How to get parameter names from a parametrized string?

What I’d like is:

param_str = "I like $thing_I_like and dislike $thing_I_dislike. What does $person_I_like like?"
get_params(param_str)  # -> ("thing_I_like", "thing_I_dislike", "person_I_like")

I’ve looked through string.Template, it does only substitution.

Is there a standard library way to do this? Because in case of regex, there would have to be a check if $smt is actually a valid Python variable name and so on.

Answer

string.Template, or one of its subclasses, stores the compiled regex that will be used to find identifiers to replace as the pattern class attribute. Therefore you can do:

>>> from string import Template
>>> s = "I like $thing_I_like and dislike $thing_I_dislike. What does $person_I_like like?"
>>> Template.pattern.findall(s)
[('', 'thing_I_like', '', ''), ('', 'thing_I_dislike', '', ''), ('', 'person_I_like', '', '')]

The groups in the result are:

• escaped (e.g. $$ -> "$");
• named (e.g. $identifier -> "identifier");
• braced (e.g. ${noun}ification -> "noun"); or
• invalid (“any other delimiter pattern (usually a single delimiter)”, e.g. $ -> "").

For your purposes, therefore, you probably want:

>>> [
...     named or braced
...     for escaped, named, braced, invalid in Template.pattern.findall(s)
...     if named or braced
... ]
['thing_I_like', 'thing_I_dislike', 'person_I_like']