extracting string.Template substitution placeholders

[Steven D'Aprano]

On Sun, 12 Jan 2014 10:08:31 -0500, Eric S. Johansson wrote:

> As part of speech recognition accessibility tools that I'm building, I'm
> using string.Template. In order to construct on-the-fly grammar, I need
> to know all of the identifiers before the template is filled in. what is
> the best way to do this?


py> import string
py> t = string.Template("$sub some $text $here")
py> t.template
'$sub some $text $here'

Now just walk the template for $ signs. Watch out for $$ which escapes 
the dollar sign. Here's a baby parser:

def get_next(text, start=0):
    while True:
        i = text.find("$", start)
        if i == -1:
            return
        if text[i:i+2] == '$$':
            start += i
            continue
        j = text.find(' ', i)
        if j == -1:
            j = len(text)
        assert i < j
        return (text[i:j], j)

start = 0
while start < len(t.template):
    word, start = get_next(t.template, start)
    print(word)


> can string.Template handle recursive expansion i.e. an identifier
> contains a template.

If you mean, recursive expand the template until there's nothing left to 
substitute, then no, not directly. You would have to manually expand the 
template yourself.


-- 
Steven