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Title:Frequently Asked Questions for Python Developers
Date: 2010-09-15
Version: 10931
Web site:


1   General Information

1.1   Where do I start?

The dev page links to various documents to help get you started.

1.2   How can I become a developer?

Contribute on a regular basis through patches and ask for commit privileges once you have demonstrated a track record of being good at fixing things.

(Raymond Hettinger commented on the School of Hard Knocks required.)

2   Version Control

2.1   Where can I learn about the version control system used, Subversion (svn)?

Subversion's (also known as svn) official web site is at . A book on Subversion published by O'Reilly Media, Version Control with Subversion, is available for free online.

With Subversion installed, you can run the help tool that comes with Subversion to get help:

svn help

The man page for svn is rather scant and not very helpful.

2.2   What do I need to use Subversion?

2.2.1   UNIX

First, you need to download Subversion. Most UNIX-based operating systems have binary packages available. Also, most packaging systems also have Subversion available.

If you have checkin rights, you need OpenSSH. This is needed to verify your identity when performing commits.

2.2.2   Windows

You have several options on Windows. One is to download Subversion itself which will give you a command-line version. Another option is to download TortoiseSVN which integrates with Windows Explorer.

If you have checkin rights, you will also need an SSH client. Download PuTTY and friends (PuTTYgen, Pageant, and Plink) for this. All other questions in this FAQ will assume you are using these tools.

Once you have both Subversion and PuTTY installed you must tell Subversion where to find an SSH client. Do this by editing %APPDATA%\Subversion\config to have the following section:

ssh="c:/path/to/putty/plink.exe" -T

Change the path to be the proper one for your system. The -T option prevents a pseudo-terminal from being created.

You can use Pageant to prevent from having to type in your password for your SSH 2 key constantly. If you prefer not to have another program running, you need to create a profile in PuTTY.

Go to Session:Saved Sessions and create a new profile named In Session:Host Name, enter In SSH/Auth:Private key file select your private key. In Connection:Auto-login username enter pythondev.

With this set up, paths are slightly different than most other settings in that the username is not required. Do take notice of this when choosing to check out a project!

2.3   How do I get a checkout of the repository (read-only or read-write)?

Regardless of whether you are checking out a read-only or read-write version of the repository, the basic command is the same:

svn checkout <URL> [PATH]

<URL> is the specified location of the project within the repository that you would like to check out (those paths are discussed later). The optional [PATH] argument specifies the local directory to put the checkout into. If left out then the tail part of <URL> is used for the directory name.

For a read-only checkout, the format of <URL> is:<path>

with <path> representing the path to the project. A list of projects can be viewed at . Any subdirectory may also be checked out individually.

For a read-write checkout (with a caveat for Windows users using PuTTY without Pageant), the format for <URL> is:


There are three critical differences between a read-only URL and a read-write URL. One is the protocol being specified as svn+ssh and not http. Next, the username pythondev is added (note that Windows users using PuTTY without Pageant should leave off pythondev@ if PuTTY was set up following the instructions in this FAQ). Lastly, note that projects was removed from the path entirely for a read-write checkout.

The repositories most people will be interested in are:

Repository read-only read-write
PEPs svn+ssh://
2.7 svn+ssh://
3.1 svn+ssh://
3.2 svn+ssh://

2.4   How do I update my working copy to be in sync with the repository?


svn update

from the directory you wish to update. The directory and all its subdirectories will be updated.

2.6   Where can I find a downloadable snapshot of the source code?

Visit to download a tarball containing a daily snapshot of the repository.

2.8   How do I verify that my commit privileges are working?

2.8.1   UNIX

If you are listed as a committer at , then you should be able to execute:


and have the following printed to your terminal:

( success ( 2 2 ( ) ( edit-pipeline svndiff1 absent-entries
  commit-revprops depth log-revprops partial-replay ) ) )

If something else is printed, then there is a problem with your SSH 2 public key and you should contact .

