Changes/Python 3 as Default

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= Python 3 as Default =
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= Python 3 as the Default Implementation =
  
 
== Summary ==
 
== Summary ==
Up until now, Fedora has used Python 2 as the default Python implementation. This change proposes switching to Python 3. The term "switching" is explained in detail in [[#Scope|Scope]] section.
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Up until now, Fedora has used Python 2 as the default Pythn implementation. This change proposes switching to Python 3. The details of the term "switching" are explained thoroughly in the [[#Scope|Scope]] section.
  
 
== Owner ==
 
== Owner ==
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For change proposals to quality as self-contained, owners of all affected packages need to be included here. Alternatively, a SIG can be listed as an owner if it owns all affected packages.  
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For change proposals to quality as self-contained, owners of all affected packages need to be included here. Alternatively, a SIG can be listed as an owner if it owns all affected packages.
This should link to your home wiki page so we know who you are.  
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This should link to your home wiki page so we know who you are.
 
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* Name: [[User:Bkabrda|Slavek Kabrda]]
 
* Name: [[User:Bkabrda|Slavek Kabrda]]
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== Current status ==
 
== Current status ==
* Targeted release: [[Releases/22 | Fedora 22 ]]  
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* Targeted release: [[Releases/22 | Fedora 22 ]]
 
* Last updated: 10-03-2013
 
* Last updated: 10-03-2013
<!-- After the change proposal is accepted by FESCo, tracking bug is created in Bugzilla and linked to this page  
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<!-- After the change proposal is accepted by FESCo, tracking bug is created in Bugzilla and linked to this page
 
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Bugzilla states meaning as usual:
 
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NEW -> change proposal is submitted and announced
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== Detailed Description ==
 
== Detailed Description ==
Python 3 is the next generation of Python programming language. It is currently mature and stable, since it has been under active development for five years - 3.0 was released in December 2008, current latest stable version is 3.3.2 released in May 2013. The reasons why to switch to Python 3 as a default are mentioned in [[#Benefit to Fedora]] section, possible issues in [[#Scope]]. This feature assumes that DNF will be the default package manager in F22, as Yum doesn't and never will work with Python 3. If DNF doesn't make it to Fedora 22, this feature has to be postponed accordingly.
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Python 3 is the next generation of Python programming language. It is currently mature and stable, since it has been under active development for five years - version 3.0 was released in December 2008, current latest stable version is 3.3.2 released in May 2013. The main reason to switch to Python 3 as the default implementation is that Python 2 is in maintenance mode, thus only bugfixes and security fixes are accepted upstream. Further reasons are mentioned in the [[#Benefit to Fedora]] section.
 +
For this Change to be carried out successfully, it is necessary that the key packages in the Fedora software stack be ported to Python 3. These are parts of the minimal buildroot, the default package manager, programs present on the LiveCD etc. More information on the packages involved can be found in [[#Dependencies]]. While porting of some packages is rather trivial, other packages need significant amount of work to get rid of the Python 2 dependence.
  
 
== Benefit to Fedora ==
 
== Benefit to Fedora ==
 
<!-- What is the benefit to the platform?  If this is a major capability update, what has changed?  If this is a new functionality, what capabilities does it bring? Why will Fedora become a better distribution or project because of this proposal?-->
 
<!-- What is the benefit to the platform?  If this is a major capability update, what has changed?  If this is a new functionality, what capabilities does it bring? Why will Fedora become a better distribution or project because of this proposal?-->
Python 2.7 (latest Python 2 release, which we also have in Fedora) is currently in maintenance mode only, which means upstream only accepts bugfixes and security fixes, but no new features are being implemented. According to upstream, Python 2.7 is the last release of Python 2 (http://www.python.org/dev/peps/pep-0404/). Support for Python 2 is guaranteed by upstream until May 2015 (http://www.python.org/dev/peps/pep-0373/#maintenance-releases), then it may continue for some time or it may not. Python 3, on the other hand, is actively developed and new features are being added every release. Moreover, there is currently no end of support date for Python 3.
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Python 2.7 (latest Python 2 release, which we also have in Fedora) is currently in maintenance mode only, which means upstream only accepts bugfixes and security fixes, but no new features are being implemented. According to upstream, Python 2.7 is the last release of Python 2 (http://www.python.org/dev/peps/pep-0404/). Support for Python 2 is guaranteed by upstream until May 2015 (http://www.python.org/dev/peps/pep-0373/#maintenance-releases), then it may continue for some time, or it may not. Python 3, on the other hand, is actively developed and new features are being added every release. Moreover, there is currently no end of support date for Python 3.
  
