lesson 113 CLI & Automation · intermediate

Settings from environment variables

Some settings belong to the machine, not to one run: where reports go, an API key, whether to send emails. Environment variables carry them, always as text, and a script reads them with a default.

You'll be able to

Read a variable with os.environ.get(name, default) and explain why os.environ[name] can fail
Convert the text to the type the program needs, and treat "0" and "no" as off
Combine sources in a clear order: command line, then environment, then a default

The idea

Every running program receives a set of named text values from whatever started it: the environment. A shell sets one for a single run with REPORT_LIMIT=50 python report.py, or for the whole session with export REPORT_LIMIT=50. Servers and schedulers set them in their configuration. They suit settings that differ per machine (a folder, a database address) and secrets that shouldn't be typed on a command line where they end up in the shell's history.

Python shows the environment as os.environ, which behaves like a dictionary from text to text. Assigning to it, as the first line below does, sets a variable for this program (and any program it starts), just as the shell would have.

Run each example to see what it prints. Running examples doesn't count toward progress.

example-1.py · Reading with and without a default

import os

os.environ["EXPORT_FOLDER"] = "exports/daily"
print(os.environ["EXPORT_FOLDER"])
print(os.environ.get("EXPORT_BATCH_LIMIT"))
print(os.environ.get("EXPORT_BATCH_LIMIT", "25"))
limit = int(os.environ.get("EXPORT_BATCH_LIMIT", "25"))
print(limit + 5)

example-2.py · Square brackets on a missing variable

import os

folder = os.environ["EXPORT_ARCHIVE_FOLDER"]
print("archiving to", folder)

A function that reads settings is easier to test when it takes the environment as a parameter. The program passes os.environ; a test passes an ordinary dictionary, so it doesn't depend on what happens to be set on the machine running it.

On/off settings need care. Every non-empty text is true in an if, including "0" and "no", so compare against the words that mean *on* after strip() and lower().

example-3.py · All of a report's settings in one place

def load_settings(env):
    limit = int(env.get("REPORT_LIMIT", "25"))
    send = env.get("SEND_EMAIL", "no").strip().lower()
    return {
        "folder": env.get("REPORT_FOLDER", "reports"),
        "limit": limit,
        "send_email": send in ["1", "true", "yes", "on"],
    }

print(load_settings({}))
print(load_settings({"REPORT_LIMIT": "100", "SEND_EMAIL": " Yes ", "REPORT_FOLDER": "/srv/reports"}))
print(load_settings({"SEND_EMAIL": "0"}))

When to use it: for settings that belong to the machine or the deployment, and for secrets. A value someone chooses per run belongs on the command line instead. When both exist, the usual order is: an explicit command-line value wins, then the environment, then the default in the code.

Mistakes to avoid: os.environ[name] for an optional setting (KeyError when it's missing); using the text without converting it ("10" > "9" is False, because text compares letter by letter); and if env.get("DEBUG"):, which is true for "0".

Review

os.environ.get(name, default) reads a setting; brackets raise KeyError when it's missing.
Environment values are always text: convert numbers, and compare on/off words after strip and lower.
Take the environment as a parameter, and use a clear order: command line, environment, default.
Next: starting other programs from Python, and keeping that call replaceable.

Practise this