A simple LISP interpreter for https://scratch.mit.edu/discuss/topic/189101/

golfedowllisp.py

import re
from operator import *
i=lambda p: p if type(p)!=tuple else{"cons":lambda a,b:(a,b),"car":lambda a:a[0],"cdr":lambda a:a[1],"cadr":lambda a:a[1][0],"+":add,"-":sub,"*":mul,"print":print}[p[0]](*map(i,p[1:]))
while 1:i(eval(re.sub("[\n ]+",",",re.sub("[a-z+\-*]+","'\g<0>'",input()).replace("'nil'","''"))or"''"))

owllisp.py

import re
from functools import reduce
from operator import mul

# Dictionary of defined functions with lots of lambdas
FUNCTIONS = {"cons": lambda a, b: (a, b),
             "car": lambda a: a[0],
             "cdr": lambda a: a[1],
             "cadr": lambda a: a[1][0],
             "+": lambda *args: sum(args),
             "-": lambda a, b: a - b,
             "*": lambda *args: reduce(mul, args, 0),
             "print": lambda arg: print(arg)}

def parse(prog):
    "Parses a S-Expression by converting it into a tuple"
    # Replace every symbol (but not numbers) with the symbol enclosed in '',
    # so that when we run eval the symbols turn into strings
    prog = re.sub("[a-zA-Z+\-*]+", "'\g<0>'", prog)
    # Replace "nil" with None
    prog = prog.replace("'nil'", "None")
    # Replace all the newlines and spaces (Lisp list seperators)
    # with ", " (Python tuple seperators)
    prog = re.sub("[\n ]+", ", ", prog)
    # prog is now valid tuple syntax, so run eval to turn it into one
    return eval(prog)

def interpret(prog):
    "Recursively interprets the parsed prog"
    # If we are interpreting a literal (int or None), return it
    if type(prog) in (int, type(None)):
        return prog
    # Else, we have a tuple
    # Run the function given by the first item in the tuple
    # with arguments given by recusivly interpreting the rest of the tuple
    elif prog[0] in FUNCTIONS:
        return FUNCTIONS[prog[0]](*tuple(map(interpret, prog[1:])))


def run(prog):
    "A helper function that parses and inteprets prog"
    return interpret(parse(prog))

if __name__ == "__main__":
    print("OwlLISP REPL")
    while True:
        prog = input("Enter your program: ")
        ret = run(prog)
        if ret:
            print(ret)

Next steps:

• Add lambda
• Add quoted lists

If you want to continue, I'd suggest to make it less code golf-y 📦 (not that you have to)