Four arithmetic operations

four_arithmetic_operations.py

def is_digit( num ) :
    if num in list( map( lambda n: str( n ), range( 10 ) ) ) :
        return True
    return False

class Tokenizer :
    def __init__( self, formula ) :
        self.formula = formula
        self.tokens = []

        self.position = 0

        token = ''
        character = ''

        while True :
            character = self.peek()

            if character == None :
                break

            if character == ' ' :
                self.next()
                continue

            if is_digit( character ) or character == '.' :
                token = token + character
                self.next()
                continue

            if token != '' :
                self.tokens.append( token )
                token = ''

            self.tokens.append( character )
            self.next()

        if token != '' :
            self.tokens.append( token )

    def peek( self ) :
        if self.position < len( self.formula ) :
            return self.formula[self.position]
        return None

    def next( self ) :
        self.position = self.position + 1

    def get_tokens( self ) :
        return self.tokens

class Parser :

    def __init__( self, tokens ) :
        self.tokens = tokens
        self.position = 0

    def peek( self ) :
        if self.position < len( self.tokens ) :
            return self.tokens[self.position]
        return None

    def poll( self ) :
        position = self.position
        self.position = self.position + 1
        return self.tokens[position]

    def parse( self ) :
        return self.expression()

    def expression( self ) :
        n = self.term()

        if self.peek() == '+' or self.peek() == '-' :
            n = {
                'type': 'BinaryExpression',
                'operator': self.poll(),
                'left': n,
                'right': self.term()
            }

        return n

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

        if self.peek() == '*' or self.peek() == '/' :
            n = {
                'type': 'BinaryExpression',
                'operator': self.poll(),
                'left': n,
                'right': self.factor()
            }

        return n

    def factor( self ) :
        if self.peek() == '(' :
            self.poll()
            n = self.expression()
            self.poll()
            return n
        else :
            return {
                'type': 'Literal',
                'value': self.poll()
            }

def evaluate( tree ) :
    if tree['type'] == 'BinaryExpression' :
        left = evaluate( tree['left'] )
        right = evaluate( tree['right'] )

        if tree['operator'] == '+' :
            return left + right
        if tree['operator'] == '-' :
            return left - right
        if tree['operator'] == '*' :
            return left * right
        if tree['operator'] == '/' :
            return left / right

    if tree['type'] == 'Literal' :
        if tree['value'].find( '.' ) == -1 :
            return int( tree['value'] )
        else :
            return float( tree['value'] )

formula = '((2 + 5.1) + 2) * 2'
tokens = Tokenizer( formula ).get_tokens()
parsed = Parser( tokens ).parse()
result = evaluate( parsed )

print( result )