Add get_object_by_path.

NEWS

@@ -18,6 +18,11 @@
   * Handle commit identity fields with multiple ">" characters.
     (Nicolas Dandrimont)
 
+ IMPROVEMENTS
+
+  * ``dulwich.porcelain.get_object_by_path`` method for easily
+    accessing a path in another tree. (Jelmer Vernooij)
+
 0.19.6	2018-08-11
 
  BUG FIXES

dulwich/porcelain.py

@@ -1385,3 +1385,23 @@ def describe(repo):
 
         # Return plain commit if no parent tag can be found
         return 'g{}'.format(latest_commit.id.decode('ascii')[:7])
+
+
+def get_object_by_path(repo, path, committish=None):
+    """Get an object by path.
+
+    :param repo: A path to the repository
+    :param path: Path to look up
+    :param committish: Commit to look up path in
+    :return: A `ShaFile` object
+    """
+    if committish is None:
+        committish = "HEAD"
+    # Get the repository
+    with open_repo_closing(repo) as r:
+        commit = parse_commit(repo, committish)
+        base_tree = commit.tree
+        (mode, sha) = tree_lookup_path(
+            r.object_store.__getitem__,
+            base_tree, path)
+        return r[sha]

dulwich/tests/test_porcelain.py

@@ -1541,3 +1541,24 @@ class HelperTests(PorcelainTestCase):
                 os.path.join(os.getcwd(), '..'), 'baz'))
         finally:
             os.chdir(cwd)
+
+
+class GetObjectBypathTests(PorcelainTestCase):
+
+    def test_simple(self):
+        fullpath = os.path.join(self.repo.path, 'foo')
+        with open(fullpath, 'w') as f:
+            f.write("BAR")
+        porcelain.add(repo=self.repo.path, paths=[fullpath])
+        porcelain.commit(
+                self.repo.path, message=b"Some message",
+                author=b"Joe <joe@example.com>",
+                committer=b"Bob <bob@example.com>")
+        self.assertEqual(
+            b"BAR",
+            porcelain.get_object_by_path(self.repo, 'foo').data)
+
+    def test_missing(self):
+        self.assertRaises(
+            KeyError,
+            porcelain.get_object_by_path, self.repo, 'foo')