Working with Environment Variables

• Environment variables are dynamic, named values provided by the operating system to running processes, enabling configuration of behavior without code modifications.
• They allow applications to adapt across development, staging, and production environments by externalizing configuration data such as API keys, file paths, and feature flags.
• Python’s os module offers simple interfaces to access and manage these variables, promoting separation of code and configuration.

import os

for key in ["HOME", "SHELL"]:
    value = os.getenv(key)
    print(f"{key} = {value if value else "Not set"}")

env_keys = list(os.environ.keys())
print(f"We have {len(env_keys)} environment variables available!")

for key in env_keys[:5]:
    print(key)

Accessing Environment Variables with os.getenv()

• The os.getenv function retrieves the value of an environment variable by key, returning None or a provided default if the key is not found.
• It prevents KeyError exceptions by offering a safe access pattern for optional configuration settings.
• Since environment variables are always strings, any expected non-string types require explicit conversion after retrieval.

import os 

os.environ["APP_API_KEY"] = "ab12cd34"

api_key = os.getenv("APP_API_KEY")
debug_mode = os.getenv("DEBUG_MODE", False)

if api_key:
    print(f"API key found: {api_key[:4]}... (masked)")
else:
    print("APP_API_KEY not set.")

print(f"Debug mode: {debug_mode}")

Accessing Environment Variables with os.environ

• os.environ behaves like a dictionary mapping environment variable names to their string values.
• Accessing a missing key via os.environ['KEY'] raises a KeyError, making it suitable for mandatory variables.
• One should guard against missing keys by checking membership or catching KeyError to handle critical configuration errors.

import os

try:
    java_home = os.environ["JAVA_HOME"]
    print(java_home)
except KeyError:
    print("JAVA_HOME environment variable not set.")

Setting Environment Variables Within Python

• While environment variables are typically set externally, os.environ can be modified at runtime to affect the current process and its children.
• Assigning to os.environ['KEY'] makes the variable available to any subprocesses spawned by the script.
• Deleting an entry from os.environ removes it for subsequent operations within the process, but changes do not persist after the script exits.

import os
import sys
import subprocess

print(f"Initial MY_CUSTOM_VAR: {os.getenv("MY_CUSTOM_VAR")}")

os.environ["MY_CUSTOM_VAR"] = "SetByOurScript"
print(f"Updated MY_CUSTOM_VAR: {os.getenv("MY_CUSTOM_VAR")}")

result = subprocess.run([
    sys.executable,
    "-c",
    """import os
print(f"Child sees MY_CUSTOM_VAR: {os.getenv("MY_CUSTOM_VAR")}")"""  
])

result.stdout

del os.environ["MY_CUSTOM_VAR"]
print(f"After deletion, MY_CUSTOM_VAR: {os.getenv("MY_CUSTOM_VAR")}")

Using dotenv to Manage Local Environment Files

• The python-dotenv library lets you keep sensitive and environment-specific values in a .env file instead of the shell.
• A .env file lives alongside your script and contains lines like KEY=value; it's loaded at runtime into os.environ.
• Install with pip install python-dotenv==1.1.0 (version included here so that we are all using the same, in other installations it may be omitted), then call load_dotenv() before any os.getenv calls.
• This approach keeps your shell clean and makes it easy to commit example .env.example files without secrets.
• Remember not to commit actual .env files with real secrets! Add them to .gitignore.

import os
from dotenv import load_dotenv

os.environ["MY_DOTENV_VAR"] = "setFromJupyter"

load_dotenv(override=True)

secret_dotenv_value = os.getenv("MY_DOTENV_VAR")

print(f"Retrieved MY_DOTENV_VAR with value {secret_dotenv_value}")

Common Pitfalls & How to Avoid Them

• Environment variable names are always case-sensitive in Python, regardless of the underlying OS; inconsistent casing leads to unexpected missing values.
• Forgetting that all environment variable values are strings can cause type errors; always convert to the intended type like int or bool after retrieval.
• Accessing a missing mandatory variable via os.environ raises KeyError; avoid unhandled errors by checking membership or catching exceptions.
• Storing highly sensitive secrets in plain environment variables carries security risks; for production use, consider managed secrets solutions like Vault or AWS Secrets Manager.

import os
from dotenv import load_dotenv

load_dotenv(override=True)

number_dotenv_value = os.getenv("MY_NUMBER_VAR")

print(type(number_dotenv_value))
# print(number_dotenv_value + 45) # Uncommenting will raise TypeError because number_dotenv_value is a string!