Deploying a python script without virtualenv

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Lately I’ve been writing a lot of python to script some common processes in a large infrastructure. Those scripts require some python libraries which are not part of the standard library nor installed by default in all systems. For example, I love how docopt manages the arguments of a python script, or using the YAML format for my config files instead of INI.

During the development stage, I usually manage those dependencies using pip and virtualenv, but I would like my deployment to require only python3 installed in the remote machine.

pip allows us to download the dependencies to a local directory running the following command:

pip3 install -r requirements.txt --upgrade --system --target python_deps
  • upgrade: we use this flag to download not only the required dependency, but all the transitive dependencies that this one requires.
  • system: by default, pip installs the packages following the user scheme, which is a specific directory structure for the interpreter to find the packages when running as a specific user. This flag modifies this behaviour, using a scheme that we point the interpreter to.
  • target: we use this flag to tell pip where we want the dependencies to be downloaded.

After installing the packages to a specific directory, we need to add that directory to our PYTHONPATH environment variable when running the interpreter:

PYTHONPATH=`pwd`/python_deps python3

To wrap all of this, we can create a simple script to run our python program, that searchs for the dependencies directory, installs them if they are not there and then runs the program with the PYTHONPATH variable set:

#!/usr/bin/env bash

# Set the PYTHONPATH environment variable. Using export will make the variable
# available for the rest of the script
export PYTHONPATH=`pwd`/python_deps

# If the python_deps directory doesn't exist, we download the dependencies there
if [[ ! -d "$PYTHONPATH" ]]; then
    pip3 install -r requirements.txt --upgrade --system --target $PYTHONPATH

python3 src/ $@

We can now upload the script and run it from any system knowing that the only thing we need is the python3 interpreter to be installed.

An improvement for this method would be to directly create a debian package, but that’s for the next post.