Start code that takes some Lisp code, returns its abstract syntax tree, and runs it.

lisp_parser.py

#!/usr/bin/env python3

import ast

lisp_code = '(first (list 1 (+ 2 3) 9))'

# find innermost list
# get its abstract syntax tree (AST)
# (run it)
# find second innermost list and iterate
# ...
# run AST of lisp_code


def make_ast(cs):
    """Return AST for flat list."""
    l = cs.strip('()').split()
    l = [l[0]] + [ast.literal_eval(i) for i in l[1:]]
    assert isinstance(l[0], str)
    return l


def run_sum(l):
    """Return sum of non-first elements."""
    assert l[0] == '+'
    return sum(l[1:])


def run_list(l):
    """Return list of non-first elements."""
    assert l[0] == 'list'
    return [i for i in l[1:]]

if __name__ == "__main__":
    # execute only if run as a script
    char_str = '(+ 2 3)'  # innermost list
    li = make_ast(char_str)
    print('Call our implementation of sum function on innermost list:')
    s = run_sum(li)
    print(s)