Fedora already has Python 3 stack, that is parallel to Python 2 stack. The are few benefits of switching the "primary" Python stack:
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Fedora already has Python 3 stack that is parallel to Python 2 stack. The are several benefits of switching the "primary" Python stack:
 
* Getting upstream support for default system version will not be limited by time.
 
* Getting upstream support for default system version will not be limited by time.
 
* Our system tools will be able to switch to Python 3, drop the burden of Python 2 support and use new features of Python 3.
 
* Our system tools will be able to switch to Python 3, drop the burden of Python 2 support and use new features of Python 3.
* As a distribution that stays close to upstream, Fedora should help Python community go forward by contributing patches and working closely with upstreams to get this accomplished. Thus this Change is meant to benefit not only Fedora, but also broad Python community.
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* As a distribution that stays close to upstream, Fedora should help Python community go forward by contributing patches and working closely with upstreams to get this accomplished. Thus this Change is meant to benefit not only Fedora, but also broader Python community.
 
* Switching to Python 3 as a default will once again push Fedora to stay as close to upstream as possible, highlighting the "Features" and "First" (although, to be honest, Arch Linux was first in this...)
 
* Switching to Python 3 as a default will once again push Fedora to stay as close to upstream as possible, highlighting the "Features" and "First" (although, to be honest, Arch Linux was first in this...)
  
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The main goal is switching to Python 3 as a default, in which state:
 
The main goal is switching to Python 3 as a default, in which state:
 
* DNF is the default package manager instead of Yum, which only works with Python 2
 
* DNF is the default package manager instead of Yum, which only works with Python 2
* Python 3 is the only Python in minimal buildroot
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* Python 3 is the only Python implementation in the minimal buildroot
* Python 3 is the only Python on LiveCD
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* Python 3 is the only Python implementation on the LiveCD
 
* Anaconda and all of its dependencies run on Python 3
 
* Anaconda and all of its dependencies run on Python 3
 
* cloud-init and all of its dependencies run on Python 3
 
* cloud-init and all of its dependencies run on Python 3
 
(see [[#Dependencies|Dependencies]] for the list of packages that need to be ported)
 
(see [[#Dependencies|Dependencies]] for the list of packages that need to be ported)
  
This will also require revisiting Python guidelines (broader discussion with community and FPC approval - TBD). The result of the discussion will reflect in this feature in further instructions for Fedora packagers.
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This will also require revisiting Python guidelines (broader discussion with community and FPC approval - TBD). The result of the discussion will reflect in this Change in further instructions for Fedora packagers.
  
 
There are basically two types of packages that need to undergo the conversion:
 
There are basically two types of packages that need to undergo the conversion:
 
* Python extension modules and libraries that provide Python bindings - assuming that there is upstream support, these can recieve python3- subpackage anytime without any damage to Fedora; we can then just utilize this subpackage when switching to Python 3 (instead of using current python- subpackage).
 
* Python extension modules and libraries that provide Python bindings - assuming that there is upstream support, these can recieve python3- subpackage anytime without any damage to Fedora; we can then just utilize this subpackage when switching to Python 3 (instead of using current python- subpackage).
* Packages that build with some sort of "embedded Python support", like gdb, or Rhythmbox with its plugins. In these cases, it makes no sense to do a python3- subpackage, since the whole package would need to be duplicated (e.g. python3-gdb), which doesn't make sense. These packages should be tested with Python 3 locally without any modifications to how they're currently built in Fedora. When we get a Koji side-tag, these packages will switch and build against Python 3 in the side tag.
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* Packages that build with some sort of "embedded Python support", like gdb, or Rhythmbox with its plugins. In these cases, it makes no sense to do a python3- subpackage, since the whole package would need to be duplicated (e.g. python3-gdb). These packages should be tested with Python 3 locally without any modifications to how they're currently built in Fedora. When we get a Koji side tag, these packages will switch and build against Python 3 in the side tag.
  
