Add the Pen protocol, window helpers and a recording fake turtle

The drawing functions take a Pen, the part of turtle.Turtle they use, so a test can
pass a fake that records every call. window.py is the only module that imports
turtle, and it does so inside the functions so that importing the package needs no Tk.

Task: MIL-002#1
Task: MIL-002#2

tests/fakes.py

@@ -0,0 +1,68 @@
+"""Test doubles that stand in for the turtle and its window."""
+
+from turtle_challenges.pen import Color
+
+
+class FakePen:
+    """A pen that records every call instead of drawing.
+
+    The heading follows `setheading` and `right`, like a real turtle that
+    starts facing east, so code that reads `heading()` behaves as in a window.
+    """
+
+    def __init__(self) -> None:
+        self.calls: list[tuple[str, tuple[object, ...]]] = []
+        self._heading = 0.0
+
+    def _record(self, name: str, *args: object) -> None:
+        self.calls.append((name, args))
+
+    def args_of(self, name: str) -> list[tuple[object, ...]]:
+        """Return the arguments of every call to `name`, in order."""
+        return [args for call, args in self.calls if call == name]
+
+    def names(self) -> list[str]:
+        """Return the name of every call, in order."""
+        return [call for call, _ in self.calls]
+
+    def forward(self, distance: float, /) -> None:
+        self._record("forward", distance)
+
+    def right(self, angle: float, /) -> None:
+        self._record("right", angle)
+        self._heading = (self._heading - angle) % 360
+
+    def penup(self) -> None:
+        self._record("penup")
+
+    def pendown(self) -> None:
+        self._record("pendown")
+
+    def color(self, color: Color, /) -> None:
+        self._record("color", color)
+
+    def circle(self, radius: float, /) -> None:
+        self._record("circle", radius)
+
+    def heading(self) -> float:
+        return self._heading
+
+    def setheading(self, to_angle: float, /) -> None:
+        self._record("setheading", to_angle)
+        self._heading = to_angle % 360
+
+    def pensize(self, width: int, /) -> None:
+        self._record("pensize", width)
+
+    def speed(self, speed: int, /) -> None:
+        self._record("speed", speed)
+
+
+class FakeWindow:
+    """A window that records whether it was told to wait for a click."""
+
+    def __init__(self) -> None:
+        self.waited_for_click = False
+
+    def exitonclick(self) -> None:
+        self.waited_for_click = True

tests/test_window.py

@@ -0,0 +1,33 @@
+import sys
+
+import pytest
+
+from turtle_challenges.window import (
+    TURTLE_UNAVAILABLE_MESSAGE,
+    TurtleUnavailableError,
+    create_pen,
+    create_window,
+)
+
+
+@pytest.fixture
+def without_turtle(monkeypatch: pytest.MonkeyPatch) -> None:
+    """Make `import turtle` fail, as on a Python without Tk."""
+    monkeypatch.setitem(sys.modules, "turtle", None)
+
+
+def test_create_window_reports_a_missing_turtle(without_turtle: None) -> None:
+    with pytest.raises(TurtleUnavailableError) as error_info:
+        create_window()
+
+    assert str(error_info.value) == TURTLE_UNAVAILABLE_MESSAGE
+    assert isinstance(error_info.value.__cause__, ImportError)
+
+
+def test_create_pen_reports_a_missing_turtle(without_turtle: None) -> None:
+    with pytest.raises(TurtleUnavailableError):
+        create_pen()
+
+
+def test_the_missing_turtle_message_names_the_debian_package() -> None:
+    assert "python3-tk" in TURTLE_UNAVAILABLE_MESSAGE