Python：160行代码写一个编辑器和解释器

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2008-06-23 14:58 by 副主编 QQbyte 评论(8) 有15502人浏览

Python

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M. Taylor写道：我在几周前开始学习Python，作为一个学习用的项目，我给自己设定的目标是编写一个简单的while语句的编辑器和解释器。写Python代码如同梦幻，工作流程如下：思考你如何解决这个问题，尝试用最高级别eval loop去解决，如果不能完成，找另外的途径，如果可行，把它放入你的模块。终于我成功了！

我用最简单的方式来写python：

stmtlist   := (statement)*
statement  :=    'if' condition stmtlist ['else' stmtlist] 'endif'
               | 'while' condition stmtlist 'endwhile'
               | label '=' expression
               | 'print' expression
condition  := expression ('=='|'!=') expression
expression := term ('+'|'-' term)*
term       := label|digit

下面是全部代码：

#****************** Lexer ************************

tokenlist = []
currtoken = ("", "", 0)
keywords = set(["while", "endwhile", "if", "else", "endif", "print", "=", "==", "!=", "+", "-"])
symboltable = dict()

def nextToken():
    global currtoken, symboltable
    if(len(tokenlist) > 0):
        s = tokenlist.pop(0)
        if s in keywords:
            currtoken = (s, "", 0)
        elif s.isdigit():
            currtoken = ("digit", "", int(s))
        elif s.isalnum():
            symboltable[s] = 0
            currtoken = ("label", s, 0)
        else:
            print "syntax error: " + s
    else:
        currtoken = ("", "", 0)

def consume(expected):
    if currtoken[0] == expected:
        nextToken()
    else:
        print "expected " + expected + " not found" 

#****************** Parser ************************

def parseFile(filename):
    inputfile = open(filename, "r")
    inputstring = inputfile.read()
    global tokenlist
    tokenlist = inputstring.split()
    nextToken()
    return doStatementList()

def doStatementList():
    stmts = []
    newstmt = []    
    while currtoken[0] in ["while", "if", "print", "label"]:
        if currtoken[0] == "while":
            # ["while", [condition], [statementlist]]
            consume("while")
            newstmt = ["while"]
            newstmt.append(doCondition())
            newstmt.append(doStatementList())
            consume("endwhile")
        elif currtoken[0] == "if":
            # ["if", [condition], [then part], [else part]]
            consume("if")
            newstmt = ["if"]
            newstmt.append(doCondition())
            newstmt.append(doStatementList())
            if currtoken[0] == "else":
                consume("else")
                newstmt.append(doStatementList())
            consume("endif")
        elif currtoken[0] == "print":
            # ["print", [expression]]
            consume("print")
            newstmt = ["print"]
            newstmt.append(doExpression())
        elif currtoken[0] == "label":
            # ["=", [expression], [expression]]
            label = [currtoken[1]]
            nextToken()
            consume("=")
            newstmt = ["="]
            newstmt.append(label)
            newstmt.append(doExpression())
        else:
            print "invalid statement: " + currtoken[0]
        stmts.append(newstmt)
    return stmts

def doCondition():
    exp = doExpression()
    # ["==|!=", [left side], [right side]]
    if currtoken[0] in ["==", "!="]:
        retval = [currtoken[0]]
        retval.append(exp)
        nextToken()
        retval.append(doExpression())
    else:
        print "expected == or != not found"
    return retval
    
def doExpression():
    term = doTerm()
    # carry the term in case there's no +|-
    exp = term
    # ["+|-", [left side], [right side]]
    while currtoken[0] in ["+", "-"]:
        exp = [currtoken[0]]
        nextToken()
        exp.append(term)
        exp.append(doExpression())
    return exp

def doTerm():
    if currtoken[0] == "label":
        retval = currtoken[1]
        nextToken()
    elif currtoken[0] == "digit":
        retval = currtoken[2]
        nextToken()
    return [retval]

#****************** Interpreter ************************
stack = []
def execStatementList(pgm):
    for stmt in pgm:
        execStatement(stmt)
        
def execStatement(stmt):
    if stmt[0] == "while":
        execCondition(stmt[1])
        while stack.pop():
            execStatementList(stmt[2])
            execCondition(stmt[1])
    elif stmt[0] == "if":
        execCondition(stmt[1])
        if stack.pop():
            execStatementList(stmt[2])
        elif len(stmt) == 4:
            execStatementList(stmt[3])
    elif stmt[0] == "=":
        execExpression(stmt[2])
        symboltable[stmt[1][0]] = stack.pop()
    elif stmt[0] == "print":
        execExpression(stmt[1])
        print "output:" + str(stack.pop())
    else:
        print "invalid statement"
    
def execCondition(cond):
    execExpression(cond[1])
    execExpression(cond[2])
    if cond[0] == "==":
        stack.append(stack.pop() == stack.pop())
    elif cond[0] == "!=":
        stack.append(stack.pop() != stack.pop())

def execExpression(exp):
    if len(exp) == 3:
        execExpression(exp[1])
        execExpression(exp[2])
        if exp[0] == "+":
            stack.append(stack.pop() + stack.pop())
        else:
            stack.append(stack.pop() - stack.pop())
    else:
        if type(exp[0]) == int:
            stack.append(exp[0])
        else:
            stack.append(symboltable[exp[0]])

来自: jroller.com

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评论共 8 条请登录后发表评论

8 楼 wu_093 2014-05-15 18:27

execExpression中

execExpression(exp[1])
execExpression(exp[2])
顺序反了

7 楼 fangzhouxing 2008-06-25 08:21

支持python,已经从java转向python开发Web应用！

6 楼 simonli 2008-06-24 15:25

Python代码简洁，优雅
接触后就爱上了

5 楼 AllenZhang 2008-06-24 13:02

语法太过于简单，不过确实写的比较精炼

4 楼 caixicai 2008-06-24 11:29

我也好想学呀。好玩吗？但是怕没时间

3 楼 shaka 2008-06-24 08:36

弄得我也对python感兴趣了

2 楼 hellas 2008-06-23 18:21

是很有趣，不过用了pyparsing的话，会更简单。

1 楼 QQbyte 2008-06-23 14:59

python代码很有趣。

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