Trivial string substitution/parser

[Graham Breed]

Duncan Booth wote:

> Also, of course, vars just needs to be something which quacks like a dict:
> it can do whatever it needs to do such as looking up a database or querying
> a server to generate the value only when it needs it, or even evaluating
> the name as an expression; in the OP's case it could call get_variable.

And in case that sounds difficult, the code is

class VariableGetter:
    def __getitem__(self, key):
        return get_variable(key)

> Anyway, the question seems to be moot since the OP's definition of 'elegant
> and lazy' includes regular expressions and reinvented wheels.

Your suggestion of subclassing string.Template will also require a
regular expression -- and a fairly hairy one as far as I can work out
from the documentation.  There isn't an example and I don't think it's
the easiest way of solving this problem.  But if Samuel really wants
backslash escaping it'd be easier to do a replace('$$','$$$$') and
replace('\\$', '$$') (or replace('\\$','\\$$') if he really wants the
backslash to persist) before using the template.

Then, if he really does want to reject single letter variable names,
or names beginning with a backslash, he'll still need to subclass
Template and supply a regular expression, but a simpler one.

> ... and in another message Graham Breed wrote:
> >     def get_variable(varname):
> >         return globals()[varname]
>
> Doesn't the mere thought of creating global variables with unknown names
> make you shudder?

Not at all.  It works, it's what the shell does, and it's easy to test
interactively.  Obviously the application code wouldn't look like
that.


                         Graham