43.1. Python 2 vs. Python 3

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:

This scheme is analogous to the recommendations in PEP 394 regarding the naming and transitioning of the python command.

It depends on the build configuration or the installed packages whether PL/Python for Python 2 or Python 3 or both are available.

Tip: The built variant depends on which Python version was found during the installation or which version was explicitly set using the PYTHON environment variable; see Section 15.4. To make both variants of PL/Python available in one installation, the source tree has to be configured and built twice.

This results in the following usage and migration strategy:

See also the document What's New In Python 3.0 for more information about porting to Python 3.

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.