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><A
NAME="PLPYTHON-PYTHON23"
>42.1. Python 2 vs. Python 3</A
></H1
><P
> PL/Python supports both the Python 2 and Python 3 language
variants. (The PostgreSQL installation instructions might contain
more precise information about the exact supported minor versions
of Python.) Because the Python 2 and Python 3 language variants
are incompatible in some important aspects, the following naming
and transitioning scheme is used by PL/Python to avoid mixing them:
<P
></P
></P><UL
><LI
><P
> The PostgreSQL language named <TT
CLASS="LITERAL"
>plpython2u</TT
>
implements PL/Python based on the Python 2 language variant.
</P
></LI
><LI
><P
> The PostgreSQL language named <TT
CLASS="LITERAL"
>plpython3u</TT
>
implements PL/Python based on the Python 3 language variant.
</P
></LI
><LI
><P
> The language named <TT
CLASS="LITERAL"
>plpythonu</TT
> implements
PL/Python based on the default Python language variant, which is
currently Python 2. (This default is independent of what any
local Python installations might consider to be
their <SPAN
CLASS="QUOTE"
>"default"</SPAN
>, for example,
what <TT
CLASS="FILENAME"
>/usr/bin/python</TT
> might be.) The
default will probably be changed to Python 3 in a distant future
release of PostgreSQL, depending on the progress of the
migration to Python 3 in the Python community.
</P
></LI
></UL
><P>
This scheme is analogous to the recommendations in <A
HREF="http://www.python.org/dev/peps/pep-0394/"
TARGET="_top"
>PEP 394</A
> regarding the
naming and transitioning of the <TT
CLASS="COMMAND"
>python</TT
> command.
</P
><P
> It depends on the build configuration or the installed packages
whether PL/Python for Python 2 or Python 3 or both are available.
</P
><DIV
CLASS="TIP"
><BLOCKQUOTE
CLASS="TIP"
><P
><B
>Tip: </B
> The built variant depends on which Python version was found during
the installation or which version was explicitly set using
the <TT
CLASS="ENVAR"
>PYTHON</TT
> environment variable;
see <A
HREF="install-procedure.html"
>Section 15.4</A
>. To make both variants of
PL/Python available in one installation, the source tree has to be
configured and built twice.
</P
></BLOCKQUOTE
></DIV
><P
> This results in the following usage and migration strategy:
<P
></P
></P><UL
><LI
><P
> Existing users and users who are currently not interested in
Python 3 use the language name <TT
CLASS="LITERAL"
>plpythonu</TT
> and
don't have to change anything for the foreseeable future. It is
recommended to gradually <SPAN
CLASS="QUOTE"
>"future-proof"</SPAN
> the code
via migration to Python 2.6/2.7 to simplify the eventual
migration to Python 3.
</P
><P
> In practice, many PL/Python functions will migrate to Python 3
with few or no changes.
</P
></LI
><LI
><P
> Users who know that they have heavily Python 2 dependent code
and don't plan to ever change it can make use of
the <TT
CLASS="LITERAL"
>plpython2u</TT
> language name. This will
continue to work into the very distant future, until Python 2
support might be completely dropped by PostgreSQL.
</P
></LI
><LI
><P
> Users who want to dive into Python 3 can use
the <TT
CLASS="LITERAL"
>plpython3u</TT
> language name, which will keep
working forever by today's standards. In the distant future,
when Python 3 might become the default, they might like to
remove the <SPAN
CLASS="QUOTE"
>"3"</SPAN
> for aesthetic reasons.
</P
></LI
><LI
><P
> Daredevils, who want to build a Python-3-only operating system
environment, can change the contents of
<A
HREF="catalog-pg-pltemplate.html"
><TT
CLASS="STRUCTNAME"
>pg_pltemplate</TT
></A
>
to make <TT
CLASS="LITERAL"
>plpythonu</TT
> be equivalent
to <TT
CLASS="LITERAL"
>plpython3u</TT
>, keeping in mind that this
would make their installation incompatible with most of the rest
of the world.
</P
></LI
></UL
><P>
</P
><P
> See also the
document <A
HREF="http://docs.python.org/py3k/whatsnew/3.0.html"
TARGET="_top"
>What's
New In Python 3.0</A
> for more information about porting to
Python 3.
</P
><P
> It is not allowed to use PL/Python based on Python 2 and PL/Python
based on Python 3 in the same session, because the symbols in the
dynamic modules would clash, which could result in crashes of the
PostgreSQL server process. There is a check that prevents mixing
Python major versions in a session, which will abort the session if
a mismatch is detected. It is possible, however, to use both
PL/Python variants in the same database, from separate sessions.
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