Development workflow
Talk is cheap. Show me the code.
—Linus Torvalds [1]
Contents
- Introduction
- How to send a patch
- Coding conventions in Sympy
- Workflow process
- Reviewing patches
- References
In SymPy we encourage collaborative work.
Everyone is welcome to join and to implement new feature, fix some bug, give general advice, etc. Also, we try to discuss everything and to review each other's work so that many eyes can see more thus raising the quality.
General discussion takes place on [email protected] mailing list and in the issues. Some discussion also takes place on IRC (our channel is #sympy at freenode).
As some of you already know, software development is not just coding. Many non-coding tasks have to be done in order to produce good code. For example: setting up infrastructure, designing, testing, documenting, assisting new developers (we are doing it here), and of course programming.
But even programming is not all about writing the code, it is about writing the code and preparing it so that the code can be included in the project.
Both producing the code and bringing it to the project are important parts of the game -- without the code there is nothing to bring in, and having the code outside is a no-win for anyone.
As already said above, we review changes. This idea was borrowed from successful projects like Linux, Python, SAGE and many more. In short, each change is first reviewed by other developers and only when it is approved is the code pushed in.
Like it takes effort to write good and clear code, reviewing other's work needs effort too. There are good practices how to do this so that reviewing is fun for both the author and the reviewer. We try to follow these good practices, and we'll try to show you how to follow them too.
When reviewing other's patches you learn a lot, so why not participate as a reviewer too? Anyone regardless of technical skill can help review code, and it's an excellent way for newcomers to learn about Sympy's development process and community.
License: New BSD License (see the LICENSE file for details) covers all files in the SymPy repository unless stated otherwise.
There are a few ways to create and send a patch.
The best way is to send a GitHub pull request against the sympy/sympy repository. We'll review it and push it in. The GitHub pull request is the preferred method, because it makes it easy for us to review and push the code in.
More quickly, but not convenient for reviewing and merging, is to create a patch-file using git alone. This way can be used if the patch has a high-priority or is significant, only one or two files are involved, or you don't have enough time to use the preferred method.
Although we are grateful for any improvements of Sympy, we strongly recommend you submit your patches as pull requests: this will greatly speed up the processing of the patch and ensure that it doesn't get forgotten due to inactivity.
The basic work-flow for both variants is a follows:
- Create your environment, if it was not created earlier.
- Create a new branch.
- Modify code and/or create tests of it.
- Be sure that all tests of SymPy pass.
- Only then commit changes.
- Create patch file, or pull request from GitHub.
All those are described in the details below Workflow process, but before you read that, it would be useful to acquaint yourself with Coding conventions in Sympy.
If you have any questions you can ask them on the mailinglist.
Follow the standard Style Guide for Python Code when writing code for SymPy, as explained at the following URLs:
In particular,
-
Use 4 spaces for indentation levels.
-
Use all lowercase function names with words separated by underscores. For example, you are encouraged to write Python functions using the naming convention
def set_some_value()
instead of the CamelCase convention.
-
Use CamelCase for class names and major functions that create objects, e.g.
class PolynomialRing(object)
Note, however, that some functions do have uppercase letters where it makes sense. For example, for matrices they are LUdecomposition or T (transposition) methods.
prime's docstring is an example of a well formatted docstring:
"""Return the nth prime. Primes are indexed as prime(1) = 2, prime(2) = 3, etc.... The nth prime is approximately n*log(n) and can never be larger than 2**n. See Also ======== sympy.ntheory.primetest.isprime, primerange, primepi References ========== .. [1] http://primes.utm.edu/glossary/xpage/BertrandsPostulate.html Examples ======== >>> from sympy import prime >>> prime(10) 29 >>> prime(1) 2 """
For more information see Writing documentation article on wiki.
SymPy uses a single codebase for Python 2 and Python 3 (the current supported versions are Python 2.6, 2.7, 3.2, and 3.3). This means that your code needs to run in both Python 2 and Python 3.
To make this easier, there are many functions in sympy.core.compatibility
that should be used when there are differences between the two Python
versions. To see what things you can import from that file, look at its
source.
If you need to use additional functions or methods that change names
from Python 2 to Python 3, it is far better to add to
sympy.core.compatibility and import from there than to bloat
individual source files with version-specific logic.
