Add docstrings to Line and remove None handling for path_arc parameter (#4223)

* In Line: simplify _account_for_buff and set default path_arc to 0.0

* In Line: add docstring

* change 0.0 to 0

* Use explicit check for path_arc == 0 in _account_for_buff

---------

Co-authored-by: Francisco Manríquez Novoa <[email protected]>

Author: irvanalhaq9

manim/mobject/geometry/line.py

@@ -65,27 +65,55 @@
 
 
 class Line(TipableVMobject):
+    """A straight or curved line segment between two points or mobjects.
+
+    Parameters
+    ----------
+    start
+        The starting point or Mobject of the line.
+    end
+        The ending point or Mobject of the line.
+    buff
+        The distance to shorten the line from both ends.
+    path_arc
+        If nonzero, the line will be curved into an arc with this angle (in radians).
+    kwargs
+        Additional arguments to be passed to :class:`TipableVMobject`
+
+    Examples
+    --------
+    .. manim:: LineExample
+        :save_last_frame:
+
+        class LineExample(Scene):
+            def construct(self):
+                line1 = Line(LEFT*2, RIGHT*2)
+                line2 = Line(LEFT*2, RIGHT*2, buff=0.5)
+                line3 = Line(LEFT*2, RIGHT*2, path_arc=PI/2)
+                grp = VGroup(line1,line2,line3).arrange(DOWN, buff=2)
+                self.add(grp)
+    """
+
     def __init__(
         self,
         start: Point3DLike | Mobject = LEFT,
         end: Point3DLike | Mobject = RIGHT,
         buff: float = 0,
-        path_arc: float | None = None,
+        path_arc: float = 0,
         **kwargs: Any,
     ) -> None:
         self.dim = 3
         self.buff = buff
         self.path_arc = path_arc
         self._set_start_and_end_attrs(start, end)
         super().__init__(**kwargs)
-        # TODO: Deal with the situation where path_arc is None
 
     def generate_points(self) -> None:
         self.set_points_by_ends(
             start=self.start,
             end=self.end,
             buff=self.buff,
-            path_arc=self.path_arc,  # type: ignore[arg-type]
+            path_arc=self.path_arc,
         )
 
     def set_points_by_ends(
@@ -112,9 +140,6 @@ def set_points_by_ends(
         """
         self._set_start_and_end_attrs(start, end)
         if path_arc:
-            # self.path_arc could potentially be None, which is not accepted
-            # as parameter.
-            assert self.path_arc is not None
             arc = ArcBetweenPoints(self.start, self.end, angle=self.path_arc)
             self.set_points(arc.points)
         else:
@@ -125,16 +150,13 @@ def set_points_by_ends(
     init_points = generate_points
 
     def _account_for_buff(self, buff: float) -> None:
-        if buff == 0:
+        if buff <= 0:
             return
-        #
         length = self.get_length() if self.path_arc == 0 else self.get_arc_length()
-        #
         if length < 2 * buff:
             return
         buff_proportion = buff / length
         self.pointwise_become_partial(self, buff_proportion, 1 - buff_proportion)
-        return
 
     def _set_start_and_end_attrs(
         self, start: Point3DLike | Mobject, end: Point3DLike | Mobject