In [1]:
!git status
This tutorial is largely based on the repository:
git@github.com:rdadolf/git-and-github.git
which was created for IACS's ac297r course, By Robert Adolf. Most of the credit goes to him.
Version control is a way of tracking the change history of a project. Even if you have never used a version control tool, you've probably already done it manually: copying and renaming project folders ("paper-1.doc", "paper-2.doc", etc.) is a form of version control.
Git is a tool that automates and enhances a lot of the tasks that arise when dealing with larger, longer-living, and collaborative projects. It's also become the common underpinning to many popular online code repositories, GitHub being the most popular.
We'll go over the basics of git, but we should point out that a lot of talented people have given git tutorials, and we won't do any better than they have. In fact, if you're interested in learning git deeply and have some time on your hands, I suggest you stop reading this and instead read the Git Book. Scott Chacon and Ben Straub have done a tremendous job, and if you want to understand both the interfaces and the mechanisms behind git, this is the place to start.
In this document, we'll go over the concepts of git that are relevant to this course. You will be using git in a particular fashion, in this course, which is described in Homework 0. Homework 0 also contains details about how to install git; so if you havent done so, please do so and come back here.
At this point, we assume you have installed git as described in HW0 and have followed the instructions there to create ssh keys for your machine. This enables you to use git without a pesky username-password combination.
If you ask 10 people, you'll get 10 different answers, but one of the commonalities is that most people don't realize how integral it is to their development process until they've started using it. Still, for the sake of argument, here are some highlights:
The first thing to understand about git is that the contents of your project are stored in several different states and forms at any given time. If you think about what version control is, this might not be surprising: in order to remember every change that's ever been made, you need to store a record of those changes somewhere, and to be able to handle multiple people changing the same code, you need to have different copies of the project and a way to combine them.
You can think about git operating on four different areas:
git add..git directory in the working directory of your project. Files are moved from the index to the local repository via the command git commit.git push, and in the other direction using git fetch.You can think of most git operations as moving code or metadata from one of these areas to another.
Forking a repository done on github. On github, go to the url https://github.com/cs109/Testing. Click the "Fork button on the upper right side. A screenshot is below.
Forking brings a repository into your own namespace. Its really a cloning process (see below), but its done between two "remotes" on the server. In other words it creates a second upstream repository on the server, called the origin.
The forking process on github will ask you where you want to fork the repository. Choose your own github id.
In my case I will choose @rahuldave, as in the screenshot above. In this tutorial, wherever you see rahuldave, substitute your own github id.
This leaves me with my own repository, rahuldave/Testing, as seen in this image
You will get a similar page.
Now that we have a fork of the cs109/Testing repository, lets clone it down to our local machines.
clone
Cloning a repository does two things: it takes a repository from somewhere (usually an upstream repository) and makes a local copy (your new local repository), and it creates the most recent copy of all of the files in the project (your new working directory). This is generally how you will start working on a project for the first time.
Cloning a repository depends a lot on the type of repository you're using. If you're cloning out of a directory on the machine you're currently on, it's just the path to the <project>.git file. For github, you can clone over ssh using the SSH clone url in the bottom right corner of the last screenshot. This assumes you have set up your ssh keys. See HW0 for how to do this if you havent done it as yet. If you havent set up your ssh keys, use https instead.
Note for windows users
bash.exe is installed along with git but isnt in your path. Convert the next cell from Raw NbConvert to code and run it to add the path to bash to your path
In [2]:
%%bash
cd /tmp
rm -rf Testing #remove if it exists
git clone git@github.com:rahuldave/Testing.git
In [3]:
%%bash
ls /tmp/Testing
We have a nice smelling fresh repository. Lets look around it.
log
Log tells you all the changes that have occured in this project as of now...
In [4]:
%%bash
cd /tmp/Testing; git log
Each one of these "commits" is a SHA hash. It uniquely identifies all actions that have happened to this repository previously. We shall soon see how to add our own actions in. In the meanwhile, lets see the "status"of our working directory.
If you ever need help on a command, you can find the git man pages by hyphenating git and the command name:
status
Status is your window into the current state of your project. It can tell you which files you have changed and which files you currently have in your staging area.
In [5]:
%%bash
cd /tmp/Testing; git status
Pay close attention to the text above. It says we are on the master branch of our local repository, and that this branch is up-to-date with the master branch of the upstream repository or remote named origin. We know this as clone brings down a copy of the remote branch: "origin/master" represents the local copy of the branch that came from the upstream repository (nicknamed "origin" in this case). Branches are different, co-existing versions of your project. Here we have encountered two of them, but remember there is a third one in the repository we forked from, and perhaps many more, depending on who else made these forks (as well as my original clones of cs109/Testing).