In general, the Python 3 names are to be preferred, but this is not an
absolute requirement. For example, the current codebase uses xrange extensively.
You should also make sure that you have:
from __future__ import print_function, division
at the top of each file. This will make sure that print is a function, and
that 1/2 does floating point division and not integer division. You should
also be aware that all imports are absolute, so import module will not
work if module is a module in the same directory as your file. You will
need to use import .module.
Creating of environment is once-only.
To install git in Linux-like systems you can do it via your native package management system:
$ yum install git
or:
$ sudo apt-get install git
In Windows systems, first of all, install Python from:
http://python.org/download/
by downloading the "Python 2.7 Windows installer" (or Python 2.6 or 2.5) and running it. Then do not forget to add Python to the $PATH environment.
On Windows and Mac OS X, the easiest way to get git is to download GitHub's software, which will install git, and also provide a nice GUI (this tutorial will be based on the command line interface). Note, you may need to go into the GitHub preferences and choose the "Install Command Line Tools" option to get git installed into the terminal.
If you do decide to use the GitHub GUI, you should make sure that any "sync does rebase" option is disabled in the settings.
Sympy development uses a few tools that are not included in a basic Python distribution. You won't really need them until you are getting ready to submit a pull request, but to save time later, you can install:
- Sphinx documentation generator (package sphinx-doc on Debian-based systems)
- Python coverage library (package python-coverage)
Git tracks who makes each commit by checking the user’s name and email. In addition, we use this info to associate your commits with your GitHub account.
To set these, enter the code below, replacing the name and email with your own (--global is optional).:
$ git config --global user.name "Firstname Lastname" $ git config --global user.email "[email protected]"
The name should be your actual name, not your GitHub username.
These global options (i.e. applying to all repositories) are placed in ~/.gitconfig. You can edit this file to add setup colors and some handy shortcuts:
[user]
name = Firstname Lastname
email = [email protected]
[color]
diff = auto
status= auto
branch= auto
interactive = true
[alias]
ci = commit
di = diff --color-words
st = status
co = checkout
log1 = log --pretty=oneline --abbrev-commit
logs = log --stat
It can be convenient in future to tune the bash prompt to display the current git branch.
The easiest way to do it, is to add the snippet below to your .bashrc or .bash_profile:
PS1="[\u@\h \W\$(git branch 2> /dev/null | grep -e '\* ' | sed 's/^..\(.*\)/{\1}/')]\$ "
But better is to use git-completion from the git source. This also has the advantage of adding tab completion to just about every git command. It also includes many other useful features, for example, promptings. To use git-completion, first download the git source code (about 27 MiB), then copy the file to your profile directory:
$ git clone git://git.kernel.org/pub/scm/git/git.git $ cp git/contrib/completion/git-completion.bash ~/.git-completion.sh
Read instructions in '~/.git-completion.sh'
Note that if you install git from the package manager in many Linux distros, this file is already installed for you. You can check if it is installed by seeing if tab completion works on git commands (try, e.g., git commi<TAB>, or git log --st<TAB>). You can also check if the PS1 commands work by doing something like:
$ PS1='\W $(__git_ps1 "%s")\$ '
And your command prompt should change to something like:
sympy master$
Note, it is important to define your PS1 using single quotes ('), not double quotes ("), or else bash will not update the branch name.
As you are going to use GitHub you should have a GitHub account. If you have not one yet then sign up at:
Then create your own fork of the SymPy project (if you have not yet). Go to the SymPy GitHub repository:
and click the “Fork” button.
Now you have your own repository for the SymPy project. If your username in GitHub is mynick then the address of the forked project will look something like:
Some tools connect to GitHub without SSH. To use these tools properly you need to find and configure your API Token.
On GitHub, click “Account Settings” then “Account Admin.”
Enter the code below, replacing the mynick and 012-api-token with your own:
$ git config --global github.user mynick $ git config --global github.token 012-api-token
Note: if you ever change your GitHub password, a new token will be created and will need to be updated.
Note: GitHub no longer uses API tokens. You can skip this step.
On your machine browse to where you would like to store SymPy, and clone (download) the latest code from SymPy's original repository (about 20 MiB):
$ git clone git://github.com/sympy/sympy.git $ cd sympy
Then assign your read-and-write repo to a remote called "github":
$ git remote add github [email protected]:mynick/sympy.git
For more information about GitHub forking and tuning see: [8], [9] and [11].