 
=== Work in Fedora 21 Timeframe ===
 
=== Work in Fedora 21 Timeframe ===
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* Proposal owners:
 
* Proposal owners:
 
** Continue the work from F21 timeframe
 
** Continue the work from F21 timeframe
** Request Koji side-tag and encourage packagers to rebuilt their packages with Python 3 there
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** Request Koji side tag and encourage packagers to rebuilt their packages with Python 3 there
** If the switch to Python 3 is achieved in the side-tag:
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** If the switch to Python 3 is achieved in the side tag:
 
*** Modify comps accordingly
 
*** Modify comps accordingly
 
*** Apply the changes to Python packaging guidelines
 
*** Apply the changes to Python packaging guidelines
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<!-- REQUIRED FOR SYSTEM WIDE CHANGES -->
 
<!-- REQUIRED FOR SYSTEM WIDE CHANGES -->
The Python 2 stack will stay in Fedora, it will just not be the default one. Depending on the modifications done to Python packaging guidelines, this probably means that Python 2 packages will stay the way they are and default tools will drag in python3- dependencies. Upstream recommends, that /usr/bin/python should point to Python 2 runtime for the time being, so if we go with that, there shouldn't be any serious compatibility impact:
+
The Python 2 stack will stay in Fedora, it will just not be the default one. Depending on the modifications done to Python packaging guidelines, this probably means that Python 2 packages will stay the way they are and default tools will drag in python3- dependencies. Upstream recommends that /usr/bin/python point to Python 2 runtime for the time being, so if we go with that, there shouldn't be any serious compatibility impact:
 
* Users will still be able to install Python 2 packages
 
* Users will still be able to install Python 2 packages
 
* Both Python 2 and 3 stacks will still live in parallel
 
* Both Python 2 and 3 stacks will still live in parallel
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== How To Test ==
 
== How To Test ==
<!-- This does not need to be a full-fledged document. Describe the dimensions of tests that this change implementation is expected to pass when it is done.  If it needs to be tested with different hardware or software configurations, indicate them.  The more specific you can be, the better the community testing can be.  
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<!-- This does not need to be a full-fledged document. Describe the dimensions of tests that this change implementation is expected to pass when it is done.  If it needs to be tested with different hardware or software configurations, indicate them.  The more specific you can be, the better the community testing can be.
  
 
Remember that you are writing this how to for interested testers to use to check out your change implementation - documenting what you do for testing is OK, but it's much better to document what *I* can do to test your change.
 
Remember that you are writing this how to for interested testers to use to check out your change implementation - documenting what you do for testing is OK, but it's much better to document what *I* can do to test your change.
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== Contingency Plan ==
 
== Contingency Plan ==
 
<!-- If you cannot complete your feature by the final development freeze, what is the backup plan?  This might be as simple as "Revert the shipped configuration".  Or it might not (e.g. rebuilding a number of dependent packages).  If you feature is not completed in time we want to assure others that other parts of Fedora will not be in jeopardy.  -->
 
<!-- If you cannot complete your feature by the final development freeze, what is the backup plan?  This might be as simple as "Revert the shipped configuration".  Or it might not (e.g. rebuilding a number of dependent packages).  If you feature is not completed in time we want to assure others that other parts of Fedora will not be in jeopardy.  -->
* Contingency mechanism: (What to do?  Who will do it?) Don't merge the koji side tag into F22, postpone the feature to F23, keep the side tag with all the work that has been done.
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* Contingency mechanism: Don't merge the koji side tag into F22, postpone the feature to F23, keep the side tag with all the work that has been done.
 
<!-- When is the last time the contingency mechanism can be put in place?  This will typically be the beta freeze. -->
 
<!-- When is the last time the contingency mechanism can be put in place?  This will typically be the beta freeze. -->
 
* Contingency deadline: Software string freeze
 
* Contingency deadline: Software string freeze
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== Release Notes ==
 
== Release Notes ==
 
<!-- The Fedora Release Notes inform end-users about what is new in the release.  Examples of past release notes are here: http://docs.fedoraproject.org/release-notes/ -->
 
<!-- The Fedora Release Notes inform end-users about what is new in the release.  Examples of past release notes are here: http://docs.fedoraproject.org/release-notes/ -->
<!-- The release notes also help users know how to deal with platform changes such as ABIs/APIs, configuration or data file formats, or upgrade concerns.  If there are any such changes involved in this change, indicate them here.  A link to upstream documentation will often satisfy this need.  This information forms the basis of the release notes edited by the documentation team and shipped with the release.  
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<!-- The release notes also help users know how to deal with platform changes such as ABIs/APIs, configuration or data file formats, or upgrade concerns.  If there are any such changes involved in this change, indicate them here.  A link to upstream documentation will often satisfy this need.  This information forms the basis of the release notes edited by the documentation team and shipped with the release.
  
