Let’s Build A Simple Interpreter

calc.py

# Token types
# EOF (end-of-file) token is used to indicate that
# there is no more input left for lexical analysis
INTEGER, PLUS, MINUS, MUL, DIV, EOF = 'INTEGER', 'PLUS', 'MINUS', 'MUL', 'DIV', 'EOF'


class Token(object):
    def __init__(self, type, value):
        # token type: INTEGER, PLUS, MINUS, 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(PLUS '+')
        """
        return 'Token({type}, {value})'.format(
            type=self.type,
            value=repr(self.value)
        )

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


class Interpreter(object):
    def __init__(self, text):
        # client string input, e.g. "3 + 5", "12 - 5", etc
        self.text = text
        # self.pos is an index into self.text
        self.pos = 0
        # current token instance
        self.current_token = None
        self.current_char = self.text[self.pos]

    def error(self):
        raise Exception('Error parsing input')

    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.
        """
        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(PLUS, '+')

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

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

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

            self.error()

        return Token(EOF, None)

    def eat(self, token_types):
        # check the current token type is contaned in the passed token
        # types and if it's true then "eat" the current token
        # and assign the next token to the self.current_token,
        # otherwise raise an exception.
        if self.current_token.type in token_types:
            self.current_token = self.get_next_token()
        else:
            self.error()

    def expr(self):
        """Parser / Interpreter

        expr -> INTEGER PLUS INTEGER
        expr -> INTEGER MINUS INTEGER
        """
        # set current token to the first token taken from the input
        self.current_token = self.get_next_token()

        # we expect the current token to be an integer
        left = self.current_token
        self.eat((INTEGER))

        # we expect the current token to be either a '+' or '-' or '*' or '/'
        op = self.current_token
        self.eat((PLUS, MINUS, MUL, DIV))

        # we expect the current token to be an integer
        right = self.current_token
        self.eat((INTEGER))
        # after the above call the self.current_token is set to
        # EOF token

        # at this point either the INTEGER PLUS INTEGER or
        # the INTEGER MINUS INTEGER sequence of tokens
        # has been successfully found and the method can just
        # return the result of adding or subtracting two integers,
        # thus effectively interpreting client input
        if op.type == PLUS:
            result = left.value + right.value
        elif op.type == MINUS:
            result = left.value - right.value
        elif op.type == MUL:
            result = left.value * right.value
        elif op.type == DIV:
            result = left.value / right.value

        return result

    def expr_arbitrary(self):
        """Parser / Interpreter

        expr -> INTEGER PLUS INTEGER MINUS INTEGER
        """
        self.current_token = self.get_next_token()

        left = self.current_token
        self.eat((INTEGER))

        while (self.current_token.value):

            op = self.current_token
            self.eat((PLUS, MINUS))

            right = self.current_token
            self.eat((INTEGER))

            if op.type == PLUS:
                left = Token(INTEGER, left.value + right.value)
            elif op.type == MINUS:
                left = Token(INTEGER, left.value - right.value)

        result = left.value
        return result


def main():
    while True:
        try:
            text = input('calc> ')
        except EOFError:
            break
        if not text:
            continue
        interpreter = Interpreter(text)
        result = interpreter.expr_arbitrary()
        print(result)


if __name__ == '__main__':
    main()