This sounds like a recipe for disaster, but in a sense, branches are just like commits. They represent a snapshot of the project, by someone, at some particular point in time. In general you will only care about your own branches, and those of the "parent" remotes you forked/cloned from.
Configuration Information is stored in a special file config, in a hidden folder called .git in your working directory. (The index and the local repository are stored there as well...more on that in a bit)
In [6]:
%%bash
cd /tmp/Testing; cat .git/config
Notice that this file tells us about a remote called "origin" which is simply the github repository we cloned from. So the process of cloning left us with a remote. The file also tells us about a branch called "master", which "tracks" a remote branch valled master at "origin".
Finally I set us up with a .gitignore file, hidden in the repository folder. It tells us what files to ignore when adding files to the index, and then comitting them to the local repository. We use this file to ignore temporary data files and such when working in our repository. Folders are indicated with a / at the end, in which case, all files in that folder are ignored.
In [7]:
%%bash
cd /tmp/Testing; cat .gitignore
You are always working on a given branch in a repository. Typically this is master. More on this later..You can know which branch you are on by typing git branch. The strred one is the one you are on.
In [8]:
%%bash
cd /tmp/Testing; git branch
Ok! Enough poking around. Lets get down to business and add some files into our folder.
Now let's say that we want to add a new file to the project. The canonical sequence is "edit–add–commit–push".
In [9]:
%%bash
cd /tmp/Testing
echo '# Hello world rahuldave' > hello.md
git status
We've added a file to the working directory, but it hasn't been staged yet.
add
When you've made a change to a set of files and are ready to create a commit, the first step is to add all of the changed files to the staging area. Add does that. Remember that what you see in the filesystem is your working directory, so the way to see what's in the staging area is with the status command. This also means that if you add something to the staging area and then edit it again, you'll also need to add the file to the staging area again if you want to remember the new changes.
In [10]:
%%bash
cd /tmp/Testing
git add hello.md
git status
Now our file is in the staging area (Index), waiting to be committed.
I will sometimes simply use git add . in the top level of the repository. This adds all new files and changed files to the index, and is particularly useful if I have created multiple new files.
commit
When you're satisfied with the changes you've added to your staging area, you can commit those changes to your local repository with the commit command. Those changes will have a permanent record in the repository from now on.
Every commit has two features you should be aware of. The first is a hash. This is a unique identifier for all of the information about that commit, including the code changes, the timestamp, and the author. The second is a commit message. This is text that you can (and should) add to a commit to describe what the changes were.
Good commit messages are important.
Commit messages are a way of quickly telling your future self (and your collaborators) what a commit was about. For even a moderately sized project, digging through tens or hundreds of commits to find the change you're looking for is a nightmare without friendly summaries.
By convention, commit messages start with a single-line summary, then an empty line, then a more comprehensive description of the changes.
This is an okay commit message. The changes are small, and the summary is sufficient to describe what happened.
This is better. The summary captures the important information (major shift, direct vs. helper), and the full commit message describes what the high-level changes were.
This. Don't do this.
In [11]:
%%bash
cd /tmp/Testing
git commit -m "Said hello to myself"
git status
The git commit -m... version is just a way to specify a commit message without opening a text editor (ipython notebook can't handle it). Otherwise you just say git commit or git commit -a (if you added a new file to the current branch on the repository)
Now we see that our branch, "master", has one more commit than the "origin/master" branch, the local copy of the branch that came from the upstream repository (nicknamed "origin" in this case). Let's push the changes.
push
The push command takes the changes you have made to your local repository and attempts to update a remote repository with them. If you're the only person working with both of these (which is how a solo GitHub project would work), then push should always succeed.
In [12]:
%%bash
cd /tmp/Testing
git push
git status
If you're working with other people, then it's possible that they have made changes to the remote repository between the time you first cloned it and now. push will fail.
In our particular case of the Testing repository, this is not going to happen, since you just cloned it and presumably havent invited anyone to collaborate with you on it (you can do this from the settings link on the right side of the repository page).
However you can imagine that the original repository cs109/testing, which you are now divorced from, has changed, and that you somehow want to pull those changes in.
That's where the next two commands come in.
remote
We have seen so far that our repository has one "remote", or upstream repository, which has been identified with the word origin, as seen in .git/config. We now wish to add another remote, which we shall call course, which points to the original repository we forked from. We want to do this to pull in changes, in-case something changed there.
