One unfortunate difficulty when working with App Engine is managing your local dependencies. You don’t have access to your Python environment so all libraries you wish to use must be vendored with your installation. That is, you need to copy all of your library code into a local folder to ship along with your app.

This usually doesn’t cause any problems but difficulties start to crop up when you manage multiple dependencies that rely on each other. For example, the official elasticsearch client requires urllib3 between version 1.8 and 2.0.

Traditionally, pip is used to install these dependencies on your behalf. The command pip install elasticsearch will automatically fetch the urllib3 dependency for you and install it to your local Python environment. By adding the -t flag you can provide a destination folder to install your libraries. As an example, we can install the elasticsearch and urllib3 libraries to the folder src/lib with the following command.

pip install elasticsearch -t src/lib

This works great for App Engine which requires the source of your libraries to be shipped with your application. Unfortunately, it starts to break down when you need to upgrade your dependencies. Installing with the -t flag does not overwrite the contents of the existing folder so running the same command again results in an error.

A solution to this can be found with some basic shell scripting. The first portion of our script installs the requested package and it’s dependencies to a temporary directory and removes any extra files that we don’t need.

pip install elasticsearch -t $TEMP_DIRECTORY
rm -r $TEMP_DIRECTORY/*.egg-info >/dev/null 2>&1
rm -r $TEMP_DIRECTORY/*.dist-info >/dev/null 2>&1

The next step is to remove the specific libraries being installed from our App Engine library directory and copy the contents of our temporary directory in their place.

for i in $(ls $TEMP_DIRECTORY); do
  rm -r $TARGET/$i >/dev/null 2>&1  # remove existing module
  cp -R $TEMP_DIRECTORY/$i $TARGET  # copy the replacement in

This code can be used as a starting point to write a more user friendly and robust script. Although this does not truly solve the problem of dependency management with App Engine it does provide a way to seamlessly vendor Python libraries and all of their dependencies with your application.