S-expression parser in python (minus value classes)

sexp.py

from string import whitespace, digits
from values import Cell, Symbol, Integer

token_boundaries = '(.)' + whitespace

# parser functions return (result, leftover) tuples

# parse a token single token, respecting token boundaries
def parse_token(s):
    # find the first token boundary character
    indices = [s.find(boundary) for boundary in token_boundaries]
    indices = [index for index in indices if index > 0]
    
    # if there are none, return the whole string with no leftover
    if not indices:
        return (s, '')
    
    # if there is one, split it out and strip off whitespace from the rest
    index = min(indices)
    return (s[:index], s[index:].lstrip(whitespace))

def parse_int(s):
    # parse the first token and turn it into an int
    val, rest = parse_token(s)
    return (Integer(int(val)), rest)

def parse_symbol(s):
    # parse the first token and turn it into a symbol
    val, rest = parse_token(s)
    
    # special nil handling
    if val == "nil":
        return (None, rest)
    
    return (Symbol(val), rest)

# accepts a string *without* the leading open paren
# like so: `first second third . cdr) ...`
def parse_cell(s):
    if s[0] == ')':
        # end of list. None ~= empty list
        return (None, s[1:].lstrip(whitespace))
    
    # get the car from the first value
    car, rest = parse_value(s)
    
    # check for a dot at the beginning of rest, indicating an improper list
    if rest[0] == '.':
        # improper list!
        # last value can be any sort of value
        cdr, rest = parse_value(rest[1:].lstrip(whitespace))
    else:
        # no dot found yet
        # parse the rest of the list into cdr
        cdr, rest = parse_cell(rest)
    
    return (Cell(car, cdr), rest)

def parse_value(s):
    # figure out what sort of thing to parse based on the first character
    if s[0] in digits:
        return parse_int(s)
    if s[0] == '(':
        # shave off this open paren, since parse_cell expects it to be gone
        return parse_cell(s[1:].lstrip(whitespace))
    else:
        # if nothing else matched, it must be a symbol
        return parse_symbol(s)

print(parse_value("52 a b"))
print(parse_value("apples oranges"))
print(parse_value("(apples oranges pear)"))
print(parse_value("(apples oranges . pear)"))
print(parse_value("(apples oranges . nil)"))
print(parse_value("(oranges (this . that))"))