feature: add cap_style and join_style properties to LineFormat

Add line cap style (flat, round, square) and join style (round, bevel,
miter) properties to LineFormat. Introduces MSO_LINE_CAP_STYLE and
MSO_LINE_JOIN_STYLE enums and models the cap attribute and
EG_LineJoinProperties choice group on CT_LineProperties.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: MHoroszowski

src/pptx/dml/line.py

@@ -3,7 +3,7 @@
 from __future__ import annotations
 
 from pptx.dml.fill import FillFormat
-from pptx.enum.dml import MSO_FILL
+from pptx.enum.dml import MSO_FILL, MSO_LINE_CAP_STYLE, MSO_LINE_JOIN_STYLE
 from pptx.util import Emu, lazyproperty
 
 
@@ -18,6 +18,23 @@ def __init__(self, parent):
         super(LineFormat, self).__init__()
         self._parent = parent
 
+    @property
+    def cap_style(self) -> MSO_LINE_CAP_STYLE | None:
+        """Cap style for this line.
+
+        Read/write. Returns a member of :ref:`MsoLineCapStyle` or |None| if no explicit value
+        has been set. Assigning |None| removes any existing value.
+        """
+        ln = self._ln
+        if ln is None:
+            return None
+        return ln.cap
+
+    @cap_style.setter
+    def cap_style(self, value: MSO_LINE_CAP_STYLE | None) -> None:
+        ln = self._get_or_add_ln()
+        ln.cap = value
+
     @lazyproperty
     def color(self):
         """
@@ -32,6 +49,36 @@ def color(self):
             self.fill.solid()
         return self.fill.fore_color
 
+    @property
+    def join_style(self) -> MSO_LINE_JOIN_STYLE | None:
+        """Join style for this line.
+
+        Read/write. Returns a member of :ref:`MsoLineJoinStyle` or |None| if no explicit value
+        has been set. Assigning |None| removes any existing value.
+        """
+        ln = self._ln
+        if ln is None:
+            return None
+        join = ln.eg_lineJoinProperties
+        if join is None:
+            return None
+        tag_name = join.tag.split("}")[-1]
+        return {"round": MSO_LINE_JOIN_STYLE.ROUND, "bevel": MSO_LINE_JOIN_STYLE.BEVEL,
+                "miter": MSO_LINE_JOIN_STYLE.MITER}[tag_name]
+
+    @join_style.setter
+    def join_style(self, value: MSO_LINE_JOIN_STYLE | None) -> None:
+        ln = self._get_or_add_ln()
+        if value is None:
+            ln._remove_eg_lineJoinProperties()
+            return
+        method_map = {
+            MSO_LINE_JOIN_STYLE.ROUND: "get_or_change_to_round",
+            MSO_LINE_JOIN_STYLE.BEVEL: "get_or_change_to_bevel",
+            MSO_LINE_JOIN_STYLE.MITER: "get_or_change_to_miter",
+        }
+        getattr(ln, method_map[value])()
+
     @property
     def dash_style(self):
         """Return value indicating line style.

src/pptx/enum/dml.py

@@ -139,6 +139,50 @@ class MSO_LINE_DASH_STYLE(BaseXmlEnum):
 MSO_LINE = MSO_LINE_DASH_STYLE
 
 
+class MSO_LINE_CAP_STYLE(BaseXmlEnum):
+    """Specifies the cap style for a line.
+
+    Example::
+
+        from pptx.enum.dml import MSO_LINE_CAP_STYLE
+
+        shape.line.cap_style = MSO_LINE_CAP_STYLE.ROUND
+
+    MS API name: `MsoLineCap` (not in MS API, maps to DrawingML `ST_LineCap`)
+    """
+
+    FLAT = (1, "flat", "A flat cap at the end of a line.")
+    """A flat cap at the end of a line."""
+
+    ROUND = (2, "rnd", "A round cap at the end of a line.")
+    """A round cap at the end of a line."""
+
+    SQUARE = (3, "sq", "A square cap at the end of a line.")
+    """A square cap at the end of a line."""
+
+
+class MSO_LINE_JOIN_STYLE(BaseXmlEnum):
+    """Specifies the join style for a line.
+
+    Example::
+
+        from pptx.enum.dml import MSO_LINE_JOIN_STYLE
+
+        shape.line.join_style = MSO_LINE_JOIN_STYLE.MITER
+
+    MS API name: Not directly in MS API, maps to DrawingML `EG_LineJoinProperties`.
+    """
+
+    ROUND = (1, "round", "A round join between two lines.")
+    """A round join between two lines."""
+
+    BEVEL = (2, "bevel", "A bevel join between two lines.")
+    """A bevel join between two lines."""
+
+    MITER = (3, "miter", "A miter join between two lines.")
+    """A miter join between two lines."""
+
+
 class MSO_PATTERN_TYPE(BaseXmlEnum):
     """Specifies the fill pattern used in a shape.
 