To establish a secure connection between your computer and GitHub see detailed instructions in [11].
If you have any problems with SSH access to GitHub, read the troubleshooting instructions at [12], or ask us in mail-list.
And now, do not forget to go to the Create separated branch instructions before modifying the code.
Typically, you will create a new branch to begin work on a new issue. Also pull request related with them.
A branch name should briefly describe the topic of the patch or pull request. If you know the issue number, then the branch name could be, for example, 1234_sequences. To create and checkout (that is, make it the working branch) a new branch
$ git branch 1234_sequences $ git checkout 1234_sequences
or in one command using
$ git checkout -b 1234_sequences
To view all branches, with your current branch highlighted, type:
$ git branch
And remember, never type the following commands in master: git merge, git commit, git rebase.
...
...
...
Do not forget that all new functionality should be tested, and all new methods, functions, and classes should have doctests showing how to use them.
Be sure that all tests of SymPy pass
To ensure everything stays in shape, let’s see if all tests pass:
$ ./bin/test $ ./bin/doctest
Each command will show a DO NOT COMMIT message if any of the tests it runs does not pass.
bin/test and bin/doctest do fast tests (those that take seconds). You'll want to run them whenever your code is supposed to work and not break anything.
You can also run bin/test --slow, to run the slow tests (those that may
take minutes each).
Code quality (unwanted spaces and indents) are checked by ./bin/test utilities too. But you can separately run this test with the help of this command:
$ ./bin/test quality
If you have trailing whitespace it will show errors. This one will fix unwanted spaces.
$ ./bin/strip_whitespace <file>
If you want to test only one set of tests try:
$ ./bin/test sympy/concrete/tests/test_products.py
But remember that all tests should pass before committing.
Note that all tests will be run when you make your pull request automatically by Travis CI, so do not worry too much about running every possible test. You can usually just run:
$ ./bin/test mod $ ./bin/doctest mod
where mod is the name of the module that you modified.
You can check what files are changed:
$ git status
Add new files to the index if necessary:
$ git add new_file.py
Check total changes:
$ git diff
You are ready to commit changes locally. A commit also contains a commit message which describes it. See the next section for guidelines on writing good commit messages. Type:
$ git commit
An editor window will appear automatically in this case. In Linux, this is vim by default. You can change what editor pops up by changing the $EDITOR shell variable.
Also with the help of option -a you can tell the command commit to automatically stage files that have been modified and deleted, but new files you have not told git about will not be affected, e.g.,:
$ git commit -a
If you want to stage only part of your changes, you can use the interactive commit feature. Just type:
$ git commit --interactive
and choose the changes you want in the resulting interface.
There are only two formatting rules for commit messages
- All lines should be 78 characters of less. This is so they can be easily read in terminals, which don't automatically line wrap things.
- There should be a single line with a summary, then an empty line, followed
by (optional) additional details. A common convention is to not end the
first line with a
., but all additional lines should (this convention probably exists to save an extra character to make it easier to fit the first line summary in 78 characters).
Here is an example commit message (from the commit [bf0e81e12a2f75711c30f0788daf4e58f72b2a41, which is part of the SymPy history):
integrals: Improved speed of heurisch() and revised tests
Improved speed of anti-derivative candidate expansion and solution
phases using explicit domains and solve_lin_sys(). The upside of
this change is that large integrals (those that generate lots of
monomials) are now computed *much* faster. The downside is that
integrals involving Derivative() don't work anymore. I'm not sure
if they really used to work properly or it was just a coincidence
and/or bad implementation. This needs further investigation.
Example:
In [1]: from sympy.integrals.heurisch import heurisch
In [2]: f = (1 + x + x*exp(x))*(x + log(x) + exp(x) - 1)/(x + log(x) + exp(x))**2/x
In [3]: %time ratsimp(heurisch(f, x))
CPU times: user 7.27 s, sys: 0.04 s, total: 7.31 s
Wall time: 7.32 s
Out[3]:
⎛ 2 x 2⋅x x 2 ⎞
log⎝x + 2⋅x⋅ℯ + 2⋅x⋅log(x) + ℯ + 2⋅ℯ ⋅log(x) + log (x)⎠ 1
──────────────────────────────────────────────────────────── + ───────────────
2 x
x + ℯ + log(x)
Previously it took 450 seconds and 4 GB of RAM to compute.
