older pythons

Lie Ryan lie.1296 at gmail.com
Thu Jul 9 13:15:45 CEST 2009

superpollo wrote:
> Adrian Dziubek wrote:
>> The recommended Debian way is update-alternatives. I find it a bit
>> unintuitive, so I have to read through the documentation every time I
>> use it, but it should be able link a chosen version of python to /usr/
>> bin/python. I don't know if it's set up by default, I have only one
>> version installed.
>> -- 
>> Adrian
> what i was asking for is about a way to *INSTALL* and mantain different
> python versions, a task i think is not unusal for developers.
> thanks anyway for reply
> bye

Installing several minor versions (e.g. 2.3, 2.4, 2.5, 3.0) in parallel
has always been supported. Just install them and it should work

However changing the system's default /usr/bin/python is NOT
recommended, especially in linux systems that rely heavily on python for
many of the system tools. If you install from source, you need to check
for some settings to ensure the installer does not install the
/usr/bin/python's symlink. If you install from a package manager, they
usually doesn't change the symlink and you'll need to run a separate
program to change the symlinks (e.g. python-updater in Gentoo, which
will also updates all installed python packages to the appropriate
version. Neat.).

The easiest way to make a specific script use a specific version of
python is by changing its hashbang (#!) line:

instead of:
#!/usr/bin/env python

#!/usr/bin/env python2.3

or whatever version it should run on.

AFAIK, no major linux distributions have officially ported to python
3.x. This means switching /usr/bin/python to python3.1 will definitely
break your system. As an alternative, you can always explicitly specify
the version number of python you want to use:

$ python2.5
Python 2.5.4 (r254:67916, Jul  5 2009, 04:12:16)
[GCC 4.3.2] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> exit()
$ python2.6
Python 2.6.2 (r262:71600, Jul  5 2009, 04:08:11)
[GCC 4.3.2] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> exit()
$ python3.0
Python 3.0.1 (r301:69556, Jul  5 2009, 04:03:20)
[GCC 4.3.2] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> exit()
$ python3.0 need3.py
happily running with python 3.0
$ python2 need3.py
SillyError: not running with python 3.0

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