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I've rewritten the TRAC interpreter from Recipe 577366 using a modern recursive descent style based on the Dragon book's lexer/parser model at the end of "Compliers: Principles, Techniques and Tools," Chapter 2, by Aho, Sethi, Ullman (1986).

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import sys
import os
import unittest

EOF = ""
SEGMENT_GAP = "\xff"

class TracException(Exception):
    pass

def list_get(list_, index, default=EOF):
    try:
        return list_[index]
    except:
        return default

class Io(object):
    def __init__(self, s=""):
        self.buffer = []
        self.buffer += s
        
    def pop(self, default=EOF):
        try:
            return self.buffer.pop(0)
        except:
            return default

    def push(self, s):
        self.buffer[0:0] += s

    def delete(self, i, j):
        del self.buffer[i:j]

    def peek(self, count=1):
        try:
            chars = self.buffer[0:0 + count]
        except:
            chars = [EOF]
        return "".join(chars)
    
    def get_trac_string(self):
        chars = []
        c = self.pop()
        while True:
            if c == '#':
                if self.peek(1) == '(' or self.peek(2) == '#(':
                    break
            elif c in "(),":
                break
            else:
                chars.append(c)
                c = self.pop()
        if c != EOF: 
            self.push(c)
        s = "".join(chars)
        return s

    def get_trac_protected_string(self):
        chars = []
        paren_count = 0
        matched = False
        c = self.pop()
        while c != EOF and not matched:
            if c == '(':
                paren_count += 1
            elif c == ')':
                paren_count -= 1
            chars.append(c)
            if paren_count != 0:
                c = self.pop()
            else:
                matched = True                 
        if not matched:
            raise TracException, "%s: can't find matching end parenthesis" %("get_trac_protected_string") 
        s = "".join(chars[1:-1])
        return s

class Tag(object):
    CLOSE_PAREN = ')'
    COMMA = ','    
    NONE = -1
    STRING = 300
    OPEN_ACTIVE_FUNC = 400
    OPEN_NEUTRAL_FUNC = 500
    DONE = 600    
    
class Token(object):
    def __init__(self, t, value=''):
        self.tag = t
        self.value = value
        
    def __str__(self):
        return "Token([%s] [%s])" % (str(self.tag), str(self.value))
    
class String(Token):
    def __init__(self, v):
        Token.__init__(self, Tag.STRING, v)
      
class Lexer(object):
    def __init__(self):
        self.lookahead = None
        self.output = ""         # last string printed to output by ps for unit testing
        self.trace = True        # flag for printing trace results of function evaluation     
        self.primitives = {"ds":self.ds, \
                           "ps":self.ps, \
                           "ss":self.ss, \
                           "cl":self.cl, \
                           "ad":self.ad, \
                           "su":self.su, \
                           "ml":self.ml, \
                           "dv":self.dv, \
                           "tn":self.tn, \
                           "tf":self.tf, \
                           "eq":self.eq \
                            }

    def initialize(self, program=""):
        self.forms = {}
        self.io = Io(program)

    def tn(self, args):
        self.trace = True
        return ""

    def tf(self, args):
        self.trace = False
        return ""

    def ds(self, args):
        key = list_get(args, 0)
        value = list_get(args, 1)
        self.forms[key] = value
        return ""

    def ps(self, args):
        try:
            s = list_get(args, 0)
            print s
            self.output = s
        except:
            pass
        return ""
    
    def ad(self, args):
        try:
            num1 = int(list_get(args, 0))
            num2 = int(list_get(args, 1))
            return str(num1 + num2)
        except:
            return ""
    
    def su(self, args):
        try:
            num1 = int(list_get(args, 0))
            num2 = int(list_get(args, 1))
            return str(num1 - num2)
        except:
            return ""
    
    def ml(self, args):
        try:
            num1 = int(list_get(args, 0))
            num2 = int(list_get(args, 1))
            return str(num1 * num2)
        except:
            return ""
    
    def dv(self, args):
        try:
            num1 = int(list_get(args, 0))
            num2 = int(list_get(args, 1))
            return str(num1 / num2)
        except:
            return ""

    def eq(self, args):
        try:
            s1 = list_get(args, 0)
            s2 = list_get(args, 1)
            eq_result = list_get(args, 2)
            neq_result = list_get(args, 3)
            if s1 == s2:
                return eq_result
            else:
                return neq_result
        except:
            return ""
    
    def ss(self, args):
        try:
            form_key = args.pop(0)
            form = self.forms[form_key]
            form_marked = form
            for i in range(len(args)):
                arg = args[i]
                marker = "%s%s" % (SEGMENT_GAP, chr(i))
                form_marked = form_marked.replace(arg, marker)
            self.forms[form_key] = form_marked
            form_list = []
            form_list += form_marked
            return ""
        except:
            return ""
            
    def cl(self, args):
        try:
            form_key = args.pop(0)
            form = self.forms[form_key]
            form_processed = form
            for i in range(len(args)):
                arg = args[i]
                marker = "%s%s" % (SEGMENT_GAP, chr(i))
                form_processed = form_processed.replace(marker, arg)
            return form_processed
        except:
            return ""