In [13]:
%%bash
cd /tmp/Testing
git remote add course git@github.com:cs109/Testing.git
cat .git/config
Notice that the master branch only tracks the same branch on the origin remote. We havent set up any connection with the course remote as yet.
Now lets figure out how to get changes from an upstream repository, be it our origin upstream that a collaborator has pushed too, or another course remote to which your memory-added head TF has posted a change!
fetch
Let's say a collaborator has pushed changes to your shared upstream repository while you were editing. Their local repository and the upstream repository now both contain their changes, but your local repository does not. To update your local repository, you run fetch.
But what if you've committed changes in the meantime? Does your local repository contain your changes or theirs? The answer is that it contains a record of both, but they are kept separate. Remember that git repositories are not copies of your project files. They store all the contents of your files, along with a bunch of metadata, but in its own internal format. This is one of the reasons.
Let's say that you and your collaborator both edited the same line of the same file at the same time in different ways. On your respective machines, you both add and commit your different changes, and your collaborator pushes theirs to the upstream repository. When you run fetch, git adds a record of their changes to your local repository alongside your own. These are called branches, and they represent different, coexisting versions of your project. The fetch command adds your collaborator's branch to your local repository, but keeps yours as well.
In [14]:
%%bash
cd /tmp/Testing
git fetch course
You can see that a copy of a new remote branch has been made below, by providing the -avv argument to git branch.
In [15]:
%%bash
cd /tmp/Testing
git branch -avv
Indeed, the way git works is by creating copies of remote branches locally. Then it just compares to these "copy" branches to see what changes have been made.
Sometimes we want to merge the changes in...precisely we want to merge the change from remotes/course/master in. In HW0 and later here, we'll show you a case where you want to simply create another branch.
merge
Having multiple branches is fine, but at some point, you'll want to combine the changes that you've made with those made by others. This is called merging.
There are two general cases when merging two branches: first, the two branches are different but the changes are in unrelated places; and second, the two branches are different and the changes are in the same locations in the same files.
The first scenario is easy. Git will simply apply both sets of changes to the appropriate places and put the resulting files into the staging area for you. Then you can commit the changes and push them back to the upstream repository. Your collaborator does the same, and everyone sees everything.
The second scenario is more complicated. Let's say the two changes set some variable to different values. Git can't know which is the correct value. One solution would be to simply use the more recent change, but this very easily leads to self-inconsistent programs. A more conservative solution, and the one git uses, is to simply leave the decision to the user. When git detects a conflict that it cannot resolve, merge fails, and git places a modified version of the offending file in your project directory. This is important: the file that git puts into your directory is not actually either of the originals. It is a new file that has special markings around the locations that conflicted. We shall not consider this case in this lab, deferring it to a time just before you work with other collaborators on your projects.
Lets merge in the changes from course/master: (The next 2-3 inputs only make sense if cs109/master is edited in-between)
In [16]:
%%bash
cd /tmp/Testing
git merge course/master
git status
We seem to be ahead of our upstream-tracking repository by 2 commits..why?
In [17]:
%%bash
cd /tmp/Testing
git log -3
Aha: one commit came from the course upstream, and one was a merge commit. In the case you had edited the README.md at the same time and comitted locally, you would have been asked to resolve the conflict in the merge (the second case above).
Lets push these changes to the origin now
In [18]:
%%bash
cd /tmp/Testing
git push
git status
You can combine a fetch and a merge together by simply doing a git pull. This will fail if you and your collaborator have worked on the same file (since you will have to merge by hand), but is a great shortcut when the files worked on are different. I use it all the times on a personal level too, to shift work between two different machines, as long as I am not working on both at the same time. The usual use case is day work on a work computer, and then evening work at home on the laptop. Read the docs if you are interested.
As you might have seen by now, everything in git is a branch. We have branches on remote (upstream) repositories, copies of remote branches in our local repository, and branches on local repositories which (so far) track remote branches (or more precisely local copies of remote repositories)
In [19]:
%%bash
cd /tmp/Testing
git branch -avv
And all of these branches are nothing but commit-streams in disguise, as can be seen above. Its a very simple model which leads to a lot of interesting version control patterns.
Since branches are so light-weight, the recommended way of working on software using git is to create a new branch for each new feature you add, test it out, and if good, merge it into master. Then you deploy the software from master. But we have been using branches under the hood. Lets now lift the hood.
branch
Branches can also be created manually, and they are a useful way of organizing unfinished changes.
