MathExpression.py

import decimal

import MathOperations


class MathExpression:

    evaluated_decimal = None

    leftside_decimal = None
    operation_string = None
    rightside_expression = None

    def __init__(self, math_expression_string):
        """Parsing the math_expression_string"""

        # Set self.leftside_decimal
        leftside_decimal_parsed_string = parse_for_leftside_decimal_string(math_expression_string)
        try:
            self.leftside_decimal = decimal.Decimal(leftside_decimal_parsed_string)
        except decimal.InvalidOperation:
            raise InvalidMathExpressionException

        # Check if there's an operation after the leftside_decimal
        operation_check_result, operation_end_index = check_for_operation(math_expression_string, len(leftside_decimal_parsed_string))
        if operation_check_result is None:
            self.evaluated_decimal = self.leftside_decimal
            return
        self.operation_string = operation_check_result

        # Set self.rightside_expression
        self.rightside_expression = MathExpression(math_expression_string[operation_end_index:])

        # Finalization of this MathExpression object
        self.evaluated_decimal = MathOperations.compute_operation(
            self.leftside_decimal,
            self.operation_string,
            self.rightside_expression.evaluated_decimal,
        )


def parse_for_leftside_decimal_string(math_expression_string):
    end_index = 0
    encountered_nonspace_character_previously = False

    while not end_index == len(math_expression_string):
        char = math_expression_string[end_index]
        if char not in ['0', '1', '2', '3', '4', '5', '6', '7', '8', '9', '-', '.', ' ']:
            break

        if char == '-':
            if encountered_nonspace_character_previously:
                break

        if not char == ' ':
            encountered_nonspace_character_previously = True

        end_index += 1

    return math_expression_string[0:end_index]


def check_for_operation(math_expression_string, start_index):
    parsed_string = ''

    end_index = start_index
    while not end_index == len(math_expression_string):
        char = math_expression_string[end_index]

        if char.isdigit() or char == '.':
            break

        if not char == ' ':
            parsed_string += char
        if char == ' ' and not parsed_string == '':
            break
        end_index += 1

    if parsed_string == '':
        return None, None
    else:
        if MathOperations.string_represents_math_operation(parsed_string):
            return parsed_string, end_index
        else:
            print('String doesnt represent operation!')
            raise InvalidMathExpressionException


class InvalidMathExpressionException(Exception):
    pass