Some things to note about this commit message:
- The first line gives a brief description of what the commit does. Tools like
git shortlogor even GitHub only show the first line of the commit by default, so it is important to convey the most important aspects of the commit in the first line. - The first line has
integrals:, which gives context to the commit. A commit won't always be seen in the context of your branch, so it is often helpful to give each commit some context. This is not required, though, as it is not hard to look at the commit metadata to see what files were modified or at the commit history to see the nearby related commits. - After the first line, there is a paragraph describing the commit in more detail. This is important, as it describes what the commit does, which might be hard to figure out just from looking at the diff. It also gives information that might not be in the diff at all, such as known issues. Such paragraphs should be written in plain English. Commit messages are intended for human readers, both for people who will be reviewing your code right now, and for people who might come across your commit in the future while researching some change in the code. Sometimes, bulleted lists are a good format to convey the changes of a commit.
- Last, there is an example. It is nice to give a concrete example in commits that add new features. This particular example is about improving the speed of something, so the example is a benchmark result.
Note that you may feel free to use Unicode characters in commit messages, such as output from the SymPy Unicode pretty printer.
Other things to do in commit messages:
- If the bug fixes an issue, reference that issue in the message. Also reference any pull requests or mailing list messages with links. This will make it easier to find related discussions about the commit in the future. You do not need to add a reference to the pull request that contains the commit. That can be found from the git log.
Try to avoid short commit messages, like "Fix", and commit messages that give no context, like "Found the bug". When in doubt, a longer commit message is probably better than a short one.
Be sure that you are in your own branch, and run:
$ git push github 1234_sequences
This will send your local changes to your fork of the SymPy repository. Then navigate to your repository with the changes you want someone else to pull:
https://github.com/mynick/sympy
Select branch, and press the Pull Request button.
After pressing the Pull Request button, you are presented with a preview page where you can enter a title and optional description, see exactly what commits will be included when the pull request is sent, and also see who the pull request will be sent to:
If you’re sending from a topic branch, the title is pre-filled based on the name of the branch. Markdown is supported in the description, so you can embed images or use preformatted text blocks.
You can switch to the Commits tab to ensure that the correct set of changes is being sent. And review the diff of all changes by switching to the Files Changed.
Once you’ve entered the title and description, made any necessary customizations to the commit range, and reviewed the commits and file changes to be sent, press the Send pull request button.
The pull request is sent immediately. You’re taken to the main pull request discussion and review page. Additionally, all repository collaborators and followers will see an event in their dashboard.
That's all.
See also Updating your pull request
If after a time you need to make changes in pull request then the best way is to add a new commit in you local repository and simply repeat push command:
$ git commit $ git push github 1234_sequences
Note that if you do any rebasing or in any way edit your commit history, you will have to add the -f (force) option to the push command for it to work:
$ git push -f github
You don't need to do this if you merge, which is the recommended way.
Sometimes, you may need to merge your branch with the upstream master. Usually you don't need to do this, but you may need to if
- Someone tells you that your branch needs to be merged because there are merge conflicts.
- sympy-bot tells you that your branch could not be merged.
- You need some change from master that was made after you started your branch.
Note, that after cloning a repository, it has a default remote called origin that points to the sympy/sympy repository. And your fork remote named as github. You can observe the remotes names with the help of this command:
$ git remote -v github [email protected]:mynick/sympy.git (fetch) github [email protected]:mynick/sympy.git (push) origin git://github.com/sympy/sympy.git (fetch) origin git://github.com/sympy/sympy.git (push)
As an example, consider that we have these commits in the master branch of local git repository:
A---B---C master
Then we have divergent branch 1234_sequences:
A---B---C master
\
a---b 1234_sequences
In the meantime the remote sympy/sympy master repository was updated too:
A---B---C---D origin/master
A---B---C master
\
a---b 1234_sequences
There are basically two ways to get up to date with a changed master: rebasing and merging. Merging is recommended.
Merging creates a special commit, called a "merge commit", that joins your branch and master together:
A---B---C------D origin/master
\ \
\ M merge
\ /
a--b 1234_sequences
Note that the commits A, B, C, and D from master and the
commits a and b from 1234_sequences remain unchanged. Only the new
commit, M, is added to 1234_sequences, which merges in the new commit
branch from master.
