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>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
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><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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