    def lexan(self):
        token = None
        while not token:
            c = self.io.pop()
            if c == '\t' or  c == '\n':
                pass # strip out white space
            elif c == '(':
                self.io.push(c)
                s = self.io.get_trac_protected_string()
                token = String(s)       
            elif c == ')':
                token = Token(Tag.CLOSE_PAREN)
            elif c == '#' and self.io.peek(1) == '(':
                self.io.delete(0,1)
                token = Token(Tag.OPEN_ACTIVE_FUNC)
            elif c == '#' and self.io.peek(2) == '#(':
                self.io.delete(0,2)
                token = Token(Tag.OPEN_NEUTRAL_FUNC)
            elif c == ',':
                token = Token(Tag.COMMA)
            elif c == EOF:
                token = Token(Tag.DONE)
            else:
                self.io.push(c)
                s = self.io.get_trac_string()
                token = String(s)       
        
        return token
            
    def func(self, tag=""):
        result = ""
        args = []
        token = self.lookahead
        if token.tag == Tag.STRING:
            args.append(token.value)        
        self.factor()
        while True:
            token = self.lookahead
            if token.tag in (Tag.STRING, Tag.OPEN_ACTIVE_FUNC, 
                             Tag.OPEN_NEUTRAL_FUNC, Tag.COMMA):
                if token.tag == Tag.STRING:
                    args.append(token.value)
                result = self.factor()
                if token.tag == Tag.OPEN_NEUTRAL_FUNC:
                    args.append(result)
            else:
                if tag:
                    result = self.eval_func(args)
                break
        return result
            
    def factor(self):
        result = None
        token = self.lookahead
        if token.tag == Tag.OPEN_ACTIVE_FUNC:
            self.match(Tag.OPEN_ACTIVE_FUNC)
            result = self.func(token.tag)
            self.match(Tag.CLOSE_PAREN, result)
        elif token.tag == Tag.OPEN_NEUTRAL_FUNC:
            self.match(Tag.OPEN_NEUTRAL_FUNC)
            result = self.func(token.tag)
            self.match(Tag.CLOSE_PAREN)
        elif token.tag == Tag.COMMA:
            self.match(Tag.COMMA)
        elif token.tag == Tag.STRING:
            self.match(Tag.STRING)
        elif self.lookahead.tag == Tag.DONE:
            self.match(Tag.DONE)
        else:
            raise TracException("factor: bad token tag - %s" % token)     
        return result
   
    def match(self, tag, result=None):
        if self.lookahead.tag == tag:
            if result != None:
                self.io.push(result)
            self.lookahead = self.lexan()
        else:
            raise TracException("match: tag %s doesn't match %s" % (tag, self.lookahead))

    def eval_func(self, args):
        result = ""
        try:
            func_name = args[0]
            args = args[1:]
            primitive = self.primitives.get(func_name, None)
            if primitive:
                result = primitive(args)
                if self.trace:
                    print "eval: %s %s -> [%s]" % (func_name, args, result)
        except Exception, e:
            raise TracException, "%s: failed - %s" %("eval", e) 
        return result
        
    def parse(self):
        self.lookahead = self.lexan()
        while self.lookahead.tag != Tag.DONE:
            self.func()
            self.match(Tag.DONE)
        print "Forms: %s" % self.forms
        print "Output: %s" % self.output
        return self
            
class TestTrac(unittest.TestCase):
        
    def setUp(self):
        pass
        
    def __test(self, program, output):
        self.lexer = Lexer()
        self.lexer.initialize(program)
        self.lexer.parse()
        self.assertEqual(self.lexer.output, output)

    def test_1_ps(self):
        self.__test("#(ps,Hello world)", "Hello world")
    
    def test_2_equal(self):
        self.__test("#(ps,#(eq,A,A,=,!=))", "=")

    def test_3_not_equal(self):
        self.__test("#(ps,#(eq,Cat,Dog,equal,not equal))", "not equal")

    def test_4_ds(self):
        self.__test("#(ds,AA,Cat)#(ps,#(cl,AA))", "Cat")

    def test_5_protect_parens(self):
        self.__test("#(ds,AA,Cat)#(ds,BB,(#(cl,AA)))#(ps,(#(cl,BB)))", "#(cl,BB)")

    def test_6_neutral_func(self):
        self.__test("#(ds,AA,Cat)#(ds,BB,(#(cl,AA)))#(ps,##(cl,BB))", "#(cl,AA)")

    def test_7_indirection(self):
        self.__test("#(ds,AA,Cat)#(ds,BB,(#(cl,AA)))#(ps,#(cl,BB))", "Cat")

    def test_8_ss(self):
        self.__test("#(ds,AA,Hello X)#(ss,AA,X)#(ps,#(cl,AA,world))", "Hello world")

    def test_9_factorial(self):
        self.__test("""
#(ds,Factorial,(#(eq,X,1,1,(#(ml,X,#(cl,Factorial,#(su,X,1)))))))
#(ss,Factorial,X)
#(ps,#(cl,Factorial,5))
""", "120")

if __name__ == "__main__":
    print __file__
    unittest.main()

This approach roughly halves the lines of code to implement the lexical analyzer portion of the interpreter than in the original algorithm specified by Calvin Mooers, TRAC's creator.

Assuming one is used to recursion, recursive descent is more comprehensible than the Mooers mechanism. Breaking the character stream into tokens allows one a better view of the overall structure.

For a discussion of the TRAC language, see http://code.activestate.com/recipes/577366-trac-interpreter-sixties-programming-language/ .