Create a simple calculator that given a string of operators (), +, -, *, / and numbers separated by spaces returns the value of that expression.
Calculator().evaluate("2 / 2 + 3 * 4 - 6") # => 7
Remember about the order of operations! Multiplications and divisions have a higher priority and should be performed left-to-right. Additions and subtractions have a lower priority and should also be performed left-to-right.