2.8.2   Windows

If you are using Pageant, you can verify that your SSH 2 key is set up properly by running:


Using the proper path to your PuTTY installation, you should get a response from the server that says:

( success ( 1 2 ( ANONYMOUS EXTERNAL ) ( edit-pipeline ) ) )

If there is a failure, run plink with -v to analyse the problem.

If you are using a profile in PuTTY, the best way to test is to try to log in through Open.

2.9   What configuration settings should I use?

Make sure the following settings are in your Subversion config file (~/.subversion/config under UNIX):

enable-auto-props = yes

* = svn:eol-style=native
*.c = svn:keywords=Id
*.h = svn:keywords=Id
*.py = svn:keywords=Id
*.txt = svn:keywords=Author Date Id Revision

The [auto-props] line specifies the beginning of the section in the config file. The svn:eol-style setting tells Subversion to check out files using the native line endings on your OS. It will also automatically convert line endings upon committal so that they are consistent across all platforms. The svn:keywords settings are to automatically substitute $keyword$ arguments in files that match the pattern. *.txt has more options so as to cover all needed keywords for PEPs.

The [miscellany] section and its one option make Subversion apply the various rules in the [auto-props] section automatically to all added or imported files into the respository.

2.10   How do I add a file or directory to the repository?

Simply specify the path to the file or directory to add and run:

svn add PATH

Subversion will skip any directories it already knows about. But if you want new files that exist in any directories specified in PATH, specify --force and Subversion will check all directories for new files.

You will then need to run svn commit (as discussed in How do I commit a change to a file?) to commit the file to the repository.

2.11   How do I commit a change to a file?

To have any changes to a file (which include adding a new file or deleting an existing one), you use the command:

svn commit [PATH]

Although [PATH] is optional, if PATH is omitted all changes in your local copy will be committed to the repository. DO NOT USE THIS!!! You should specify the specific files to be committed unless you are absolutely positive that all outstanding modifications are meant to go in this commit.

To abort a commit that you are in the middle of, leave the message empty (i.e., close the text editor without adding any text for the message). Subversion will confirm if you want to abort the commit.

If you do not like the default text editor Subversion uses for entering commmit messages, you may specify a different editor in your Subversion config file with the editor-cmd option in the [helpers] section.

2.12   How do I delete a file or directory in the repository?

Specify the path to be removed with:

svn delete PATH

Any modified files or files that are not checked in will not be deleted in the working copy on your machine.

2.13   What files are modified locally in my working copy?


svn status [PATH]

will list any differences between your working copy and the repository. Some key indicators that can appear in the first column of output are:

A Scheduled to be added
D Scheduled to be deleted
M Modified locally
? Not under version control

2.15   How do I revert a file I have modified back to the version in the respository?


svn revert PATH

will change PATH to match the version in the repository, throwing away any changes you made locally. If you run:

svn revert -R .

from the root of your local repository it will recursively restore everything to match up with the main server.

2.16   How do I find out who edited or what revision changed a line last?

You want:

svn blame PATH

This will output to stdout every line of the file along with what revision number last touched that line and who committed that revision. Since it is printed to stdout, you probably want to pipe the output to a pager:

svn blame PATH | less

2.17   How can I see a list of log messages for a file or specific revision?

To see the log messages for a specific file, run:

svn log PATH

That will list all messages that pertain to the file specified in PATH.

If you want to view the log message for a specific revision, run:

svn log --verbose -r REV

With REV substituted with the revision number. The --verbose flag should be used to get a listing of all files modified in that revision.

2.18   How can I edit the log message of a committed revision?


svn propedit -r <revision> --revprop svn:log

Replace <revision> with the revision number of the commit whose log message you wish to change.

2.19   How do I get a diff between the repository and my working copy for a file?

The diff between your working copy and what is in the repository can be had with:

svn diff PATH

This will work off the current revision in the repository. To diff your working copy with a specific revision, do:

svn diff -r REV PATH

Finally, to generate a diff between two specific revisions, use:

svn diff -r REV1:REV2 PATH

Notice the : between REV1 and REV2.

