Andrew McNabb

gitlib: Low-level Git Library

This document serves multiple purposes. First, it shows how Git works on a low level. Second, it provides unit testing. To do unit testing, just run "python tests/gitlib_test.py". To see the output of all of the tests, do "python tests/gitlib_test.py -v".

Init

Sorry for the noise. We need to import some modules before we get started:

>>> import tempfile, os, subprocess
>>> import gitlib

Repositories

gitlib can create a new Git repository, although this is rarely necessary:

>>> path = tempfile.mkdtemp()
>>> repo = gitlib.Repository(path)
>>> repo.create()

Git Objects

You can create blobs.

>>> blob = gitlib.Blob()
>>> blob.text = 'This is a test.\n'
>>> blob.freeze()
>>> repo.write(blob)

You can add them to trees.

>>> TESTFILE1 = 'test.txt'
>>> tree = gitlib.Tree()
>>> tree.add_file(TESTFILE1, blob.name, 'blob')
True
>>> tree.freeze()
>>> repo.write(tree)

Which can be added to commits.

>>> commit = gitlib.Commit()
>>> commit.tree = tree.name
>>> commit.author = 'Andrew McNabb <amcnabb@mcnabbs.org> 1219616814 -0600'
>>> commit.message = 'First Commit!\n'
>>> commit.freeze()
>>> repo.write(commit)

Which can be saved to branches.

>>> repo.save_head(commit.name, None)
True
>>>

Branches

Our commit created a new tree in the repository, which is now the head of the "master" branch. If we want to look at the tree/commit, we can do that:

>>> head = repo.find_head()
>>>

The variable head holds a string representing a hexadecimal number. This number is the SHA-1 hash of the commit. It is used as a unique identifier. We can use the id of this commit to find the id of the file we just added to the repository, and we can open this file as a file-like object:

>>> fileid = repo.getname(TESTFILE1, treename=head)
>>> blob2 = repo.getblob(fileid)
>>> print blob2.contents,
This is a test.
>>>

We can add a new file to the repository, but until we create a new tree with a reference to it, it's just a dangling object.

>>> blob = gitlib.Blob('Second test file.\nTesting.\n')
>>> repo.write(blob)

Indexes

If we want to change a repository, we need to make our changes in an index, and then save the index as a new tree. Let's walk through the process.

>>> TESTFILE2 = 'test2.txt'
>>> index = repo.readtree(head)
>>> index.add(TESTFILE2, blob.name)
>>> newtree = index.write()

But now the new tree (whose id is in newtree), is still a dangling object. We still need to make a commit. In the following commit, we specify the id of the tree that we're committing, as well as the parent commits for this commit (in this case just head), and the changelog string.

>>> newcommit = repo.commit(newtree, [head], 'committing with gitlib')

We still have a dangling commit object! The last step to tie in all of these new objects is to save the commit's id to the master head ref.

>>> repo.save_head(newcommit, head)
True
>>>

Now we can retrieve our new file, which has been properly committed.

>>> fileid = repo.getname(TESTFILE2)
>>> repo.gettype(fileid)
'blob'
>>> f = repo.getblob(fileid)
>>> print f,
Second test file.
Testing.
>>>

Merging

TODO: Add documentation for merging.

Cleanup

>>> subprocess.check_call(('rm', '-rf', path))
0
>>>
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