Develop Reference

ipython version mismatches python version in docker

I'm building a docker with the following Dockerfile:
FROM ubuntu:18.04
RUN \
apt-get update -y && \
apt-get install python3.8 -y && \ <----- I ask for python3.8
apt-get install python3-pip -y && \
pip3 install ipython
When I ran the image I was surprised to see that the version of ipython is 3.6.9:
$ docker build --tag versions --file .\Dockerfile.txt .
$ docker run -d -t --name ver versions
ba9bd772bc6d247a6c83f2bf932a6c5172c23f00e1e6a35f14878608d0f35f89
$ docker exec -it ver bash
# ipython
Python 3.6.9 (default, Jan 26 2021, 15:33:00)

The package python3-pip depends on python3, and the default python3 for ubuntu 18.04 is version 3.6.
There are at least three options.
Use a python base image
The official python base images include pip. If possible, I would use one of these.
python:3.8 - includes compilers to install compiled packages
python:3.8-slim - does not include compilers
Install pip with the get-pip.py script
One can install pip without the system package manager, for example with the script get-pip.py:
wget https://bootstrap.pypa.io/get-pip.py
python3.8 get-pip.py
Use ubuntu 20.04
As #NickODell comments, ubuntu 20.04 uses 3.8 as the default python. But you will have less fine-grained control over the python version than if you use one of the official python docker images.

Can CPython be compiled with Clang?

I'm trying to build CPython using Clang, with very specific requirements:
Python 2.7.14
CentOS 6.9 (but settling for Ubuntu 16.04), x64
LLVM 5.0.0
I tried setting env variable CC to my clang executable location (i.e. /opt/llvm/5/bin/clang, but the ./configure command fails with the following:
configure: error: C compiler cannot create executables
What flags should I set to make this build work?

As the commenters on OP's post pointed out, you can compile cpython with clang. Here are reproducible instructions, in the form of a Dockerfile.
FROM ubuntu:16.04
WORKDIR /opt/cpython-2.7.14
RUN apt-get update -qq \
&& apt-get install --yes build-essential curl \
# Install clang
&& curl -fsSL https://releases.llvm.org/5.0.0/clang+llvm-5.0.0-linux-x86_64-ubuntu16.04.tar.xz \
| tar xJ -C /usr/local --strip-components 1 \
&& curl -fsSL https://www.python.org/ftp/python/2.7.14/Python-2.7.14.tgz \
| tar xz --strip-components 1 \
&& CC=/usr/local/bin/clang ./configure --prefix /usr/local/cpython-2.7.14 \
&& make \
&& make install
ENTRYPOINT ["/usr/local/cpython-2.7.14/bin/python"]
docker build --tag cpython:2.7 .
docker run --rm cpython:2.7 --version
# Python 2.7.14
It is difficult to say what OP's original issue was, because it seemed like a problem with the clang installation. Looking at the configure logs would provide more information.
Disclaimer: Python 2 has reached its end of life, and ubuntu 16.04 reaches its end of life in April 2021.

tried setting env variable CC to my clang executable location
On Linux, define an alias in Bash as follows:
alias cc="clang"
On my system I set it as alias cc="clang-11"
Also, be sure to have installed all clang and llvm packages on the system.

My python installation is broken/corrupted. How do I fix it?

I followed these instructions on my RedHat Linux version 7 server (which originally just had Python 2.6.x installed):
beginning of instructions
install build tools
sudo yum install make automake gcc gcc-c++ kernel-devel git-core -y
install python 2.7 and change default python symlink
sudo yum install python27-devel -y
sudo rm /usr/bin/python
sudo ln -s /usr/bin/python2.7 /usr/bin/python
yum still needs 2.6, so write it in and backup script
sudo cp /usr/bin/yum /usr/bin/_yum_before_27
sudo sed -i s/python/python2.6/g /usr/bin/yum
sudo sed -i s/python2.6/python2.6/g /usr/bin/yum
should display now 2.7.5 or later:
python -V
end of instructions
The above commands and comments were taken from:
http://www.lecloud.net/post/61401763496/install-update-to-python-2-7-and-latest-pip-on
The python -v command returned this:
-bash: python: command not found
Now it is as if I have no Python installed. I don't want yum to break. I tried installing Python 3.4.
whereis python shows this:
python: /usr/bin/python2.6 /usr/bin/python2.6-config /usr/bin/python /usr/lib/python2.6 /usr/lib64/python2.6 /usr/local/bin/python2.7 /usr/local/bin/python3.4m-config /usr/local/bin/python2.7-config /usr/local/bin/python3.4 /usr/local/bin/python3.4m /usr/local/lib/python2.7 /usr/local/lib/python3.4 /usr/include/python2.6 /usr/share/man/man1/python.1.gz
What should I do now? I want a working installation of Python. For certain things I'm doing, I need it to be 2.7 or higher. I want yum to still work.

Do
sudo update-alternatives --remove-all python
sudo ln -sf /usr/bin/python2.7 /usr/bin/python

I got the same issue while upgrading ubuntu 18 to 19, this made it:
sudo rm /usr/bin/python
sudo ln -s /usr/bin/python2.7 /usr/bin/python
do-release-upgrade
From:
https://bugs.launchpad.net/ubuntu/+source/ubuntu-release-upgrader/+bug/1825655

For me, nothing worked except this one:
unlink /usr/bin/python3
ln -s /usr/bin/python3.7 /usr/bin/python3
Credit: https://josephgeis.dev/2020/04/upgrading-to-ubuntu-20-04-python3/