2.20   How do I undo the changes made in a recent committal?

Assuming your bad revision is NEW and OLD is the equivalent of NEW - 1, then run:

svn merge -r NEW:OLD PATH

This will revert all files back to their state in revision OLD. The reason that OLD is just NEW - 1 is you do not want files to be accidentally reverted to a state older than your changes, just to the point prior.

Note: PATH here refers to the top of the checked out repository, not the full pathname to a file. PATH can refer to a different branch when merging from the head, but it must still be the top and not an individual file or subdirectory.

2.21   How do I update to a specific release tag?


svn list svn+ssh://

or visit:

to get a list of tags. To switch your current sandbox to a specific tag, run:

svn switch svn+ssh://

To just update to the revision corresponding to that tag without changing the metadata for the repository, note the revision number corresponding to the tag of interest and update to it, e.g.:

svn update -r 39619

2.22   Why should I use svn switch?

If you picture each file/directory in Subversion as uniquely identified by a 2-space coordinate system [URL, revision] (given a checkout, you can use "svn info" to get its coordinates), then we can say that "svn up -r N" (for some revision number N) keeps the url unchanged and changes the revision to whatever number you specified. In other words, you get the state of the working copy URL at the time revision N was created. For instance, if you execute it with revision 39619 within the trunk working copy, you will get the trunk at the moment 2.4.2 was released.

On the other hand, "svn switch" moves the URL: it basically "moves" your checkout from [old_URL, revision] to [new_URL, HEAD], downloading the minimal set of diffs to do so. If the new_URL is a tag URL (e.g. .../tags/r242), it means any revision is good, since nobody is going to commit into that directory (it will stay unchanged forever). So [/tags/r242, HEAD] is the same as any other [/tags/r242, revision] (assuming of course that /tags/r242 was already created at the time the revision was created).

If you want to create a sandbox corresponding to a particular release tag, use svn switch to switch to [/tags/some_tag, HEAD] if you don't plan on doing modifications. On the other hand if you want to make modifications to a particular release branch, use svn switch to change to [/branches/some_branch, HEAD].

(Written by Giovanni Bajo on python-dev.)

2.23   How do I create a branch?

The best way is to do a server-side copy by specifying the URL for the source of the branch, and the eventual destination URL for the new branch:


You can then checkout your branch as normal. You will want to prepare your branch for future merging from the source branch so as to keep them in sync. To find out how to do that, read How do I merge between branches?.

2.25   How do I prepare a new branch for merging?

You need to initialize a new branch by having discover the revision number that the branch was created with. Do this with the command: init

Then check in the change to the root of the branch. This is a one-time operation (i.e. only when the branch is originally created, not when each developer creates a local checkout for the branch).

2.26   How do I merge between branches?

In the current situation for Python there are four branches under development, meaning that there are three branches to merge into. Assuming a change is committed into trunk as revision 0001, you merge into the 2.x maintenance by doing:

# In the 2.x maintenance branch checkout. merge -r 0001
svn commit -F svnmerge-commit-message.txt  # r0002

To pull into py3k:

# In a py3k checkout. merge -r 0001
svn commit -F svnmerge-commit-message.txt  # r0003

The 3.x maintenance branch is a special case as you must pull from the py3k branch revision, not trunk:

# In a 3.x maintenance checkout. merge -r 0003  # Notice the rev is the one from py3k!
svn resolved .
svn commit -F svnmerge-commit-message.txt

2.27   How do I block a specific revision from being merged into a branch?

With the revision number that you want to block handy and, go to your checkout of the branch where you want to block the revision and run: block -r <revision #>

This will modify the repository's top directory (which should be your current directory) and create svnmerge-commit-message.txt which contains a generated log message.

If the command says "no available revisions to block", then it means someone already merged the revision.

