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Environment Variables

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Tip

If you already know what "environment variables" are and how to use them, feel free to skip this.

An environment variable (also known as "env var") is a variable that lives outside of the Python code, in the operating system, and could be read by your Python code (or by other programs as well).

Environment variables could be useful for handling application settings, as part of the installation of Python, etc.

Create and Use Env Vars

You can create and use environment variables in the shell (terminal), without needing Python:

// You could create an env var MY_NAME with
$ export MY_NAME="Wade Wilson"

// Then you could use it with other programs, like
$ echo "Hello $MY_NAME"

Hello Wade Wilson

// Create an env var MY_NAME
$ $Env:MY_NAME = "Wade Wilson"

// Use it with other programs, like
$ echo "Hello $Env:MY_NAME"

Hello Wade Wilson

Read env vars in Python

You could also create environment variables outside of Python, in the terminal (or with any other method), and then read them in Python.

For example you could have a file main.py with:

import os

name = os.getenv("MY_NAME", "World")
print(f"Hello {name} from Python")

Tip

The second argument to os.getenv() is the default value to return.

If not provided, it's None by default, here we provide "World" as the default value to use.

Then you could call that Python program:

// Here we don't set the env var yet
$ python main.py

// As we didn't set the env var, we get the default value

Hello World from Python

// But if we create an environment variable first
$ export MY_NAME="Wade Wilson"

// And then call the program again
$ python main.py

// Now it can read the environment variable

Hello Wade Wilson from Python

// Here we don't set the env var yet
$ python main.py

// As we didn't set the env var, we get the default value

Hello World from Python

// But if we create an environment variable first
$ $Env:MY_NAME = "Wade Wilson"

// And then call the program again
$ python main.py

// Now it can read the environment variable

Hello Wade Wilson from Python

As environment variables can be set outside of the code, but can be read by the code, and don't have to be stored (committed to git) with the rest of the files, it's common to use them for configurations or settings.

You can also create an environment variable only for a specific program invocation, that is only available to that program, and only for its duration.

To do that, create it right before the program itself, on the same line:

// Create an env var MY_NAME in line for this program call
$ MY_NAME="Wade Wilson" python main.py

// Now it can read the environment variable

Hello Wade Wilson from Python

// The env var no longer exists afterwards
$ python main.py

Hello World from Python

Tip

You can read more about it at The Twelve-Factor App: Config.

Types and Validation

These environment variables can only handle text strings, as they are external to Python and have to be compatible with other programs and the rest of the system (and even with different operating systems, as Linux, Windows, macOS).

That means that any value read in Python from an environment variable will be a str, and any conversion to a different type or any validation has to be done in code.

You will learn more about using environment variables for handling application settings in the Advanced User Guide - Settings and Environment Variables.

PATH Environment Variable

There is a special environment variable called PATH that is used by the operating systems (Linux, macOS, Windows) to find programs to run.

The value of the variable PATH is a long string that is made of directories separated by a colon : on Linux and macOS, and by a semicolon ; on Windows.

For example, the PATH environment variable could look like this:

/usr/local/bin:/usr/bin:/bin:/usr/sbin:/sbin

This means that the system should look for programs in the directories:

  • /usr/local/bin
  • /usr/bin
  • /bin
  • /usr/sbin
  • /sbin
C:\Program Files\Python312\Scripts;C:\Program Files\Python312;C:\Windows\System32

This means that the system should look for programs in the directories:

  • C:\Program Files\Python312\Scripts
  • C:\Program Files\Python312
  • C:\Windows\System32

When you type a command in the terminal, the operating system looks for the program in each of those directories listed in the PATH environment variable.

For example, when you type python in the terminal, the operating system looks for a program called python in the first directory in that list.

If it finds it, then it will use it. Otherwise it keeps looking in the other directories.

Installing Python and Updating the PATH

When you install Python, you might be asked if you want to update the PATH environment variable.

Let's say you install Python and it ends up in a directory /opt/custompython/bin.

If you say yes to update the PATH environment variable, then the installer will add /opt/custompython/bin to the PATH environment variable.

It could look like this:

/usr/local/bin:/usr/bin:/bin:/usr/sbin:/sbin:/opt/custompython/bin

This way, when you type python in the terminal, the system will find the Python program in /opt/custompython/bin (the last directory) and use that one.

Let's say you install Python and it ends up in a directory C:\opt\custompython\bin.

If you say yes to update the PATH environment variable, then the installer will add C:\opt\custompython\bin to the PATH environment variable.

C:\Program Files\Python312\Scripts;C:\Program Files\Python312;C:\Windows\System32;C:\opt\custompython\bin

This way, when you type python in the terminal, the system will find the Python program in C:\opt\custompython\bin (the last directory) and use that one.

So, if you type:

$ python

The system will find the python program in /opt/custompython/bin and run it.

It would be roughly equivalent to typing:

$ /opt/custompython/bin/python

The system will find the python program in C:\opt\custompython\bin\python and run it.

It would be roughly equivalent to typing:

$ C:\opt\custompython\bin\python

This information will be useful when learning about Virtual Environments.

Conclusion

With this you should have a basic understanding of what environment variables are and how to use them in Python.

You can also read more about them in the Wikipedia for Environment Variable.

In many cases it's not very obvious how environment variables would be useful and applicable right away. But they keep showing up in many different scenarios when you are developing, so it's good to know about them.

For example, you will need this information in the next section, about Virtual Environments.