python解释器实现及其嵌入式应用：一个四则运算计算器的实现

这是《Let’s build a simple interpreter》中第四部分的练习

原文参考

一 BNF

expr→term((PLUS/MINUS)term)∗

term→factor((MUL/DIV)factor)∗

factor→INTERGER

二 BNF到代码实现

按照如下规则即可将BNF转换成代码：

1. 每个非终结符对应一个函数
2. 每个终结符对应读取token
3. ‘*’对应while循环
4. ‘|’对应if…else

2.1 Token类实现

class Token(object):
def __init__(self, type, value):
# token type: INTEGER, MUL, DIV, or EOF
        self.type = type
# token value: non-negative integer value, '*', '/', or None
        self.value = value

def __str__(self):
"""String representation of the class instance.

 Examples:
 Token(INTEGER, 3)
 Token(MUL, '*')
 """
return 'Token({type}, {value})'.format(
            type=self.type,
            value=repr(self.value)
        )

def __repr__(self):
return self.__str__()

2.2 lexer类实现

class Lexer(object):
def __init__(self, text):
# client string input, e.g. "3 * 5", "12 / 3 * 4", etc
        self.text = text
# self.pos is an index into self.text
        self.pos = 0
        self.current_char = self.text[self.pos]

def error(self):
raise Exception('Invalid character')

def advance(self):
"""Advance the `pos` pointer and set the `current_char` variable."""
        self.pos += 1
if self.pos > len(self.text) - 1:
            self.current_char = None  # Indicates end of input
else:
            self.current_char = self.text[self.pos]

def skip_whitespace(self):
while self.current_char is not None and self.current_char.isspace():
            self.advance()

def integer(self):
"""Return a (multidigit) integer consumed from the input."""
        result = ''
while self.current_char is not None and self.current_char.isdigit():
            result += self.current_char
            self.advance()
return int(result)

def get_next_token(self):
"""Lexical analyzer (also known as scanner or tokenizer)

 This method is responsible for breaking a sentence
 apart into tokens. One token at a time.
 """
while self.current_char is not None:

if self.current_char.isspace():
                self.skip_whitespace()
continue

if self.current_char.isdigit():
return Token(INTEGER, self.integer())

if self.current_char == '*':
                self.advance()
return Token(MUL, '*')

if self.current_char == '/':
                self.advance()
return Token(DIV, '/')

if self.current_char == '+':
                self.advance()
return Token(PLUS, '+')

if self.current_char == '-':
                self.advance()
return Token(MINUS, '-')

            self.error()

return Token(EOF, None)

2.3 interpreter类实现

class Interpreter(object):
def __init__(self, lexer):
        self.lexer = lexer
# set current token to the first token taken from the input
        self.current_token = self.lexer.get_next_token()

def error(self):
raise Exception('Invalid syntax')

def eat(self, token_type):
# compare the current token type with the passed token
# type and if they match then "eat" the current token
# and assign the next token to the self.current_token,
# otherwise raise an exception.
if self.current_token.type == token_type:
            self.current_token = self.lexer.get_next_token()
else:
            self.error()

def factor(self):
"""Return an INTEGER token value.

 factor : INTEGER
 """
        token = self.current_token
        self.eat(INTEGER)
return token.value

def term(self):
        result = self.factor()

while self.current_token.type in (MUL, DIV):
            token = self.current_token
if token.type == MUL:
                self.eat(MUL)
                result = result * self.factor()
elif token.type == DIV:
                self.eat(DIV)
                result = result / self.factor()
return result

def expr(self):
"""Arithmetic expression parser / interpreter.

 expr : factor ((MUL | DIV) factor)*
 factor : INTEGER
 """
        result = self.term()

while self.current_token.type in (PLUS, MINUS):
            token = self.current_token
if token.type == PLUS:
                self.eat(PLUS)
                result = result + self.term()
elif token.type == MINUS:
                self.eat(MINUS)
                result = result - self.term()

return result