[PonyORM-list] Pony ORM 0.6 Release Candidate 1

[Alexey Malashkevich]

Hi all,

One of the most often requests that we are getting from Pony users is
adding support for Python 3. And today we are happy to announce that we’ve
just finished development and have prepared Pony 0.6 Release Candidate with
Python 3 support!

In order to install this release candidate version you need to execute the
following command:

    pip install pony==0.6rc1

If you don’t specify the version explicitly, pip will install the latest
stable release 0.5.4

The same Pony code runs both on Python 2 (≥ 2.6) and Python 3 (≥ 3.3). If
you clone Pony git repository from GitHub you can use it on Python 2 and
Python 3 without any modifications.

Now let’s see what changes we had to make along the way.


Unicode strings
----------------------

As you know, Python 3 has some differences from Python 2 when it comes to
strings. Python 2 provides two string types – str (byte string) and unicode
(unicode string), whereas in Python 3 the str type represents unicode
strings and the unicode type has gone.

When we were working on adding Python 3 support to Pony, one of our goals
was to have the same Pony entity declarations working on both Python 2 and
3 in the same manner. In order to achieve this we had to make one
non-backward compatible change – treat both str and unicode types as they
are unicode strings in both Python 2 and 3. Pony just adds unicode as an
alias to str in Python 3. When you importing * from pony.orm you will get
this alias along with other stuff.

    attr1 = Required(str)
    #   is now the same as
    attr2 = Required(unicode)

    attr3 = Required(LongStr)
    #   is now the same as
    attr4 = Required(LongUnicode)

Before this release, Pony stored values of str and unicode attributes as
unicode in the database, but for str attributes it had to convert unicode
to byte string on reading from the database. Starting with the Pony Release
0.6 the attributes of str type in Python 2 behave as if they were declared
as unicode attributes. There is no difference now if you specify str or
unicode as the attribute type – you will have unicode string in Python and
in the database.

The same thing is with the LongUnicode and LongStr. LongStr now is an alias
to LongUnicode. This type uses unicode in Python and in the database.


Byte sequences
----------------------

If you need to represent a byte sequence in Python 2, you can use the
buffer type. In Python 3 the buffer type has gone, and Pony uses the bytes
type which was added in Python 3 to represent binary data. But for the sake
of backward compatibility we still keep buffer as an alias to the bytes
type in Python 3. If you’re importing * from pony.orm you will get this
alias too.

If you want to write code which can run both on Python 2 and Python 3, you
should use the buffer type for binary attributes. If your code is for
Python 3 only, you can use bytes instead:

    attr1 = Required(buffer) # Python 2 and 3

    attr2 = Required(bytes) # Python 3 only

It would be cool if we could use the bytes type as an alias to buffer in
Python 2, but unfortunately it is impossible, because Python 2.6 adds bytes
as a synonym for the str type (Btw, there is a good read regarding this
decision)


Porting from previous Pony releases to 0.6
----------------------------------------------------------

In most cases, you don’t need to do anything in order to port your code to
Pony ORM 0.6. Pony entity declarations and queries should work correctly by
default both in Python 2 and Python 3.

If you declare string attribute as unicode, it will work correctly in
Python 2 and Python 3, because Pony adds unicode as an alias to str in
Python 3.

If you declare string attribute as str and keep only ASCII data there, it
will not require any change and will work both on Python 2 and Python 3.
But now that attribute will accept and return unicode values. In Python 2,
if you assign an ASCII byte string to such attribute, the value will be
automatically converted to unicode.

If you use str attribute type for storing byte strings, you should use
buffer or bytes attribute type instead. The bytes type can be used in
Python 3 only, it cannot be used correctly in Python 2. The buffer type can
be used both in Python 2 and Python 3 with the same meaning. In Python 3
Pony just adds buffer as an alias to bytes.


Declaring Pony entities which work in both Python 2 and 3
-------------------------------------------------------------------------------

It does not matter if you use type str or unicode in you string attributes
– both will do the same, because Pony adds unicode as an alias to str in
Python 3. But for aesthetical reasons we recommend to keep consistency and
assign the same type for all of you string attributes. Starting with this
release we prefer to use the str as a string type, because it looks more
naturally in Python 3.

For BLOB attributes the preferred type is buffer. In Python 3 Pony adds
buffer as an alias to bytes. Don’t use bytes if you want to run your code
both in Python 2 and Python 3, because in Python 2 bytes is an alias to
str, and has different meaning.


pymysql adapter for MySQL database added
-------------------------------------------------------------

Now Pony can use pymysql adapter in Python 2 and 3. By default, in Python
2, Pony uses MySQLdb for accessing MySQL databases. Now it falls back to
pymysql if MySQLdb is not available.

Since MySQLdb doesn’t work in Python 3, here you can use only pymysql
adapter.


Other changes
----------------------

In this release we’ve fixed the following bugs:

Fixes #74: Wrong FK column type when using sql_type on foreign ID column
Fixes #75: MappingError for self-referenced entities in a many-to-many
relationship
Fixes #80: “Entity NoneType does not belong to database” when using to_dict

As always, we appreciate your feedback. Try to use Pony in Python 3 and
share your experience with us in our email list or by email at team [at]
ponyorm.org

Best,
Alexey
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