How to use scl enable to run python3 app in dockerfile

Question

Im trying to run a python3 application in a docker container using CentOS 7 as the base image. So if I'm just playing with it interactively, I type scl enable rh-python36 bash

That obviously switches my standard python2 environment to the python3.6 environment I install earlier(in the Dockerfile) Now, earlier in the dockerfile I run the following:

SHELL ["scl", "enable", "rh-python36"] (and many variations of this)

This enables me to do all of my pip installations in this python3 environment. However, when I actually want to run my app.py with CMD, it defaults to the python2. I've tried playing with ENTRYPOINT and variations of CMD, but I cant seem to make the python3 environment active when the container finally runs. How can I get this running correctly with python3?

Here's the dockerfile:

FROM centos:7
RUN mkdir -p /usr/src/app && \
  yum install -y centos-release-scl && \
  yum install -y rh-python36 && \
  yum install -y rh-python36-python-tkinter
SHELL ["scl", "enable", "rh-python36"]
WORKDIR /usr/src/app
COPY . .
WORKDIR /usr/src/app/codeBase
RUN pip install --no-cache-dir -r /usr/src/app/codeBase/requirements.txt
EXPOSE 9000
CMD ["python",  "run.py"]

I've also tried the alias solution, but I'm afraid it doesnt change the python exe for the CMD: Here's the totally runnable version with that that still prints out python 2.7.5:

FROM centos:7
RUN mkdir -p /usr/src/app && \  
  yum install -y centos-release-scl && \
  yum install -y rh-python36 && \
  yum install -y rh-python36-python-tkinter
WORKDIR /usr/src/app
RUN alias python=$(find / -type f -name "python*" | grep "python3.6$")
CMD ["python",  "-V"]

It just seems as though none of this persists in the new shell created with CMD

Answer 1

SHELL is completely the wrong Dockerfile command for this. You'll probably want to put that in a RUN stanza instead.

The purpose of SHELL is to define the shell used to execute RUN commands. So something like

SHELL ["sh", "-c"] # The default
RUN echo "foo"

ends up running

sh -c 'echo "foo"'

Of course, replacing SHELL with a command which doesn't support this use case will simply break the RUN command for you.

Maybe try something like

FROM centos:7
RUN mkdir -p /usr/src/app && \
  yum install -y centos-release-scl && \
  yum install -y rh-python36 && \
  yum install -y rh-python36-python-tkinter
WORKDIR /usr/src/app
COPY . .
WORKDIR /usr/src/app/codeBase
RUN scl enable rh-python36 pip install --no-cache-dir -r ./requirements.txt
EXPOSE 9000
CMD ["scl", "enable", "rh-python36", "python",  "run.py"]

Answer 2

Note:

I am leaving this answer up as context to the question, but this does not solve the issue. My attempt was to modify where python is pointing, but if executing in a RUN command, the shell will exit and you'll lose the alias . Modifying the PYTHONPATH via bash is also a no-go, per this question . Thanks to @tripleee for the catch

Unfortunately, it looks like yum install -y rh-python36 puts that python3.6 in a really weird spot:

find / -type f -name "python*" | grep "python3.6$"
/opt/rh/rh-python36/root/usr/bin/python3.6

You can use that to alias your python command in your Dockerfile :

RUN mkdir -p /usr/src/app && \
  yum install -y centos-release-scl && \
  yum install -y rh-python36 && \
  yum install -y rh-python36-python-tkinter

# Here
RUN alias python=$(find / -type f -name "python*" | grep "python3.6$")

Which should allow you to retain your CMD and also ties it to the correct site-packages :

import sys

sys.path
['', '/opt/rh/rh-python36/root/usr/lib64/python36.zip', '/opt/rh/rh-python36/root/usr/lib64/python3.6', '/opt/rh/rh-python36/root/usr/lib64/python3.6/lib-dynload', '/opt/rh/rh-python36/root/usr/lib64/python3.6/site-packages', '/opt/rh/rh-python36/root/usr/lib/python3.6/site-packages']

It looks like this also might remove the need to scl enable that python environment, as you can then just use python -m pip... :

# No scl has been run
python -m pip install requests

# running python in the terminal
import requests

r = requests.get('http://google.com')
# runs like a charm

Answer 3

So I figured this out thanks to the answer from C.Nivs and an answer here . The alias works in an interactive shell, but not for the CMD. What I ended up doing was similar, only in my case I'm creating a new executable in /usr/bin that calls the special python36 exe:

RUN echo -e '#!/bin/bash\n$(find / -type f -name "python*" | grep "python3.6$") "$@"' > /usr/bin/py3 && \
    chmod +x /usr/bin/py3
CMD ["py3",  "-V"]

now py3 runs a script calling the python3 install specifically with whatever argument

Answer 4

Configuring the shell environment variables by overriding them all to use scl_source, as seen from following "Method #2" from Austin Dewey's blog works for me.

FROM centos:7
SHELL ["/usr/bin/env", "bash", "-c"]
RUN \
    yum install -y centos-release-scl && \
    yum install -y rh-python38-python-devel
ENV \
    BASH_ENV="/usr/bin/scl_enable" \
    ENV="/usr/bin/scl_enable" \
    PROMPT_COMMAND=". /usr/bin/scl_enable"
RUN echo -e "\n\
unset BASH_ENV PROMPT_COMMAND ENV\n\
source scl_source enable rh-python38\n\
" > /usr/bin/scl_enable
RUN \
    python3 -m ensurepip && \
    python3 -m pip install --upgrade pip && \
    python3 -m pip install setuptools wheel

docker build --tag python:3.8-test - < Dockerfile
docker run --rm -i -t python:3.8-test
python --version

Python 3.8.11