Changing a File and Commit it
=============================

Now we have a first commit, the next one will show a difference.

As seen in the introduction, it's about making a path in a tree point to a
new blob. The old blob will remain to compute the diff. The tree is altered
and the new commit'task is to point to this new version.

In the following examples, we will need a ``repo`` and an initial commit
(``c1``) similar to the one made in the previous chapter::

  >>> from dulwich.repo import Repo
  >>> from dulwich.objects import Blob, Tree
  >>> repo = Repo.init("myrepo", mkdir=True)
  >>> blob = Blob.from_string("My file content\n")
  >>> tree = Tree()
  >>> tree.add(0100644, "spam", blob.id)
  >>> repo.object_store.add_object(blob)
  >>> repo.object_store.add_object(tree)
  >>> c1_id = repo.do_commit("Initial commit.",
  ...     committer="JaneExample <jane@example.com>", tree=tree.id)

Let's first build the blob::

  >>> from dulwich.objects import Blob
  >>> spam = Blob.from_string("My new file content\n")
  >>> spam.id
  '16ee2682887a962f854ebd25a61db16ef4efe49f'

An alternative is to alter the previously constructed blob object::

  >>> blob.data = "My new file content\n"
  >>> blob.id
  '16ee2682887a962f854ebd25a61db16ef4efe49f'

In any case, update the blob id known as "spam". You also have the
opportunity of changing its mode::

  >>> tree["spam"] = (0100644, spam.id)

Now let's record the change::

  >>> from dulwich.objects import Commit
  >>> from time import time
  >>> c2 = Commit()
  >>> c2.tree = tree.id
  >>> c2.parents = [c1_id]
  >>> c2.author = c2.committer = "John Doe <john@example.com>"
  >>> c2.commit_time = c2.author_time = int(time())
  >>> c2.commit_timezone = c2.author_timezone = 0
  >>> c2.encoding = "UTF-8"
  >>> c2.message = 'Changing "spam"'

In this new commit we record the changed tree id, and most important, the
previous commit as the parent. Parents are actually a list because a commit
may happen to have several parents after merging branches.

Remain to record this whole new family::

  >>> repo.object_store.add_object(spam)
  >>> repo.object_store.add_object(tree)
  >>> repo.object_store.add_object(c2)

You can already ask git to introspect this commit using ``git show`` and the
value of ``c2.id`` as an argument. You'll see the difference will the
previous blob recorded as "spam".

The diff between the previous head and the new one can be printed using
write_tree_diff::

  >>> from dulwich.patch import write_tree_diff
  >>> import sys
  >>> write_tree_diff(sys.stdout, repo.object_store, repo[c1_id].tree, tree.id)
  diff --git a/spam b/spam
  index c55063a..16ee268 100644
  --- a/spam
  +++ b/spam
  @@ -1,1 +1,1 @@
  -My file content
  +My new file content

You won't see it using git log because the head is still the previous
commit. It's easy to remedy::

  >>> repo.refs['refs/heads/master'] = c2.id

Now all git tools will work as expected.