To check in the new metadata, run:

svn ci -F svnmerge-commit-message.txt

2.28   How do I include an external svn repository (external definition) in the repository?

Before attempting to include an external svn repository into Python's repository, it is important to realize that you can only include directories, not individual files.

To include a directory of an external definition (external svn repository) as a directory you need to edit the svn:externals property on the root of the repository you are working with using the format of:

local_directory remote_repositories_http_address

For instance, to include Python's sandbox repository in the 'sandbox' directory of your repository, run svn propedit svn:externals . while in the root of your repository and enter:


in your text editor. The next time you run svn update it will pull in the external definition.

2.29   How can I create a directory in the sandbox?

Assuming you have commit privileges and you do not already have a complete checkout of the sandbox itself, the easiest way is to use svn's mkdir command:

svn mkdir svn+ssh://<directory>

That command will create the new directory on the server. To gain access to the new directory you then checkout it out (substitute mkdir in the command above with checkout).

If you already have a complete checkout of the sandbox then you can just use svn mkdir on a local directory name and check in the new directory itself.

3   SSH

3.1   How do I generate an SSH 2 public key?

All generated SSH keys should be sent to pydotorg for adding to the list of keys.

3.1.1   UNIX


ssh-keygen -t rsa

This will generate a two files; your public key and your private key. Your public key is the file ending in .pub.

3.1.2   Windows

Use PuTTYgen to generate your public key. Choose the "SSH2 DSA" radio button, have it create an OpenSSH formatted key, choose a password, and save the private key to a file. Copy the section with the public key (using Alt-P) to a file; that file now has your public key.

3.2   Is there a way to prevent from having to enter my password for my SSH 2 public key constantly?

3.2.1   UNIX

Use ssh-agent and ssh-add to register your private key with SSH for your current session. The simplest solution, though, is to use KeyChain, which is a shell script that will handle ssh-agent and ssh-add for you once per login instead of per session.

3.2.2   Windows

Running Pageant will prevent you from having to type your password constantly. If you add a shortcut to Pageant to your Autostart group and edit the shortcut so that the command line includes an argument to your private key then Pageant will load the key every time you log in.

3.3   Can I make check-ins from machines other than the one I generated the keys on?

Yes, all you need is to make sure that the machine you want to check in code from has both the public and private keys in the standard place that ssh will look for them (i.e. ~/.ssh on Unix machines). Please note that although the key file ending in .pub contains your user name and machine name in it, that information is not used by the verification process, therefore these key files can be moved to a different computer and used for verification. Please guard your keys and never share your private key with anyone. If you lose the media on which your keys are stored or the machine on which your keys are stored, be sure to report this to at the same time that you change your keys.

4   Compilation

4.1   How do I create a debug build of Python?

A debug build, sometimes called a "pydebug" build, has extra checks and bits of information to help with developing Python.

4.1.1   UNIX

The basic steps for building Python for development is to configure it and then compile it.

Configuration is typically:

./configure --prefix=/dev/null --with-pydebug

More flags are available to configure, but this is the minimum you should do. This will give you a debug version of Python along with a safety measure to prevent you from accidentally installing your development version over your system install. If you are developing on OS X for Python 2.x and will not be working with the OS X-specific modules from the standard library, then consider using the --without-toolbox-glue flag to faster compilation time.

Once configure is done, you can then compile Python.:

make -s

This will build Python with only warnings and errors being printed to stderr. If you are using a multi-core machine you can use the -j flag along with the number of cores your machine has (e.g. with two cores, you would want make -s -j2) to compile multiple files at a time.

Once Python is done building you will then have a working build of Python that can be run in-place; ./python on most machines, ./python.exe on OS X.

4.1.2   Windows

For VC 9, the PCbuild directory contains the build files. For older versions of VC, see the PC directory. For a free compiler for Windows, go to .

To build from the GUI, load the project files and press F7. Make sure to choose the Debug build. If you want to build from the command line, run the build_env.bat file to get a terminal with proper environment variables. From that terminal, run:

build.bat -c Debug

Once built you will want to set Python as a startup project. F5 will launch the interpreter as well as double-clicking the binary.