Release Notes are not required for initial draft of the Change Proposal but has to be completed by the Change Freeze.  
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Release Notes are not required for initial draft of the Change Proposal but has to be completed by the Change Freeze.
 
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<!-- When your change proposal page is completed and ready for review and announcement -->
 
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<!-- remove Category:ChangePageIncomplete and change it to Category:ChangeReadyForWrangler -->
 
<!-- remove Category:ChangePageIncomplete and change it to Category:ChangeReadyForWrangler -->
<!-- The Wrangler announces the Change to the devel-announce list and changes the category to Category:ChangeAnnounced (no action required) -->  
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<!-- The Wrangler announces the Change to the devel-announce list and changes the category to Category:ChangeAnnounced (no action required) -->
 
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<!-- After review, the Wrangler will move your page to Category:ChangeReadyForFesco... if it still needs more work it will move back to Category:ChangePageIncomplete-->
  

Revision as of 07:14, 7 October 2013

Contents

Python 3 as the Default Implementation

Summary

Up until now, Fedora has used Python 2 as the default Pythn implementation. This change proposes switching to Python 3. The details of the term "switching" are explained thoroughly in the Scope section.

Owner

  • Name: Slavek Kabrda
  • Email: bkabrda@redhat.com
  • Release notes owner:

Current status

Detailed Description

Python 3 is the next generation of Python programming language. It is currently mature and stable, since it has been under active development for five years - version 3.0 was released in December 2008, current latest stable version is 3.3.2 released in May 2013. The main reason to switch to Python 3 as the default implementation is that Python 2 is in maintenance mode, thus only bugfixes and security fixes are accepted upstream. Further reasons are mentioned in the #Benefit to Fedora section. For this Change to be carried out successfully, it is necessary that the key packages in the Fedora software stack be ported to Python 3. These are parts of the minimal buildroot, the default package manager, programs present on the LiveCD etc. More information on the packages involved can be found in #Dependencies. While porting of some packages is rather trivial, other packages need significant amount of work to get rid of the Python 2 dependence.

Benefit to Fedora

Python 2.7 (latest Python 2 release, which we also have in Fedora) is currently in maintenance mode only, which means upstream only accepts bugfixes and security fixes, but no new features are being implemented. According to upstream, Python 2.7 is the last release of Python 2 (http://www.python.org/dev/peps/pep-0404/). Support for Python 2 is guaranteed by upstream until May 2015 (http://www.python.org/dev/peps/pep-0373/#maintenance-releases), then it may continue for some time, or it may not. Python 3, on the other hand, is actively developed and new features are being added every release. Moreover, there is currently no end of support date for Python 3.

Fedora already has Python 3 stack that is parallel to Python 2 stack. The are several benefits of switching the "primary" Python stack:

  • Getting upstream support for default system version will not be limited by time.
  • Our system tools will be able to switch to Python 3, drop the burden of Python 2 support and use new features of Python 3.
  • As a distribution that stays close to upstream, Fedora should help Python community go forward by contributing patches and working closely with upstreams to get this accomplished. Thus this Change is meant to benefit not only Fedora, but also broader Python community.
  • Switching to Python 3 as a default will once again push Fedora to stay as close to upstream as possible, highlighting the "Features" and "First" (although, to be honest, Arch Linux was first in this...)

Scope

The main goal is switching to Python 3 as a default, in which state:

  • DNF is the default package manager instead of Yum, which only works with Python 2
  • Python 3 is the only Python implementation in the minimal buildroot
  • Python 3 is the only Python implementation on the LiveCD
  • Anaconda and all of its dependencies run on Python 3
  • cloud-init and all of its dependencies run on Python 3

(see Dependencies for the list of packages that need to be ported)

This will also require revisiting Python guidelines (broader discussion with community and FPC approval - TBD). The result of the discussion will reflect in this Change in further instructions for Fedora packagers.

