Add Snake.reset

Refs #19
Task: MIL-003#4

src/snake_game/snake.py

@@ -3,9 +3,10 @@
 
 The class follows the lectures "Create a Snake Class & Move to OOP", "Animating
 the Snake Segments on Screen" and "Controlling the Snake with Keypresses", and
-the day-21 steps in which the snake grows and the game ends. It keeps the lectures'
+the day-21 steps in which the snake grows, and the day-24 step in which it is reset
+after a collision. It keeps the lectures'
 names: `Snake`, `segments`, `create_snake`, `add_segment`, `extend`, `move`, `head`,
-`up`, `down`, `left` and `right`. The only differences are that the snake gets
+`reset`, `up`, `down`, `left` and `right`. The only differences are that the snake gets
 the function that makes a segment as a parameter, so that a test can pass a fake
 and needs no window, and how a reversal is refused (see Snake).
 """
@@ -20,6 +21,7 @@ from snake_game.constants import (
     LEFT,
     MOVE_DISTANCE,
     RIGHT,
+    SEGMENT_PARKING_POSITION,
     SEGMENT_SHAPE,
     STARTING_POSITIONS,
     TAIL_COLLISION_DISTANCE,
@@ -204,6 +206,20 @@ class Snake:
         self.head.forward(MOVE_DISTANCE)
         self._direction_of_travel = self.head.heading()
 
+    def reset(self) -> None:
+        """! @brief Start again with a new snake of three segments.
+
+        Every old segment is sent far outside the screen, so it can no longer be
+        seen, the list is emptied, and the snake is created again at the starting
+        positions. The new head is the first new segment, and it points right.
+        """
+        for segment in self.segments:
+            segment.goto(*SEGMENT_PARKING_POSITION)
+        self.segments.clear()
+        self.create_snake()
+        self.head = self.segments[0]
+        self._direction_of_travel = self.head.heading()
+
     def hits_wall(self) -> bool:
         """! @brief Tell whether the head has passed the wall on any side.
 

tests/test_snake.py

@@ -407,3 +407,61 @@ def test_a_snake_of_four_segments_cannot_touch_its_tail_in_a_small_square() -> N
         snake.move()
 
     assert not snake.hits_tail()
+
+
+def test_reset_sends_every_old_segment_off_the_screen() -> None:
+    snake, created = make_snake()
+    snake.extend()
+    old = list(created)
+
+    snake.reset()
+
+    assert len(old) == len(constants.STARTING_POSITIONS) + 1
+    assert all(
+        segment.position() == constants.SEGMENT_PARKING_POSITION for segment in old
+    )
+
+
+def test_reset_makes_the_snake_again_at_the_starting_positions() -> None:
+    snake, created = make_snake()
+    snake.extend()
+    snake.extend()
+    old_count = len(created)
+
+    snake.reset()
+
+    assert len(snake.segments) == len(constants.STARTING_POSITIONS)
+    assert positions(snake.segments) == [
+        (float(x), float(y)) for x, y in constants.STARTING_POSITIONS
+    ]
+    assert snake.segments == created[old_count:]
+
+
+def test_reset_makes_the_first_new_segment_the_head() -> None:
+    snake, created = make_snake()
+    old_head = snake.head
+
+    snake.reset()
+
+    new_head = created[len(constants.STARTING_POSITIONS)]
+    assert snake.head is snake.segments[0] is new_head
+    assert snake.head is not old_head
+    assert ("shape", (constants.HEAD_SHAPE,)) in new_head.calls
+
+
+def test_reset_snake_moves_right_from_the_starting_positions() -> None:
+    snake, _ = make_snake_moving(constants.UP)
+
+    snake.reset()
+    snake.move()
+
+    assert snake.head.position() == (constants.MOVE_DISTANCE, 0)
+
+
+def test_reset_forgets_the_direction_of_the_last_move() -> None:
+    snake, _ = make_snake_moving(constants.DOWN)
+
+    snake.reset()
+    snake.up()  # allowed again: the new snake moves right, not down
+
+    assert snake.head.heading() == constants.UP