Parse Binary Expression

binary_parser.py

# operator precedence table
PRECEDENCE = {
    '<': 10, '>': 10, '<=': 10, '>=': 10, '==': 10, '!=': 10,
    '+': 20, '-': 20,
    '*': 30, '/': 30, '%': 30,
}

# parse 1 + 2 * 3
def parse_binary_right(self, prec, left):
    # prec starts with default 0
    # left operand is 1
    while True:
        # on first iteration the current token is +
        # query its precedence to get 20
        left_prec = self.get_precedence()
        if left_prec < prec:
            return left

        op_value = self.token.value
        self._data.next()  # consume current + token

        # recurse to get right operand
        right = self.parse_element()
        
        # current token is now *, with precedence 30
        if left_prec < self.get_precedence():
            # operator has higher precedence so recurse to get right operand
            right = self.parse_binary_right(left_prec + 1, right)

        # after recursion right operand is now a binary AST: Binary(2 * 3)
        # final binary expression is created here as Binary(1 + Binary(2 * 3))
        left = Binary(op_value, left, right)