There are basically two types of packages that need to undergo the conversion:

  • Python extension modules and libraries that provide Python bindings - assuming that there is upstream support, these can recieve python3- subpackage anytime without any damage to Fedora; we can then just utilize this subpackage when switching to Python 3 (instead of using current python- subpackage).
  • Packages that build with some sort of "embedded Python support", like gdb, or Rhythmbox with its plugins. In these cases, it makes no sense to do a python3- subpackage, since the whole package would need to be duplicated (e.g. python3-gdb). These packages should be tested with Python 3 locally without any modifications to how they're currently built in Fedora. When we get a Koji side tag, these packages will switch and build against Python 3 in the side tag.

Work in Fedora 21 Timeframe

  • Proposal owners:
    • Discussing changes in Python packaging guidelines with Fedora community and FPC
    • Helping upstreams with porting to Python 3
    • Introducing python3- packages where appropriate, testing packages that only build with Python once (e.g. gdb, Rhythmbox)
  • Other developers:
    • Hopefully the same as proposal owners.
  • Release engineering:
    • Nothing in F21 timeframe
  • Policies and guidelines:
    • As mentioned above, this will require a discussion with community and FPC and preparation of changes to Python packaging guidelines. The changes related to the actual switch should however be merged in F22 timeframe and only if the switch is successful.

Work in Fedora 22 Timeframe

  • Proposal owners:
    • Continue the work from F21 timeframe
    • Request Koji side tag and encourage packagers to rebuilt their packages with Python 3 there
    • If the switch to Python 3 is achieved in the side tag:
      • Modify comps accordingly
      • Apply the changes to Python packaging guidelines
      • Ask relengs to merge the side tag into F22
    • Else:
      • Postpone the Change for another release
      • Do not merge side tag into F22
      • Do not apply changes to Python packaging guidelines
  • Other developers:
    • Introduce python3- subpackages where appropriate, build against Python 3 in the side tag where appropriate
  • Release engineering:
    • Create the Koji side tag
    • Merge the side tag back in F22 if the switch is achieved
  • Policies and guidelines:
    • Apply the changes to Python guidelines if the switch is achieved

Upgrade/compatibility impact

The Python 2 stack will stay in Fedora, it will just not be the default one. Depending on the modifications done to Python packaging guidelines, this probably means that Python 2 packages will stay the way they are and default tools will drag in python3- dependencies. Upstream recommends that /usr/bin/python point to Python 2 runtime for the time being, so if we go with that, there shouldn't be any serious compatibility impact:

  • Users will still be able to install Python 2 packages
  • Both Python 2 and 3 stacks will still live in parallel

Libraries that are built with Python only once (like gdb) may force users to rewrite their custom scripts and plugins (see e.g. https://bugzilla.redhat.com/show_bug.cgi?id=1014549#c3).

How To Test

  • No special hardware is needed.
  • Using mock for testing is strongly recommended. During rebuilding in the Koji side tag, things may get unstable and you'd be risking breaking your system.
  • Install the packages built against Python 3 from the Koji side tag and test if they still work as expected.
  • When the switch is prepared in the side tag, just try installing the packages from there and removing Python 2 from your system. Again, look for any problems that may be related to the switch.

User Experience

Users shouldn't notice any changes, except that packages in minimal buildroot and on LiveCD will be python3-, not python-. Anaconda deps will be python3- as well. /usr/bin/python will still point to Python 2 and depending on result of discussions with FPC, "yum install python-foo" will still install Python 2 version of the package.

Dependencies

See https://fedoraproject.org/wiki/User:Churchyard/python3 (our tracking page with notes) or https://bugzilla.redhat.com/show_bug.cgi?id=1014209 (tracking bug).

Contingency Plan

  • Contingency mechanism: Don't merge the koji side tag into F22, postpone the feature to F23, keep the side tag with all the work that has been done.
  • Contingency deadline: Software string freeze
  • Blocks release? No

Documentation

http://docs.python.org/dev/howto/pyporting.html

http://docs.python.org/3/howto/cporting.html

https://wiki.gnome.org/PyGObject/IntrospectionPorting

Release Notes