Rebasing essentially takes the commits from 1234_sequences and reapplies
them on the latest master, so that it is as if you had made them from the
latest version of that branch instead. Since these commits have a different
history, they are different (they will have different SHA1 hashes, and will
often have different content):
A---B---C---D---a'---b' origin/master
Rebasing is required if you want to edit your commit history (e.g., squash commits, edit commit messages, remove unnecessary commits). But note that since this rewrites history, it is possible to lose data this way, and it makes it harder for people reviewing your code, because they can no longer just look at the "new commits"; they have to look at everything again, because all the commits are effectively new.
There are several advantages to merging instead of rebasing. Rebasing reapplies each commit iteratively over master, and if the state of the files changed by that commit is different from when it was originally made, the commit will change. This means what you can end up getting commits that are broken, or commits that do not do what they say they do (because the changes have been "rebased out"). This can lead to confusion if someone in the future tries to test something by checking out commits from the history. Finally, merge conflict resolutions can be more difficult with rebasing, because you have to resolve the conflicts for each individual commit. With merging, you only have to resolve the conflicts between the branches, not the commits. It is quite common for a merge to not have any conflicts but for a rebase to have several, because the conflicts are "already resolved" by later commits.
Merging keeps everything intact. The commits you make are exactly the same, down to the SHA1 hash, which means that if you checkout a commit from a merged branch, it is exactly the same as checking it out from a non-merged branch. What it does instead is create a single commit, the merge commit, that makes it so that the history is both master and your branch. This commit contains all merge conflict resolution information, which is another advantage over rebasing (all merge conflict resolutions when rebasing are "sifted" into the commits that caused them, making them invisible).
Since this guide is aimed at new git users, you should be learning how to merge.
First merge your local repository with the remote:
$ git checkout master $ git pull
This results in:
A---B---C---D master
\
a---b 1234_sequences
Then merge your 1234_sequences branch from 1234_sequences:
$ git checkout 1234_sequences $ git merge master
If the last command tells you that conflicts must be solved for a few indicated files.
If that's the case then the marks >>> and <<< will appear at those files. Fix the code with >>> and <<< around it to what it should be. You must manually remove useless pieces, and leave only new changes from your branch.
Then be sure that all tests pass:
$ ./bin/test $ ./bin/doctest
and commit:
$ git commit
So the result will be like that (automatic merging c):
A---B---C-------D master
\ \
a---b---M 1234_sequences
Note: merging is recommended over rebasing.
The final aim, that we want to obtain is:
A---B---C---D master
\
a---b 1234_sequences
The way to do it is first of all to merge local repository with the remote sympy/sympy:
$ git checkout master $ git pull
So we obtain:
A---B---C---D master
\
a---b 1234_sequences
Then:
$ git checkout 1234_sequences $ git rebase master
Note that this last one will require you to fix some merge conflicts if there are changes
to the same file in master and 1234_sequences. Open the file that it tells you is wrong,
fix the code with >>> and <<< around it to what it should be.
Then be sure that all tests pass:
$ ./bin/test $ ./bin/doctest
Then do:
$ git add sympy/matrices/your_conflict_file $ git rebase --continue
(git rebase will also guide you in this).
The only time when it is recommended to rebase instead of merge is when you need to edit your commit messages, or remove unnecessary commits.
Note, it is much better to get your commit messages right the first time. See the section on writing good commit messages above.
Consider these commit messages:
$ git log --oneline 7bbbc06 bugs fixing 4d6137b some additional corrections. 925d88fx sequences base implementation.
Then run rebase command in interactive mode:
$ git rebase --interactive 925d88fx
Or you can use other ways to point to commits, e.g. `git rebase --interactive HEAD^^` or `git rebase --interactive HEAD~2`.
A new editor window will appear (note that order is reversed with respect to the git log command):
pick 4d6137b some additional corrections. pick 7bbbc06 bugs fixing # Rebase 925d88f..7bbbc06 onto 925d88f # # Commands: # p, pick = use commit # r, reword = use commit, but edit the commit message # e, edit = use commit, but stop for amending # s, squash = use commit, but meld into previous commit # f, fixup = like "squash", but discard this commit's log message
To edit a commit message, change pick to reword (or on old versions of git, to edit) for those that you want to edit and save that file.
