## Observer Pattern

### Ex:

Create a program that simulates a stock market. The program should consist of two classes: $**`$Stock$`**$ and $**`$Investor$`**.$

The $**`$Stock$`**$ class should have the following properties:

$-**`$symbol$`**($string$)$: The stock symbol $($e$.$g$.,$ "AAPL"$).$

$-**`$price$`**($float$)$: The current price of the stock.

$-**`$investors$`**($list$)$: A list of $**`$Investor$`**$ objects that have invested in the stock.

$-**`$register$\_$investor$($self$,$ investor$)`**$: A method that adds an $**`$Investor$`**$ object to the list of investors.

$-**`$unregister$\_$investor$($self$,$ investor$)`**$: A method that removes an $**`$Investor$`**$ object from the list of investors.

$-**`$update$\_$price$($self$,$ price$)`**$: A method that updates the price of the stock and notifies all investors that have invested in the stock.

The $**`$Investor$`**$ class should have the following properties:

$-**`$name$`**($string$)$: The name of the investor.

$-**`$stocks$`**($list$)$: A list of $**`$Stock$`**$ objects that the investor has invested in$.$

$-**`$update$($self$,$ stock, price$)`**$: A method that is called by a $**`$Stock$`**$ object to notify the investor of a change in price.

Here are some additional requirements for the program:

$-$ The program should allow the user to create multiple $**`$Stock$`**$ objects and multiple $**`$Investor$`**$ objects.

$-$ The user should be able to register an $**`$Investor$`**$ object with a $**`$Stock$`**$ object and unregister an $**`$Investor$`**$ object from a $**`$Stock$`**$ object.

$-$ The user should be able to update the price of a $**`$Stock$`**$ object and see the changes reflected in all the $**`$Investor$`**$ objects that have invested in that stock.

To complete the exercise, you will need to implement the observer pattern in your program. The $**`$Stock$`**$ class should be the subject, and the $**`$Investor$`**$ class should be the observer. When the price of a $**`$Stock$`**$ object is updated, the $**`$Stock$`**$ object should notify all the $**`$Investor$`**$ objects that have invested in that stock.