What changed in version 3.0

Python 3 support

This version of Reahl runs on Python 3 (>= 3.3) as well on Python 2.7. In order to achieve this, some major changes were necessary.

Updated dependencies

Many of the versions of other packages Reahl 2.1 depends on do not support Python 3. Thus, almost all dependencies were upgraded to versions compatible with Python 3. Most of these should go unnoticed to users of Reahl, barring the exception discussed next.

The minor upgrades are:

Name Old version New version
Babel 0.9 1.3
python-dateutil 1.5 2.2
docutils 0.8 0.12
psycopg2 2.4 2.5
alembic 0.5 0.6
lxml 3.2 3.3
WebTest 1.4 2.0
selenium 2.25 2.42
pillow 1.7.8 2.5
cssmin 0.1 0.2
BeautifulSoup 3.2 BeautifulSoup4 4.3
webob 3.2 4.3

SqlAlchemy and Elixir/Declarative

SqlAlchemy was upgraded from version 0.7 to 0.9. Unfortunately Elixir does not support these newer versions of SqlAlchemy, prompting us to implement the framework, domain code and all examples using SqlAlchemy’s declarative extension instead.

Elixir has not been maintained for a while now, while Declarative is actively maintained as part of SqlAlchemy. Declarative follows a more conservative approach though, which means that you have to specify much more SQL-related info in your Python code than is needed when using Elixir. While it would be great to be able to circumvent the verbosity of Declarative, specifying things explicitly involves quite a bit less magic “under the hood” and getting rid of that magic has quite a few positive effects. The better support one gets on mailing lists of SqlAlchemy for Declarative (as opposed to Elixir being largely defunct) is also a major plus.

This is a major change which is necessary to make Python 3 support possible. The switch from Elixir to Declarative impacts model code built on previous versions of Reahl, using Elixir.

We provide two upgrade paths for users with code based on Elixir:

Use Reahl 3.0, but stay on Elixir:

Reahl 3.0 can be used in conjunction with a few components of Reahl 2.1: the Elixir implementation itself, and its small number of dependencies. Following this path means there are no code changes on your part, but you have to stay on Python 2.7 yourself.

In order to follow this route, specify the elixir keyword to pip when upgrading or installing Reahl, eg:

pip install --upgrade "reahl[web,elixir,postgresql]"


On Windows, first use pip uninstall to uninstall any reahl package that is currently installed, then do:

easy_install "reahl[web,elixir,postgresql]"

For your own eggs, ensure that their .reahlproject files are updated to explicitly mention versions of the 2.1 packages used:

<egg name="reahl-sqlalchemysupport" version="2.1"/>
<egg name="reahl-web-elixirimpl" version="2.1"/>
<egg name="reahl-domain" version="2.1"/>
<egg name="reahl-domainui" version="2.1"/>

Switch to using Declarative:

If you do not have any of your own code written using Elixir, following this route is simple. Just specify the declarative keyword to easy_install when upgrading or installing Reahl (you also have to manually uninstall the old reahl-web-elixirimpl):

pip install --upgrade reahl[web,declarative,postgresql]
pip uninstall reahl-web-elixirimpl


On Windows, first use pip uninstall to uninstall any reahl package that is currently installed, then do:

easy_install "reahl[web,declarative,postgresql]"

If you do have code of your own written in terms of Elixir, you will have to change that code to use Declarative instead. In order to understand what you need to do, there is no better guide than Declarative’s own documentation.

If you have a database running in production that was made with your own code written in terms of Elixir, you will also have to provide database migrations to change the underlying schema from what it was using Elixir, to what it is using Declarative.


In the instructions above, you will note that doc is never specified in the keywords to pip install. That would install the Reahl documentation and examples – typically not present on a production system. The documentation (which includes examples) cannot be installed together with Elixir.

You may want to uninstall reahl-doc first if you have it installed, but want to stay on Elixir.

Elixir to Declarative database migration guide

If you have a database running in production, and you want to upgrade that to a newer version of Reahl, you should always migrate that database schema (and possibly data) to be compatible with the newer versions of components you will be using going forward. With the change from Elixir to Declarative, Reahl includes quite a large number of such database migrations.


The Sqlite database itself does not support migration of existing data very well, and as a result migration is only possible on PostgreSQL databases. See these posts for more information on the issue: one of the last bullets of goals of alembic and Christopher Allan’s post about the issue.

If you decide to make the switch from Elixir to Declarative and you did not have any of your own code written in terms of Elixir, you can simply migrate such an existing database with the command:

reahl-control migratedb /path/to/config/directory

(In fact, you should always run this command when upgrading Reahl for a project with an existing database.)

If you do have code of your own written in terms of Elixir, and you decide to make the switch with us, you will have to write code to make such database migration possible for your own code.

Please see the Elixir to Declarative database schema migration guide for details on how to do this, or feel free to ask for help on the mailing list.

Other changes

Moved modules

As a rule, a component named reahl-xxx would contain a package reahl.xxx, with possibly sub modules, such as reahl.xxx.yyy. For a small number of components, this is not true. Specifically, reahl-domain includes reahl.partymodel, reahl.workflowmodel and reahl.systemaccountmodel that do not fit this structure.

In this version, these have been moved to reahl.domain.partymodel, reahl.domain.workflowmodel and reahl.domain.systemaccountmodel respectively. Older imports will continue to work for now, but will eventually be removed.

Session scoped classes

Classes that are @session_scoped used to have an attribute ‘session’. This has been renamed to ‘user_session’.

The relationship between a Party and a SystemAccount

Previously, a Party always had a SystemAccount. It makes more sense to be able to have Party objects with or without SystemAccounts. Hence, the relationship was changed. Now, a Party does not have any knowledge of a SystemAccount, but a SystemAccount has an ‘owner’, which is a Party.

Behind-the-scenes changes to UploadedFile

reahl.component.modelinterface.UploadedFile instances are constructed by the framework, not user code. Previously it was constructed with a file-like object from which it effectively could read the contents of the file from the client-connected socket. It is now constructed with the entire contents of the file. The file contents are always handled as bytes with no attempt to deduce character encoding.

A naming convention for database objects

The new SqlAlchemy provides a mechanism for dictating the naming convention used to name database objects (constraints, etc). Using this means the same conventions will be used across different database backends – something that will make migrations easier in future. Reahl 3.0 now uses these conventions. The reahl.sqlalchemysupport.sqlalchemysupport module includes a few functions that compute the names for such database objects according to the new naming convention. This is helpful during database migrations.