add mid property

Add minimal changelog

Add pr number to changelog

Add test tree method

Black linter

Fix flake8 lint

python/CHANGELOG.rst

@@ -18,13 +18,16 @@
 - Add ``Table.drop_metadata`` to make clearing metadata from tables easy.
   (:user:`jeromekelleher`, :pr:`2944`)
 
+- Add ``Interval.mid`` and ``Tree.mid`` properties to return the midpoint of the interval.
+  (:user:`currocam`, :pr:`2960`)
+
 **Bugfixes**
 
 - Fix to the folded, expected allele frequency spectrum (i.e.,
   `TreeSequence.allele_frequency_spectrum(mode="branch", polarised=False)`,
   which was half as big as it should have been. (:user:`petrelharp`,
   :user:`nspope`, :pr:`2933`)
-  
+
 --------------------
 [0.5.6] - 2023-10-10
 --------------------

python/tests/test_highlevel.py

@@ -4009,6 +4009,7 @@ def test_apis(self):
         assert t1.get_parent_dict() == t1.parent_dict
         assert t1.get_total_branch_length() == t1.total_branch_length
         assert t1.span == t1.interval.right - t1.interval.left
+        assert t1.mid == t1.interval.left + (t1.interval.left - t1.interval.right) / 2
         # node properties
         root = t1.get_root()
         for node in t1.nodes():
@@ -4131,6 +4132,9 @@ def test_interval(self):
             assert tree.interval.span == pytest.approx(
                 breakpoints[i + 1] - breakpoints[i]
             )
+            assert tree.interval.mid == pytest.approx(
+                breakpoints[i] + (breakpoints[i + 1] - breakpoints[i]) / 2
+            )
 
     def verify_tree_arrays(self, tree):
         ts = tree.tree_sequence

python/tskit/trees.py

@@ -89,6 +89,13 @@ def span(self) -> float | int:
         """
         return self.right - self.left
 
+    @property
+    def mid(self) -> float | int:
+        """
+        The middle point of this interval, simply ``left+(right-left)/2``.
+        """
+        return self.left + (self.right - self.left) / 2
+
 
 class EdgeDiff(NamedTuple):
     interval: Interval
@@ -1686,6 +1693,19 @@ def span(self):
         """
         return self.interval.span
 
+    @property
+    def mid(self):
+        """
+        Returns the midpoint of the genomic interval that this tree represents
+        the history of. This is defined as :math:`(l + (r - l) / 2)`, where
+        :math:`(l, r)` is the genomic interval returned by
+        :attr:`~Tree.interval`.
+
+        :return: The genomic distance covered by this tree.
+        :rtype: float
+        """
+        return self.interval.mid
+
     def get_sample_size(self):
         # Deprecated alias for self.sample_size
         return self.sample_size