This is easily fixed by installing the python27 package via yum. It should install in /usr/bin, and may overwrite the /usr/bin/python symlink that should be pointing to 2.6. If it did (just run ls -l python* in /usr/bin to see), remove the symlink and point it back to 2.6. Next create a symlink for /usr/local/bin/python pointing at /usr/bin/python2.7. Finally, modify your ~/.bashrc or ~/.bash_profile (whichever you use) to have /usr/local/bin before /usr/bin in your PATH:
export PATH=/usr/local/bin:$PATH
at the very end of the file. This way, /usr/bin/python remains linked to Python 2.6, which is what the system expects, and when you run python at the command line it'll start up 2.7. You shouldn't have to make any changes to the yum script either - just blanket replacing python with python2.6 without understanding what you're doing is not a very good idea.
I'd also recommend installing Python 3.4 in /usr/local/bin if possible, where the binary will be named python3 by convention. Even if it installs in /usr/bin, you'll still have the choice of running python3 or python3.4 to specify which version you want. I work on a CentOS system that has each version of Python from 2.4 up to 3.4 installed, all in /usr/local/bin (I'm sure this was done manually, and not via yum), while the only python* in /usr/bin is 2.6. I couldn't find a python3 package for RedHat (I may not have been looking hard enough), so I'd recommend building the latest version from source (3.4.3 as of this writing). Unzip the tarball in a suitable directory, check out the README file, then, in the Python-3.4.3 directory, run ./configure --help to see what the options are, and if you need to change anything. As long as you have gcc installed, and don't need to link to any weird math libraries or anything, you should just be able to run:
./configure
make
make test
sudo make install
and it'll install to /usr/local/bin. Check the messages at the end of the make step, as it'll list any modules it wasn't able to build there. Fails usually happen because you don't have a required library installed, so look in setup.py in the base directory in the detect_modules() function (starting on line 449, and stretching all the way down to line 1564). Install both the lib and the -devel packages so you get the necessary headers.
This same process can also be followed if you want to install the latest 2.7.9, instead of RH's 2.7.5. One of the major (in my eyes) advantages of 2.7.9 is that pip is installed by default, making third-party module installation that much easier.
Good luck!

How do you setup python3.2 on ubuntu 14.04 for testing with tox?

Trying to use tox to run tests before pushing, but I keep running into errors like:
ERROR: py26: InterpreterNotFound: python2.6
ERROR: py32: InterpreterNotFound: python3.2
ERROR: py34: InterpreterNotFound: python3.3
apt-cache search isn't offering any packages that look like they will help. How do you load all these versions of the interpreter for ubuntu14.04?

Obviously, Ubuntu doesn't ship all historic versions of Python. But you can use deadsnakes PPA which has everything from 2.3 to 3.4.
For one project I used drone.io CI service with, I had the following tox section I ran before actual test envs.
[testenv:setupdrone]
whitelist_externals = /bin/bash
commands =
bash -c "echo 'debconf debconf/frontend select noninteractive' | sudo debconf-set-selections"
bash -c "sudo add-apt-repository ppa:fkrull/deadsnakes &> /dev/null"
bash -c "sudo apt-get update &> /dev/null"
bash -c "sudo apt-get -y install python2.6 python3.4 &> /dev/null"

Installing second python on Debian

So I have Debian machine for my Django production server.
I need to install second python (2.7.1) to use with virtualenv.
But it always write I don't have some modules, then I have to search manually, apt-install them and rebuild. Is there either a way to resolve the dependencies for building, or pre-compiled .deb with python 2.7.1 for Debian Squeeze?
Sorry if this is much of a noobie question, I googled, honestly.

Get the Python 2.7.1 sources and compile it manually:
configure --prefix=/path/to/python-2.7
make; make install

Python 2.7 is available for wheezy (testing), so you should be able to install it by adding the testing repository and doing some APT pinning.
1) add the repository in /etc/apt/sources.list
deb http://ftp.us.debian.org/debian testing main contrib non-free
2) do the actual pinning in /etc/apt/preferences
Package: *
Pin: release n=testing
Pin-Priority: 100
A Pin-Priority of under 500 basically means that no packages from testing are installed automatically, so you won't have problems with other packages.
3) install python2.7 from testing:
aptitude -t testing install python2.7
(or apt-get if you don't have aptitude)

Here is two methods for Debian GNU/Linux 6.0.7 (on 18/07/2013):
The classic
Install dependencies
aptitude -y install build-essential python-pip libmysqlclient-dev libadns1-dev \
python-dev libreadline-dev libgdbm-dev zlib1g-dev libsqlite3-dev \
libssl-dev libbz2-dev libncurses5-dev libdb-dev
Download python
cd /tmp
wget http://python.org/ftp/python/2.7.5/Python-2.7.5.tar.xz
unxz -c Python*xz | tar xpf -
Compile
cd Python*
./configure --prefix=/opt/python2.7.5 --enable-shared
make
Install
make install
echo "/opt/python2.7.5/lib" > /etc/ld.so.conf.d/libpython2.7.conf
ldconfig
Test
/opt/python2.7.5/bin/python -c "print('Ok')"
Upgrade pip virtualenv
easy_install pip
pip -v install --upgrade distribute==0.7.3
pip -v install --upgrade virtualenv==1.9.1
Create an user and its virtualenv
adduser user_app --home /opt/user_app
su user_app
virtualenv --no-site-packages --verbose -p /opt/python2.7.5/bin/python $HOME
Test again
su user_app
cd
source bin/activate
python -c "import sys; print sys.version"
The "pythonic"
Use the package pyenv.
pyenv install 2.7.5

Installing a chroot-environment with debootstrap could be also a fast and secure solution.
It uses about 300mb
debootstrap wheezy /opt/debian7
chroot /opt/debian7
apt-get install python2.7

You can install and switch python versions using pythonbrew I installed python 2.7.3 and python 2.7.9 in Debian 6 and Debian 7 and works fine.
You can follow this tutorial pythonbrew howto