To squash two commits together, change pick to squash. To remove a commit, just delete the line with the commit.
To edit a commit, change pick to edit.
After that, git will drop you back into your editor for every commit you want to reword, and into the shell for every commit you wanted to edit:
$ (Change the commit in any way you like.) $ git commit --amend "your new message" $ git rebase --continue
For commits that you want to edit, it will stop. You can then do:
$ git reset --mixed HEAD^
This will "uncommit" all the changes from the commit. You can then recommit them however you want. When you are done, remember to do:
$ git rebase --continue
Most of this sequence will be explained to you by the output of the various commands of git. Continue until it says:
Successfully rebased and updated refs/heads/master.
If at any point you want to abort the rebase, do:
$ git rebase --abort
Warning: this will run git reset --hard, deleting any uncommitted
changes you have. If you want to save your uncommitted changes, run git
stash first, and then run git stash pop when you are done.
Coding's only half the battle in software development: our code also has to be thoroughly reviewed before release. Reviewers thus are an integral part of the development process. Note that you do not have to have any special pull or other privileges to review patches: anyone with Python on his/her computer can review.
Pull requests (the preferred avenue for patches) for sympy are located here. Feel free to view any open pull request. Each contains a Discussion section for comments, Commits section for viewing the author's commit files and documentation, and Diff section for viewing all the changes in code. To browse the raw code files for a commit, select a commit in the Commits section and click on the "View file" link to view a file.
Based on your level of expertise, there are two ways to participate in the review process: manually running tests and using sympy-bot. Whichever option you choose, you should also make sure that the committed code complies with the Writing documentation guidelines.
If you prefer to test code manually, you will first have to set up your environment as described in the Workflow process section. Then, you need to obtain the patched files. If you're reviewing a pull request, you should get the requested branch into your sympy folder. Go into your folder and execute (<username> being the username of the pull requester and <branchname> being the git branch of the pull request):
$ git remote add <username> git://github.com/<username>/sympy.git $ git fetch <username> $ git checkout -b <branchname> <username>/<branchname>
After obtaining the pull request or patch, go to your sympy root directory and execute:
$ ./bin/test $ ./bin/doctest
If there are any problems, notify the author in the pull request by commenting.
A good option for both new and veteran code reviewers is the sympy-bot program, which automatically tests code in pull requests and posts the results in the appropriate pull requests. To run sympy-bot, download the archive and decompress it. Go into the folder and execute:
$ ./sympy-bot list
This will list all open pull requests on GitHub. To review a pull request and run all tests, execute:
$ ./sympy-bot review <pullrequest_number>
Alternatively, to review all open pull requests, execute:
$ ./sympy-bot review all
The review command will post the results of all tested pull requests in the appropriate Github page. For more information on sympy-bot, visit the readme.
A pull request or patch must meet the following requirements during review before being considered as ready for release.
-
- All tests must pass.
-
- Rationale: We need to make sure we're not releasing buggy code.
- If new features are being implemented and/or bug fixes are added, tests should be added for them as well.
-
- The reviews (at least 1) must all be positive.
-
- Rationale: We'd like everyone to agree on the merits of the patch.
- If there are conflicting opinions, the reviewers should reach a consensus.
-
- The patch must have been posted for at least 24 hours.
-
- Rationale: This gives a chance for everyone to look at the patch.
This page is based upon present SymPy pages [2-6], GitHub help [8-9], [11-12] and inspired by Sage guide [10]:
| [1] | http://lkml.org/lkml/2000/8/25/132 |
| [2] | http://docs.sympy.org/dev/sympy-patches-tutorial.html#quick-start |
| [3] | http://sympy.org/development.html |
| [4] | https://github.com/sympy/sympy/wiki |
| [5] | https://github.com/sympy/sympy/wiki/Pushing-patches |
| [6] | https://github.com/sympy/sympy/wiki/Getting-the-bleeding-edge |
| [7] | https://github.com/sympy/sympy/wiki/Git-hg-rosetta-stone |
| [8] | http://help.github.com/pull-requests/ |
| [9] | http://help.github.com/fork-a-repo/ |
| [10] | http://sagemath.org/doc/developer/ |
| [11] | (1, 2) http://help.github.com/linux-set-up-git/ |
| [12] | http://help.github.com/troubleshooting-ssh/ |