The branch command has two forms. The first:
git branch
simply lists all of the branches in your local repository. If you run it without having created any branches, it will list only one, called master. This is the default branch. You have seen the use of git branch -avv to show all branches.
The other form creates a branch with a given name:
It's important to note that the other branch is not active. If you make changes, they will still apply to the master branch, not my-new-branch. To change this, you need the next command.
checkout
Checkout switches the active branch. Since branches can have different changes, checkout may make the working directory look very different. For instance, if you have added new files to one branch, and then check another branch out, those files will no longer show up in the directory. They are still stored in the .git folder, but since they only exist in the other branch, they cannot be accessed until you check out the original branch.
You can combine creating a new branch and checking it out with the shortcut:
Ok so lets try this out on our repository....
In [20]:
%%bash
cd /tmp/Testing
git branch mybranch1
See what branches we have created...
In [21]:
%%bash
cd /tmp/Testing
git branch
Jump onto the mybranch1 branch...
In [22]:
%%bash
cd /tmp/Testing
git checkout mybranch1
git branch
Notice that it is bootstrapped off the master branch and has the same files.
In [23]:
%%bash
cd /tmp/Testing
ls
You could have created this branch using git checkout -b mybranch1. Lets see the status here.
In [24]:
%%bash
cd /tmp/Testing
git status
Lets add a new file...note that this file gets added on this branch only
In [25]:
%%bash
cd /tmp/Testing
echo '# Hello world' > world.md
git status
We add the file to the index, and then commit the files to the local repository on the mybranch branch.
In [26]:
%%bash
cd /tmp/Testing
git add .
git status
In [27]:
%%bash
cd /tmp/Testing
git commit -m "Added another test file to demonstrate git features" -a
git status
Ok we have committed. Lets try to push!
In [28]:
%%bash
cd /tmp/Testing
git push
Oops that failed. Why? git didnt know what to push to on origin, and didnt want to assume we wanted to call the branch mybranch1 on the remote. We need to tell that to git explicitly, as it tells us to.
In [29]:
%%bash
cd /tmp/Testing
git push --set-upstream origin mybranch1
Aha, now we are set my with both a remote and a local for mybranch1
In [30]:
%%bash
cd /tmp/Testing
git branch -avv
In [31]:
%%bash
cd /tmp/Testing
git checkout master
Now suppose for a second that this mybranch1 was created by someone else. We wanted to get it down using fetch and play. But we called pull, which did an automatic merge for us.
In [32]:
%%bash
cd /tmp/Testing
git pull origin mybranch1
In [33]:
%%bash
cd /tmp/Testing
git status
oops, that was not what we wanted. We undo it using git reset, to go back to the state at the last commit.
In [34]:
%%bash
cd /tmp/Testing
git reset --hard origin/master
In [35]:
%%bash
cd /tmp/Testing
git status
The last git command I want to talk about, since it might come handy in the course, is the situation in which you dont want to merge an entire branch from the upstream, but just one file from it. There is a direct use case for it. Indeed, suppose I've made an error in this lab and want to correct it. So i fix it in the upstream. In the meanwhile you have edited some other files. You dont care to manually ignore my older copies of those files. So you want to fix just one file from this new branch. This is how you do it.
First you fetch from the remote (i ought to be doing this with the course remote but just want to illustrate it here)
In [36]:
%%bash
cd /tmp/Testing
git fetch origin
In [37]:
%%bash
cd /tmp/Testing
git branch -avv
In [38]:
%%bash
cd /tmp/Testing
git checkout origin/mybranch1 -- world.md
Why does the syntax have origin/mybranch1? Remember that multiple remotes may have the same branch...so you want to be specific. So we check out origin's mybranch1, and only want world.md.
In [39]:
%%bash
cd /tmp/Testing
git status
Note that the file as automatically added to the index. We'll commit and push.
In [40]:
%%bash
cd /tmp/Testing
git commit -m "want world in master"
In [41]:
%%bash
cd /tmp/Testing
git status
In [42]:
%%bash
cd /tmp/Testing
git push
Note that in git versions > 2.0, which you must use, git push will push the current branch to its appropriate remote, all of which can be seen through git branch -avv or by looking at the config file.
In [43]:
%%bash
cd /tmp/Testing
cat .git/config
To make hw0.ipynb easily accessible, we added it to the public lab repo, so that you can read it even without having a github account. (Otherwise we would have a chicken and egg problem.). This is because our homework repository is private, and we have set it up so that your repositories are private as well.