5   Editors and Tools

5.1   What support is included in Python's source code for Vim?

Within the Misc/Vim directory you will find two files to help you when editing Python code. One is python.vim, which is a generated syntax highlight file for Python code. This file is updated much more frequently as it contains syntax highlighting for keywords as they are added to the source tree. See the top of the file to find out how to use the file.

The other file for Vim is a vimrc file that supports PEP 7 and 8 coding standards. All settings are specific to Python and C code and thus will not affect other settings. There are also some settings which are helpful but turned off by default at the end of the file if one cares to use non-essential settings. Once again, see the top of the file to learn how to take advantage of the file.

5.2   What support is included in Python's source code for gdb?

The Misc/gdbinit file contains several helpful commands that can be added to your gdb session. You can either copy the commands into your own .gdbinit file or, if you don't have your own version of the file, simply symlink ~/.gdbinit to Misc/gdbinit.

5.3   Can I run Valgrind against Python?

Because of how Python uses memory, Valgrind requires setting some suppression rules to cut down on the false positives (which still occur, suggesting one typically should know how Python uses memory before running Valgrind against Python). See Misc/README.valgrind for more details.

6   Patches

6.1   How to make a patch?

If you are using subversion (anonymous or developer) you can use subversion to make the patches for you. Just edit your local copy and enter the following command:

svn diff | tee ~/name_of_the_patch.diff

Else you can use the diff util which comes with most operating systems (a Windows version is available as part of the cygwin tools).

6.2   How do I apply a patch?

For the general case, to apply a patch go to the directory that the patch was created from (usually /dist/src/) and run:

patch -p0 < name_of_the_patch.diff

The -p option specifies the number of directory separators ("/" in the case of UNIX) to remove from the paths of the files in the patch. -p0 leaves the paths alone.

6.3   How do I undo an applied patch?

Undoing a patch differs from applying one by only a command-line option:

patch -R -p0 < name_of_the_patch.diff

Another option is to have 'patch' create backups of all files by using the -b command-line option. See the man page for 'patch' on the details of use.

6.4   How to submit a patch?

Please consult the patch submission guidelines at .

6.5   How to test a patch?

Firstly, you'll need to get a checkout of the source tree you wish to test the patch against and then build python from this source tree.

Once you've done that, you can use Python's extensive regression test suite to check that the patch hasn't broken anything.

In general, for thorough testing, use:

python -m test.regrtest -uall

For typical testing use:

python -m test.regrtest

For running specific test modules:

python -m test.regrtest test_mod1 test_mod2

NB: Enabling the relevant test resources via -uall or something more specific is especially important when working on things like the networking code or the audio support - many of the relevant tests are skipped by default.

For more thorough documentation, read the documentation for the test package at

If you suspect the patch may impact other operating systems, test as many as you have easy access to. You can get help on alternate platforms by contacting the people listed on, who have volunteered to support a particular operating system.

6.6   How to change the status of a patch?

To change the status of a patch or assign it to somebody else you have to have the Developer role in the bug tracker. Contact one of the project administrators if the following does not work for you.

Click on the patch itself. In the screen that comes up, there is a drop-box for "Assigned To:" and a drop-box for "Status:" where you can select a new responsible developer or a new status respectively. After selecting the appropriate victim and status, hit the "Submit Changes" button at the bottom of the page.

Note: If you are sure that you have the right permissions and a drop-box does not appear, check that you are actually logged in to Roundup!

7   Bugs

7.1   Where can I submit/view bugs for Python?

The Python project uses Roundup for bug tracking. Go to for all bug management needs. You will need to create a Roundup account for yourself before submitting the first bug report; anonymous reports have been disabled since it was too difficult to get in contact with submitters. If you previously had used SourceForge to report Python bugs, you can use Roundup's "Lost your login?" link to obtain your Roundup password.