lisp parser

lis.py

def lex(inp):
    tkns = list()
    acc = list()
    for ch in inp:
        if ch.isspace() and acc:
            tkns.append(''.join(acc))
            acc = list()
        elif ch == '(' or ch == ')':
            if acc:
                tkns.append(''.join(acc))
                acc = list()
            tkns.append(ch)
        elif not ch.isspace():
            acc.append(ch)
    if acc:
        tkns.append(''.join(acc))
    return tkns

def parse(tkns):
    exps = list()
    while tkns:
        if tkns[0] == ')':
            break
        elif tkns[0] == '(':
            e, tkns = parse(tkns[1:])
            if not tkns or tkns[0] != ')':
                raise Exception("Expected ')'")
            exps.append(e if len(e) != 1 else e[0])
        else:
            exps.append(tkns[0])
        tkns = tkns[1:]
    return exps, tkns

def parse(inp):
    return parse(lex(inp))[0]