Add random_walk with tests

200 steps of equal length, each in a random heading of 0, 90, 180 or 270 degrees and
a random RGB color, with a thick line and the fastest speed.

Task: MIL-004#1

tests/test_constants.py

@@ -9,6 +9,10 @@ def test_challenge_names_are_unique() -> None:
     assert len(set(constants.CHALLENGE_NAMES)) == len(constants.CHALLENGE_NAMES)
 
 
+def test_walk_headings_are_the_four_compass_directions() -> None:
+    assert constants.WALK_HEADINGS == (0, 90, 180, 270)
+
+
 def test_palette_has_only_non_empty_names() -> None:
     assert constants.COLOR_PALETTE
     assert all(name for name in constants.COLOR_PALETTE)

tests/test_random_walk.py

@@ -0,0 +1,110 @@
+import random
+
+import pytest
+
+from tests.fakes import FakePen
+from turtle_challenges.constants import (
+    COLOR_CHANNEL_MAX,
+    COLOR_CHANNEL_MIN,
+    FASTEST_SPEED,
+    WALK_HEADINGS,
+    WALK_PEN_SIZE,
+    WALK_STEP_DISTANCE,
+    WALK_STEPS,
+)
+from turtle_challenges.random_walk import random_walk
+
+
+def test_random_walk_sets_pen_size_and_speed_before_the_first_move() -> None:
+    pen = FakePen()
+
+    random_walk(pen, rng=random.Random(1))
+
+    assert pen.calls[0] == ("pensize", (WALK_PEN_SIZE,))
+    assert pen.calls[1] == ("speed", (FASTEST_SPEED,))
+
+
+def test_random_walk_takes_the_default_number_of_equal_steps() -> None:
+    pen = FakePen()
+
+    random_walk(pen, rng=random.Random(2))
+
+    assert pen.args_of("forward") == [(WALK_STEP_DISTANCE,)] * WALK_STEPS
+
+
+def test_random_walk_faces_only_the_four_headings() -> None:
+    pen = FakePen()
+
+    random_walk(pen, rng=random.Random(3))
+
+    headings = {args[0] for args in pen.args_of("setheading")}
+    assert headings <= set(WALK_HEADINGS)
+    assert len(headings) > 1
+
+
+def test_random_walk_gives_every_step_an_rgb_color() -> None:
+    pen = FakePen()
+
+    random_walk(pen, steps=30, rng=random.Random(4))
+
+    colors = [args[0] for args in pen.args_of("color")]
+    assert len(colors) == 30
+    for color in colors:
+        assert isinstance(color, tuple)
+        assert len(color) == 3
+        assert all(
+            COLOR_CHANNEL_MIN <= channel <= COLOR_CHANNEL_MAX for channel in color
+        )
+
+
+def test_random_walk_picks_color_and_heading_before_each_move() -> None:
+    pen = FakePen()
+
+    random_walk(pen, steps=2, rng=random.Random(5))
+
+    assert pen.names()[2:] == ["color", "setheading", "forward"] * 2
+
+
+def test_random_walk_is_repeatable_with_the_same_seed() -> None:
+    first = FakePen()
+    second = FakePen()
+
+    random_walk(first, steps=25, rng=random.Random(8))
+    random_walk(second, steps=25, rng=random.Random(8))
+
+    assert first.calls == second.calls
+
+
+def test_random_walk_with_no_steps_only_sets_up_the_pen() -> None:
+    pen = FakePen()
+
+    random_walk(pen, steps=0)
+
+    assert pen.names() == ["pensize", "speed"]
+
+
+def test_random_walk_works_without_a_generator() -> None:
+    pen = FakePen()
+
+    random_walk(pen, steps=3)
+
+    assert len(pen.args_of("forward")) == 3
+
+
+def test_random_walk_rejects_negative_steps() -> None:
+    pen = FakePen()
+
+    with pytest.raises(ValueError, match="steps"):
+        random_walk(pen, steps=-1)
+
+    assert pen.calls == []
+
+
+@pytest.mark.parametrize("distance", [0, -3])
+def test_random_walk_rejects_a_distance_that_is_not_positive(distance: float) -> None:
+    pen = FakePen()
+
+    with pytest.raises(ValueError, match="distance"):
+        random_walk(pen, distance=distance)
+
+    assert pen.calls == []