===============
zodb.py3migrate
===============

Reason for this package
=======================

It is not possible to use a ``ZODB`` database with both Python 2 and Python 3.
There are the following reasons for this behavior:

#. Given you have a database created using Python 2. When reading this database
   using Python 3 all string
   objects will be read as Python 3 string objects (aka ``unicode``) requiring
   them to be ASCII encoded. If actual binary data is stored in such a ``str`` object its class
   has to inherit ``zodbpickle.binary``, so it will be unpickled as ``binary``.
#. Because of reason #1 ``ZODB`` 4.x used different magic bytes (the
   first bytes of the database file) for the different Python versions and does
   not allow to open a database with a Python version which does not match
   the magic bytes.

Upgrade workflow
================

#. Install ``zodb.py3migrate`` into your project, so it has access to your
   persistent classes when reading objects from the ZODB.

#. Use the entry point ``bin/zodb-py3migrate path/to/Data.fs`` to run the
   script.

   * The script shows a list of attributes and keys grouped by their class that
     need to be converted because they are binary strings with an encoding different from ASCII. You have to decide for each attribute if it really
     is a binary string (e. g. an image), or if it should be text, i. e.
     converted to ``unicode``.

     * Attributes that should remain binary strings must become an instance of
       ``zodbpickle.binary`` so ZODB knows that this string are to be treated
       as binary data.

     * Since ZODB will read all non-marked binary strings as unicode, all text
       fields to ``unicode`` and adjust the class accordingly.

   * Your ZODB is Python 3 ready if the script does not output any attribute,
     since all binary strings were either converted to ``unicode`` or
     marked as binary.

   * .. note:: The displayed total number of objects in the ``ZODB`` is only an
               approximation as returned by the ``FileStorage`` API.

#. Convert binary attributes in your code base to Python 3.

   * Mark actual binary attributes with ``zodbpickle.binary``. This way they
     will become compatible with Python 2 and Python 3,  so you can continue
     using Python 2 without writing incompatible data.

   * If you only need Python 3 compatibility it is sufficient to use ``bytes``.

#. Convert the ``PythonScript`` objects inside the ZODB to be Python 3
   compatible code. (Find all of them using the ``Find`` tab in the ZMI.)

#. Convert existing data in the ZODB for Python 3 compatibility by calling
   ``bin/zodb-py3migrate path/to/Data.fs --config=conversion.ini``.

   * .. warning:: This call changes the database file in place, so only call
                  it on a copy of your live ZODB.

   * Example for conversion config file, i. e. ``conversion.ini`` in example
     call above.

     .. code-block:: pacmanconf

         [zodbpickle.binary]
         foo.bar.Baz.image

         [utf-8]
         foo.bar.Baz.text
         foo.bar.Baz.title
         BTrees.OOBTree.OOBTree['7b6d22fa-594e']
         persistent.mapping.PersistentMapping['number-1']

         [latin-1]
         foo.bar.Baz.legacy
         persistent.list.PersistentList[1]

   * All entries of the section ``[zodbpickle.binary]`` will be wrapped into
     ``zodbpickle.binary``, whereas all entries of other sections will be
     decoded to unicode using the section name as encoding.

   * You might use the result of the analyse call described in step #2 to build
     the config file.

   * To convert a value inside a ``BTree`` or in a ``PersistentMapping``, the
     ``repr()`` of the key must be given in square brackets. (See example
     above.)

   * To convert a value inside a ``PersistentList``, the
     index of the value must be given in square brackets. (See example above.)

   * .. note::
               * The conversion only changes values, but not keys, since this
                 would break the application code.

               * Converting values in a ``BTree.*.*TreeSet`` is not (yet)
                 supported.

               * Converting values nested in non-persistent objects is not
                 supported.

   * After converting binary strings to ``zodbpickle.binary``, your
     application needs the ``zodbpickle`` package as install dependency.
     Otherwise the converted objects will be broken.

#. Call ``bin/zodb-py3migrate-magic path/to/Data.fs Python3`` to convert the
   magic bytes of the database file to claim Python 3 compatibility. (Use
   ``Python2`` as second argument to revert back to Python 2.)

   * .. warning:: This call changes the database file in place, so only call
                  it on a copy of your live ZODB.

Example calls
=============

Analyze the ZODB:

.. code-block:: none

    $ bin/zodb-py3migrate path/to/Data.fs
    Found 3 binary fields: (number of occurrences)
      foo.bar.Baz.image is string (12)
      foo.bar.Baz.legacy is iterable (1)
      foo.bar.Baz.text is string (23)

Convert the ZODB:

.. code-block:: none

    $ bin/zodb-py3migrate path/to/Data.fs --config=conversion.ini
    Converted 2 binary fields: (number of occurrences)
      foo.bar.Baz.image (12)
      foo.bar.Baz.text (23)
