Using Docker to run tox without adding Pythons to your system

If you would like to test python applications in a repeatable way, especially if for interpreters beyond the ones you keep on your development system, there are some ways to pull that off -- I've usually fallen back on pyenv, personally -- but they're all a bit less than optimal. One of the worst things about them is trying to get them to play nicely with tox, which is an amazing way to run cross-python tests.

Naturally, it occurred to me that using Docker might be a good way to do this, and so I put together a Dockerfile that sets up basically the omni-interpreter runtime, and configured it to run tox on a mounted /src directory.

Once you've got this dockerfile built, then you can run tox tests that depend on Pythons starting with 2.5 through 3.4, as well as pypy in both the 2.x and 3.x flavors. Technically it also supports Jython, but due to some issues with Jython and virtualenvs, I don't presently have that working (pull requests welcome).

Here's literally all it takes to run it:

$ cd ~/projects/MyProject
$ docker run --rm -v `pwd`:/src chrisr/pybuilder:latest
# tox output happens for your whole tox file

Some caveats apply if you are running docker by way of boot2docker, though; because of the nature of the boot2docker filesystem, hardlinks do not work, and distutils -- up until Python 2.7.8/3.4.2 -- fails if it can't hardlink files. Working around this requires a filthy hack in your, which depends on a flag in this Dockerfile to trigger it:

# need to kill off link if we're in docker builds
if os.environ.get('PYTHON_BUILD_DOCKER', None) == 'true':

If you're running docker natively on a linux, however, this won't be necessary.

The dockerfile to build this image looks like this:

FROM ubuntu:14.04
MAINTAINER Chris Rose <>

# ensure the base image has what we need
RUN apt-get update && \
    DEBIAN_FRONTEND=noninteractive apt-get -yqq install \
    build-essential python-pip software-properties-common \
    openjdk-7-jdk && \
    add-apt-repository ppa:fkrull/deadsnakes && \
    apt-get update

# install legacy python versions
RUN DEBIAN_FRONTEND=noninteractive apt-get -yqq install \
    python2.5 python2.6 python2.7 python3.1 python3.2 python3.3 python3.4

# add Jython installer
# ADD jython-installer-2.7-b4.jar /tmp/
ADD /tmp/jython-installer-2.7-b4.jar

# install pypy versions
# ADD pypy-2.5.0-linux64.tar.bz2 /opt/
# ADD pypy3-2.4.0-linux64.tar.bz2 /opt/
RUN mkdir -p /opt
ADD /tmp/
RUN cd /opt && tar -xf /tmp/pypy3-2.4.0-linux64.tar.bz2
ADD /tmp/
RUN cd /opt && tar -xf /tmp/pypy-2.5.0-linux64.tar.bz2

# install Jython version
RUN java -jar /tmp/jython-installer-2.7-b4.jar -d /opt/jython-2.7-b4 -s -t all
ENV PATH /opt/jython-2.7-b4/bin:$PATH
# bootstrap jython JAR cache
RUN jython

# make PyPy available
ENV PATH /opt/pypy-2.5.0-linux64/bin:/opt/pypy3-2.4.0-linux64/bin:$PATH


# install tox
RUN pip install tox

ADD /tools/


ENTRYPOINT ["/tools/"]
CMD ["tox"]

The entry point is pretty simple:

find /src \( -name __pycache__ -o -name '*.pyc' \) -delete
exec "$@"

Its purpose is to remove all .pyc files that might reference absolute paths on the host filesystem; otherwise the interpreter barfs rather frequently.

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