Nevertheless, we want you to get acquainted with the workflow you must execute in order to obtain and submit homeworks.
Let me first describe the steps by which you gain access to the homework.
cs109-students.cs109-students/2015hw. All students have read-only access to this repository. It will serve the job of the course remote, like above. Any changes after the homework has gone out will be made here.cs109-students organization, which will be of the form cs109-students/userid-2015hw. Only you and the cs109 staff have access to this repository, thus ensuring the privacy of your homework.cs109-students/userid-2015hw. The branches are, unimaginatively named: hw0, hw1,...,hw5. (For the curious, the way this works is by us creating one remote per student for a local clone of our cs109-students/2015hw repository, and pushing the new branch to it. We only push to a new branch each time as we dont want to be messing with a branch you have already worked on.). There is master branch too, which will have some instructions, but nothing very exciting. You will never work on this branch.So now, how to you obtain and submit the homework? You wont be forking here.
git clone git@github.com:cs109-students/userid-2015hw.git (for ssh users) or git clone https://github.com/cs109-students/userid-2015hw.git (for https users). Substitute your own userid for userid.course to track the read-only "guru" repository. The command for this is: git remote add course git@github.com:cs109-students/2015hw.git or git remote add course https://github.com/cs109-students/2015hw.git. This well help to incorporate any changes, just like above.master branch, and perhaps a hw0 branch. In either case you should first do git fetch origin hw0, which fetches from your remote repository on github the hw0 branch. Then you issue git checkout -b hw0 origin/hw0. This command makes a new local branch hw0 on your machine which tracks the hw0 branch on your remote.hw0 branch. This is where you will work on homework 0. Start the ipython notebook in the repository and run the homework. The file you will use is hw0.ipynb. DO NOT run the notebook ending in _original.ipynb. These are simply copies of the homework. We made these copies so that you can update them from our course remote in case we make any changes. You will now engage in the "add/commit/push" cycle as described above. (The push will only push to the remote hw0 branch.)hw0.ipynb. (In actuality we wont grade homework 0 but check that you submitted it. But we will be using this mechanism to grade the homeworks from homework 1 onwards.)git@github.com:cs109-students/userid-2015hw.git on github with the name hw1. You will now repeat the process from step 3 onwards: git fetch origin hw1 followed by git checkout -b hw1 origin/hw1. Then you work on the hw1 branch, and engage in the "add/commit/push" cycle by running hw1.ipynb. And so on...Once again, in case we make changes, you can incorporate them into your repo by doing: git fetch course; git checkout course/hw0 -- hw0_original.ipynb. An "add/commit/push" cycle will make sure these changes go into your fork as well. If you intend to work on the changed file hw0_original.ipynb, simply copy the file to hw0.ipynb and work on it.
Remember that we will be looking for files hw0.ipynb, hw1.ipynb,...,hw5.ipynb as the semester goes on.
This process is summarized in the diagram above.
The homework repos are private, shared between you and the course staff. DO NOT share them or add any collaborators. That will violate the Honor Policy.
Lets talk a bit about how labs and sections work in this course:
(Sections are 2 hours long. The first hour will be spent going over the lab, while the second if an office hour, where you can ask your TA questions about the homework, the lectures, the subject matter, and even the lab).
The labs will be made available on public github repositories, with naming schemes like cs109/2015lab1.
This is how you ought to work with them (our github tutorial has an example of this process on the cs109/testing repository):
git@github.com:rahuldave/2015lab1.git and https://github.com/rahuldave/2015lab1.git respectively. /Applications/Utilities/Terminal.app or equivalent on mac and git-bash.exe on windows). Change (cd) into an appropriate folder and clone by doing git clone url where the url is the one in step 3.course. The command for this, for example, for the first lab is: git remote add course git@github.com:cs109/2015lab1.git or git remote add course https://github.com/cs109/2015lab1.git_original.ipynb. These are simply copies of the labs. We made these copies so that you can update them from our course remote in case we make any changes.For Lab 1 I'd start with pythonpandas, followed by babypython, and finally git. The git notebook can be run under the ipython notebook. But the git commands can also be run directly on a terminal, which is probably the best place to do them...you can keep the notebook on the side to read as you follow along). So after once having read the tutorial, as described earlier, you now get to work through it.
When you follow along, you can add in your own notes, and try your own variations. As you are doing this, dont forget to continue doing the "add/commit/push" cycle, so that you save and version your changes, and push them to your fork. This typically looks like:
- git add .
