docstring is like a manual or descrption, I find it can be very helpful.

Espeically, if I work with someone else, i might not be able to understand his or her
code, but with docstring, I would have a better understanding of his or her intention.
even if I work alone, for the beginner level of coder, it would be helpful to
write down the docstring to organize the plans or orders.

yahtzee.py

@@ -10,12 +10,21 @@ class Yachtzee:
         self.dice = [Die() for num in range(5)]
 
     def play(self):
+        """While there is still avaiable goals, keep playing the game.
+        When the game is over, print the final score"""
         print("Welcome to Yachtzee!")
         while self.count_unused_goals() > 0:
             self.play_round()
         print(f"Your final score was {self.score}")
 
     def play_round(self):
+        """Show the image of an equal sign 80 times in a row.
+        Define the remaining rolls is three. 
+        Roll all five dice. 
+        Show the result of the row.
+        Ask the user to choose the goal. 
+        Mark the goal as used. 
+        Add score from this round to the total score"""
         print("=" * 80)
         self.rolls_left = 3
         for die in self.dice:
@@ -26,10 +35,18 @@ class Yachtzee:
         self.score += goal.score(self.dice)
 
     def show_status(self):
+        """show the list numbers on the dice with comma and a space.
+        Show the score and the remaining rolls."""
         dice = ', '.join([str(die) for die in self.dice])
         print(f"Score: {self.score}. Rolls left: {self.rolls_left}. Dice: {dice}.")
 
     def choose_goal(self):
+        """Build the selection menu for the user, and ask the user to select 
+        one option. 
+        If there is no avaiable roll, then the option for 'reroll' will not be shown.
+        If the user's choice is 're-roll', then the user roll the dice again, and
+        this fuction gets called again.
+        If the user choose the goal, it will return the selected goal."""
         options = []
         unused_goals = self.get_unused_goals()
         for goal in unused_goals:
@@ -46,6 +63,9 @@ class Yachtzee:
             return unused_goals[choice]
 
     def get_choice(self, options):
+        """Ask the user the input. And print the avaiable choices. 
+        Check if the choice is valid, if not, ask again. if valid, then return
+        the choice"""
         print("What would you like to do?")
         for i, option in enumerate(options):
             print(f"{i}. {option}")
@@ -56,6 +76,9 @@ class Yachtzee:
         return int(choice)
 
     def option_choice_is_valid(self, choice, options):
+        """if the users provide non-digit or a number that is less than zero
+        or a number that are greater than the maxium option number, then return
+        false. If valid, return true."""
         if not choice.isdigit():
             return False
         if int(choice) < 0:
@@ -65,9 +88,12 @@ class Yachtzee:
         return True
 
     def count_unused_goals(self):
+        """get list of unused goals, and returen its length. """
         return len(self.get_unused_goals())
 
     def get_unused_goals(self):
+        """Iterate through all possible goals, and return a list containing 
+        only the unused goals."""
         unused_goals = []
         for goal in self.goals:
             if not goal.used:
@@ -75,6 +101,9 @@ class Yachtzee:
         return unused_goals
 
     def reroll(self):
+        """Subtract 1 from the avaiable rolls, then ask the user which dice he 
+        or she wants to roll
+        And then, roll them"""
         self.rolls_left -= 1
         choices = self.get_reroll_choices()
         dice_to_reroll = self.get_dice_to_reroll(choices)
@@ -82,6 +111,8 @@ class Yachtzee:
             die.roll()
 
     def get_dice_to_reroll(self, choice_ints):
+        """ Loop through all dies, if the die face is in the list of the number
+        the user wants to reroll, then add it to the reroll """
         dice_to_reroll = []
         for die in self.dice:
             if die.face in choice_ints:
@@ -90,6 +121,8 @@ class Yachtzee:
         return dice_to_reroll
 
     def get_reroll_choices(self):
+        """Asks the user to input the numbers to reroll, check the selection is
+        valid, and if it is, return it. """
         print("Which dice do you want to re-roll?")
         choices = input("> ")
         while not self.reroll_choices_are_valid(choices):
@@ -99,6 +132,9 @@ class Yachtzee:
         return choice_ints
         
     def reroll_choices_are_valid(self, choices_str):
+        """Check the user input is valid choice for the die number, for example, 
+        if the input is not digit, return false. Then, check the input number
+        appears on the die face."""
         if not choices_str.isdigit():
             return False
         choice_ints = [int(digit) for digit in choices_str]