src/pptx/oxml/shapes/shared.py

@@ -5,6 +5,7 @@
 from typing import TYPE_CHECKING, Callable
 
 from pptx.dml.fill import CT_GradientFillProperties
+from pptx.enum.dml import MSO_LINE_CAP_STYLE
 from pptx.enum.shapes import PP_PLACEHOLDER
 from pptx.oxml.ns import qn
 from pptx.oxml.simpletypes import (
@@ -264,8 +265,19 @@ class CT_LineProperties(BaseOxmlElement):
     )
     prstDash = ZeroOrOne("a:prstDash", successors=_tag_seq[5:])
     custDash = ZeroOrOne("a:custDash", successors=_tag_seq[6:])
+    eg_lineJoinProperties = ZeroOrOneChoice(
+        (
+            Choice("a:round"),
+            Choice("a:bevel"),
+            Choice("a:miter"),
+        ),
+        successors=_tag_seq[9:],
+    )
     del _tag_seq
     w = OptionalAttribute("w", ST_LineWidth, default=Emu(0))
+    cap: MSO_LINE_CAP_STYLE | None = OptionalAttribute(  # pyright: ignore[reportAssignmentType]
+        "cap", MSO_LINE_CAP_STYLE
+    )
 
     @property
     def eg_fillProperties(self):

tests/dml/test_line.py

@@ -7,7 +7,7 @@
 from pptx.dml.color import ColorFormat
 from pptx.dml.fill import FillFormat
 from pptx.dml.line import LineFormat
-from pptx.enum.dml import MSO_FILL, MSO_LINE
+from pptx.enum.dml import MSO_FILL, MSO_LINE, MSO_LINE_CAP_STYLE, MSO_LINE_JOIN_STYLE
 from pptx.oxml.shapes.shared import CT_LineProperties
 from pptx.shapes.autoshape import Shape
 
@@ -17,6 +17,69 @@
 
 
 class DescribeLineFormat(object):
+    @pytest.mark.parametrize(
+        ("spPr_cxml", "expected_value"),
+        [
+            ("p:spPr", None),
+            ("p:spPr/a:ln", None),
+            ("p:spPr/a:ln{cap=rnd}", MSO_LINE_CAP_STYLE.ROUND),
+            ("p:spPr/a:ln{cap=sq}", MSO_LINE_CAP_STYLE.SQUARE),
+            ("p:spPr/a:ln{cap=flat}", MSO_LINE_CAP_STYLE.FLAT),
+        ],
+    )
+    def it_knows_its_cap_style(
+        self, spPr_cxml: str, expected_value: MSO_LINE_CAP_STYLE | None
+    ):
+        line = LineFormat(element(spPr_cxml))
+        assert line.cap_style == expected_value
+
+    @pytest.mark.parametrize(
+        ("spPr_cxml", "value", "expected_cxml"),
+        [
+            ("p:spPr{a:b=c}", MSO_LINE_CAP_STYLE.ROUND, "p:spPr{a:b=c}/a:ln{cap=rnd}"),
+            ("p:spPr/a:ln{cap=rnd}", MSO_LINE_CAP_STYLE.SQUARE, "p:spPr/a:ln{cap=sq}"),
+            ("p:spPr/a:ln{cap=sq}", None, "p:spPr/a:ln"),
+        ],
+    )
+    def it_can_change_its_cap_style(
+        self, spPr_cxml: str, value: MSO_LINE_CAP_STYLE | None, expected_cxml: str
+    ):
+        spPr = element(spPr_cxml)
+        LineFormat(spPr).cap_style = value
+        assert spPr.xml == xml(expected_cxml)
+
+    @pytest.mark.parametrize(
+        ("spPr_cxml", "expected_value"),
+        [
+            ("p:spPr", None),
+            ("p:spPr/a:ln", None),
+            ("p:spPr/a:ln/a:round", MSO_LINE_JOIN_STYLE.ROUND),
+            ("p:spPr/a:ln/a:bevel", MSO_LINE_JOIN_STYLE.BEVEL),
+            ("p:spPr/a:ln/a:miter", MSO_LINE_JOIN_STYLE.MITER),
+        ],
+    )
+    def it_knows_its_join_style(
+        self, spPr_cxml: str, expected_value: MSO_LINE_JOIN_STYLE | None
+    ):
+        line = LineFormat(element(spPr_cxml))
+        assert line.join_style == expected_value
+
+    @pytest.mark.parametrize(
+        ("spPr_cxml", "value", "expected_cxml"),
+        [
+            ("p:spPr{a:b=c}", MSO_LINE_JOIN_STYLE.ROUND, "p:spPr{a:b=c}/a:ln/a:round"),
+            ("p:spPr/a:ln", MSO_LINE_JOIN_STYLE.BEVEL, "p:spPr/a:ln/a:bevel"),
+            ("p:spPr/a:ln/a:round", MSO_LINE_JOIN_STYLE.MITER, "p:spPr/a:ln/a:miter"),
+            ("p:spPr/a:ln/a:miter", None, "p:spPr/a:ln"),
+        ],
+    )
+    def it_can_change_its_join_style(
+        self, spPr_cxml: str, value: MSO_LINE_JOIN_STYLE | None, expected_cxml: str
+    ):
+        spPr = element(spPr_cxml)
+        LineFormat(spPr).join_style = value
+        assert spPr.xml == xml(expected_cxml)
+
     def it_knows_its_dash_style(self, dash_style_get_fixture):
         line, expected_value = dash_style_get_fixture
         assert line.dash_style == expected_value