Summary of the Python 3 switch

SageMath has been running on top of Python 2 since the beginning, until version 8.9.

Starting from version 9.0 (to be released later in 2019), SageMath will be running on top of Python 3.

The change from Python2 to Python3 has already been made by all major scientific software based on Python, including NumPy, SciPy and Jupyter. It becomes more and more common for scientific Python software to work only under Python3. Moreover, the official end of life for Python 2 is the end of 2019.

For all these reasons, it is necessary for Sagemath to switch to Python 3. After a lot of work and a lot of time, this is now possible and will be done in the 9.0 version.

This will not be without pain for the users of sage. Python 3 has some major backward incompatibilities with Python2, and adapting the code takes some care and some work.

Some basic instructions for doing that can be found in the page Python3-compatible code.

Some more specific details follow.

Old notebooks

The "legacy Sage notebook", which was used by Sage for a long time, has been replaced by the Jupyter notebook XXX years ago. Because the legacy notebook is no longer maintained, it is not compatible with Python 3 and will not be available in versions 9.0 and higher of SageMath.

All people that still have old-style notebooks are urged to migrate them to Jupyter notebooks now.

Compiling with Python 2

* After version 9.0, you can still build and use SageMath with Python2, as follows.

make configure
./configure --with-python=2
make build

Beware that you will need to call the second line again if you ever call "make distclean".

This will work until version 9.1 at least. Then the backward compatibility with Python 2 will no longer be ensured.