Michele Simionato]

This is somewhat in between a bug report and a feature request.
I was using the new string.Template class and I have run into a
few issues that I traced back to the usage of the idiom

'%s' % val

in the 'convert' helper function in Template.substitute.

I do not understand why '%s' % val was used instead of just
str(val).

'%s' % val in unfortunate since it results in the following
surprising (for me) behavior:

>>> from string import Template as T

>>> T("$obj").substitute(obj=("hello",))
'hello'

[not '("hello",)']

>>> T("$obj").substitute(obj=())
TypeError: not enough arguments for format string

[not '()']

Is this intended behavior? It is surprising since it is
different from what one would expect from old-fashioned string
interpolation

>>> "%(obj)s" % dict(obj=("hello",))
"('hello',)"
>>> "%(obj)s" % dict(obj=())
'()'

which is the behaviour I find most useful. Also, I do not like
that different expressions such as
T("$obj").substitute(obj=("hello",))
and T("$obj").substitute(obj="hello") give the same output
(this potentially hides type bugs).

So, take this as a bug report if the behavior is not intended and
as a feature request if the current behaviour is the intended
one ;)

            Michele Simionato

P.S. at the end, the problem is that string interpolation with
positional arguments is somewhat of a hack, but fixing this will
have to wait until Python 3000 ...