Get keys from template

Question

I would like to get a list of all possible keyword arguments a string template might use in a substitution.

Is there a way to do this other than re?

Answer 1

The string.Template class has the pattern that is uses as an attribute. You can print the pattern to get the matching groups

>>> print string.Template.pattern.pattern

    \$(?:
      (?P\$) |   # Escape sequence of two delimiters
      (?P[_a-z][_a-z0-9]*)      |   # delimiter and a Python identifier
      {(?P[_a-z][_a-z0-9]*)}   |   # delimiter and a braced identifier
      (?P)              # Other ill-formed delimiter exprs
    )

And for your example,

>>> string.Template.pattern.findall("$one is a $lonely $number.")
[('', 'one', '', ''), ('', 'lonely', '', ''), ('', 'number', '', '')]

As you can see above, if you do ${one} with braces it will go to the third place of the resulting tuple:

>>> string.Template.pattern.findall('${one} is a $lonely $number.')
[('', '', 'one', ''), ('', 'lonely', '', ''), ('', 'number', '', '')]

So if you want to get all the keys, you'll have to do something like:

>>> [s[1] or s[2] for s in string.Template.pattern.findall('${one} is a $lonely $number.$$') if s[1] or s[2]]
['one', 'lonely', 'number']