Repository: gitpython-developers/GitPython

Issue:
RFE: Give Commit objects a replace method that returns new Commit with modified attributes
The `datetime` objects produced by Python's `datetime` module are immutable, much like `Commit` objects. The `datetime` module provides a convenient `replace` function that returns a new object based on an old with one or more attributes replaced.  E.g, I can write:

```
>>> d = datetime.datetime.now()
>>> d.replace(hour=0, minute=0, second=0)
datetime.datetime(2021, 2, 15, 0, 0, 0, 318021)
```

I would like to see the same api on `Commit` objects, so that one could write:

```
>>> newcommit = oldcommit.replace(message='This is a test', author='Cookie Monster')
```



Discussion:

Patch:
diff --git a/git/objects/commit.py b/git/objects/commit.py
index 302798cb..45e6d772 100644
--- a/git/objects/commit.py
+++ b/git/objects/commit.py
@@ -136,6 +136,41 @@ class Commit(base.Object, Iterable, Diffable, Traversable, Serializable):
     def _get_intermediate_items(cls, commit):
         return commit.parents
 
+    @classmethod
+    def _calculate_sha_(cls, repo, commit):
+        '''Calculate the sha of a commit.
+
+        :param repo: Repo object the commit should be part of
+        :param commit: Commit object for which to generate the sha
+        '''
+
+        stream = BytesIO()
+        commit._serialize(stream)
+        streamlen = stream.tell()
+        stream.seek(0)
+
+        istream = repo.odb.store(IStream(cls.type, streamlen, stream))
+        return istream.binsha
+
+    def replace(self, **kwargs):
+        '''Create new commit object from existing commit object.
+
+        Any values provided as keyword arguments will replace the
+        corresponding attribute in the new object.
+        '''
+
+        attrs = {k: getattr(self, k) for k in self.__slots__}
+
+        for attrname in kwargs:
+            if attrname not in self.__slots__:
+                raise ValueError('invalid attribute name')
+
+        attrs.update(kwargs)
+        new_commit = self.__class__(self.repo, self.NULL_BIN_SHA, **attrs)
+        new_commit.binsha = self._calculate_sha_(self.repo, new_commit)
+
+        return new_commit
+
     def _set_cache_(self, attr):
         if attr in Commit.__slots__:
             # read the data in a chunk, its faster - then provide a file wrapper
@@ -375,13 +410,7 @@ class Commit(base.Object, Iterable, Diffable, Traversable, Serializable):
                          committer, committer_time, committer_offset,
                          message, parent_commits, conf_encoding)
 
-        stream = BytesIO()
-        new_commit._serialize(stream)
-        streamlen = stream.tell()
-        stream.seek(0)
-
-        istream = repo.odb.store(IStream(cls.type, streamlen, stream))
-        new_commit.binsha = istream.binsha
+        new_commit.binsha = cls._calculate_sha_(repo, new_commit)
 
         if head:
             # need late import here, importing git at the very beginning throws


Test patch:
diff --git a/test/test_commit.py b/test/test_commit.py
index 7260a233..2fe80530 100644
--- a/test/test_commit.py
+++ b/test/test_commit.py
@@ -101,6 +101,26 @@ class TestCommit(TestCommitSerialization):
         assert isinstance(commit.author_tz_offset, int) and isinstance(commit.committer_tz_offset, int)
         self.assertEqual(commit.message, "Added missing information to docstrings of commit and stats module\n")
 
+    def test_replace_no_changes(self):
+        old_commit = self.rorepo.commit('2454ae89983a4496a445ce347d7a41c0bb0ea7ae')
+        new_commit = old_commit.replace()
+
+        for attr in old_commit.__slots__:
+            assert getattr(new_commit, attr) == getattr(old_commit, attr)
+
+    def test_replace_new_sha(self):
+        commit = self.rorepo.commit('2454ae89983a4496a445ce347d7a41c0bb0ea7ae')
+        new_commit = commit.replace(message='Added replace method')
+
+        assert new_commit.hexsha == 'fc84cbecac1bd4ba4deaac07c1044889edd536e6'
+        assert new_commit.message == 'Added replace method'
+
+    def test_replace_invalid_attribute(self):
+        commit = self.rorepo.commit('2454ae89983a4496a445ce347d7a41c0bb0ea7ae')
+
+        with self.assertRaises(ValueError):
+            commit.replace(badattr='This will never work')
+
     def test_stats(self):
         commit = self.rorepo.commit('33ebe7acec14b25c5f84f35a664803fcab2f7781')
         stats = commit.stats