- git commit -a
- git push
In case we make changes, you can incorporate them into your repo by doing: git fetch course; git checkout course/master -- labname_original.ipynb where labname.ipynb is the lab in question. An "add/commit/push" cycle will make sure these changes go into your fork as well. If you intend to work on the changed file, simply copy the file to another one and work on it. Or you could make a new branch. Remember that this fork is YOUR repository, and you can do to it what you like. Your fork is public.
The diagram below should help elucidate the above and serve as a command cheat-sheet.
In [ ]:
Creating a repository can be done with git command line tools, or it can be done on github. On your github home, click on the 'Repositories' tab, then click the green 'New' button. You'll be brought to a page that asks for some basic project information.
The repository name needs to be unique, but only amongst your github repositories. Despite what github suggests, try to pick something at least a little representative of your project.
The description field is just a short text field that will accompany your repository's name on the github website.
If you're creating a new repository, check the "Initialize this repository with a README" box. This is a little bit of a misnomer. If you don't do this, you won't be able to clone your repository at all. Instead, you'll have to manually create a new repository on your computer and upload that to github. Suffice to say, unless you've got an existing git repository that was hosted somewhere besides github, always check this box.
You're free to click the .gitignore or license buttons, too, but there's no requirement to do so. You can easily create either later. For our class I use a python .gitignore from the dropdown menu
In addition to hosting git repositories, github also provides web hosting for each user and project. Github provides some documentation, but it's easier to create one than it is to figure out what to do with it next.
The easiest (and most limited) mechanism for creating a project page is to use the automatic page generator. Github's documentation does a fantastic job of guiding you through this, so I won't spend time on it. It's a 30-second process.
To edit the page, you can checkout the newly created gh-pages branch in your project and edit the index.html file. A much easier way to edit the page is simply to go through the automatic page generator tool again. It might seem counterintuitive, but github saves the content that you added the first time around, so you can make small changes without too much trouble.
In the end, automatically generated pages are a great replacement for a README.md file, but not much beyond that. You are limited to a single index.html file, and the templates are fixed. If you want a larger site or more customization options, you're better off doing something else.
The next natural step is to use plain old static HTML. Regardless of how you create your site, github uses the gh-pages branch of your project to find your website files. One way to create this branch is to click on the "branch" button on your repository page:
Alternately, if you've already clone your repository somewhere, you can do it on the command line:
When you push changes to this branch, they will immediately be reflected on the web at http://your-username.github.io/your-projectname.
You can use any static HTML, handwritten or otherwise.
Commit early, commit often.
Git is more effective when used at a fine granularity. For starters, you can't undo what you haven't committed, so committing lots of small changes makes it easier to find the right rollback point. Also, merging becomes a lot easier when you only have to deal with a handful of conflicts.
Commit unrelated changes separately.
Identifying the source of a bug or understanding the reason why a particular piece of code exists is much easier when commits focus on related changes. Some of this has to do with simplifying commit messages and making it easier to look through logs, but it has other related benefits: commits are smaller and simpler, and merge conflicts are confined to only the commits which actually have conflicting code.
Do not commit binaries and other temporary files.
Git is meant for tracking changes. In nearly all cases, the only meaningful difference between the contents of two binaries is that they are different. If you change source files, compile, and commit the resulting binary, git sees an entirely different file. The end result is that the git repository (which contains a complete history, remember) begins to become bloated with the history of many dissimilar binaries. Worse, there's often little advantage to keeping those files in the history. An argument can be made for periodically snapshotting working binaries, but things like object files, compiled python files, and editor auto-saves are basically wasted space.
Ignore files which should not be committed
Git comes with a built-in mechanism for ignoring certain types of files. Placing filenames or wildcards in a .gitignore file placed in the top-level directory (where the .git directory is also located) will cause git to ignore those files when checking file status. This is a good way to ensure you don't commit the wrong files accidentally, and it also makes the output of git status somewhat cleaner.
Always make a branch for new changes
While it's tempting to work on new code directly in the master branch, it's usually a good idea to create a new one instead, especially for team-based projects. The major advantage to this practice is that it keeps logically disparate change sets separate. This means that if two people are working on improvements in two different branches, when they merge, the actual workflow is reflected in the git history. Plus, explicitly creating branches adds some semantic meaning to your branch structure. Moreover, there is very little difference in how you use git.
Write good commit messages
I cannot understate the importance of this.
Seriously. Write good commit messages.