Environment variables in Python & Docker

Using environment variables is a useful way of providing inputs into programs. It can be used to:

• add configurations
• insert secret variables

These variables are frequently used when deploying applications with your cloud service provider and/or Docker. It is definitely something you should have knowledge of since it is used everywhere, regardless of programming language or system.

To list existing environment variables in bash, you can open a terminal and type:

printenv

To set an environment variable in bash, you simply type:

export MY_ENV_VARIABLE="MY VALUE"

To verify that it now exists, type:

echo $MY_ENV_VARIABLE

And “MY VALUE” should be printed.

Python

To access these variables in Python we can use the os package. There are two ways to access them. Here I will use one of the variables I found on my system (MacBook, bash) when typing printenv (TERM_PROGRAM):

Great, we know how to access the variables, but inserting variables through the terminal using export KEY=VALUE is rather inconvenient. Instead, we would like to have a way to change multiple variables easily and insert them into the software smoothly.

Enter .env-files. These files are handy for inserting env-variables by only changing a single file. Each line in the file will describe a variable to value-mapping, for instance:

VARIABLE1 = HELLO
VARIABLE2 = "WORLD"
VARIABLE3 = ${VARIABLE1} ${VARIABLE2}

Here the second variable was quoted, but the result will be the same whether you type with or without quotes. The third variable uses previously defined environment variables using ${<variable_name>}. All variables will by default be loaded as strings. The names for these files are usually “.env”. If we have multiple files we could add a prefix or suffix and switch between them. Now, imagine we have a file called .env with the content specified above. To insert these into Python locally, we can use the python-dotenv package. Start by installing it:

pip install python-dotenv

Then only a single function call is necessary to load the variables from the file:

NOTE: if you have secrets in a .env file, don’t forget to add it to .gitignore to avoid it from being pushed to git.

Docker

When deploying applications docker is often used. With it, there are more appropriate options for inserting environment variables than using python-dotenv. Here I’m going to talk about local development. When you deploy your container(s) to the cloud the insertion of env-variables would depend on what provider and deployment option that is used.

I will use docker-compose here. While it is not necessary, I believe it’s a great tool regardless if you create one or multiple docker containers. For the demo project the following file structure is used:

There is a quite a few files, but they all contain very little code. In the docker file we have:

A python base-image is imported, here alpine because it is lightweight. The app folder is then copied and finally the python file, main.py, is run. In the .env file we have:

MY_SECRET_VARIABLE = "I'm batman!"

As described, the variable is “secret”, env-variables that are not secret can be hardcoded in the docker-compose file, which is shown below:

Note the comment describing the difference between the env_file option and the environment option and using the filename .env specifically as opposed to something else.

Next, a .dockerignore file is added. This is to avoid adding the .env-file into the image since it might contain sensitive information. The content is thus only:

.env

Finally, we have the main.py file that will be run. Here the environment variables are just printed:

To run everything the following expression is typed in the terminal:

docker-compose -f docker-compose.yml up --build

And the result is:

Summary

Environment variables are important because they are a common medium for injecting configurations into our software. While in this article Python was used, environment variables can be used with most programming languages. Good luck!

If you’re interested in reading more articles about Python, check out my reading list below:

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