recurse center, pair programming

lisp_parser.py

def create_tokens(source):
    """convert source to tokens.
    
    convert a string "(first (list 1 (+ 2 3) 9))" into a python list 
    ['(', 'first', '(', 'list', '1', '(', '+', '2', '3', ')', '9', ')', ')'].
    
    this conversion allows us to recursively inspect each element in the list.
    """
    return source.replace('(',' ( ').replace(')',' ) ').split()

def create_ast(tokens):
    """convert tokens to ast.
    
    For example, if your code is given "(first (list 1 (+ 2 3) 9))", it could 
    return a nested array like ["first", ["list", 1, ["+", 2, 3], 9]].
    """
    
    # pop off first token and examine, first in first out
    token = tokens.pop(0)

    # whenever we encounter an open parenthesis, we create a new list
    if '(' == token:
        L = []
        while tokens[0] != ')':  # break when we see a closed paren
            L.append(create_ast(tokens))  # recursively read tokens
        tokens.pop(0)  # pop off ')' since we dont need it for our ast
        return L
    
    # base case
    else:
        return atom(token)
    
def atom(token):
    """
    if we have a string "2", convert it to a python integer 2. otherwise, keep
    it as a python string.
    """
    try: 
        return int(token)
    except ValueError:
        return str(token)