Environment variables usage in Python - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is an environment variable?

An environment variable is a named value stored outside a program that can affect how the program runs. It is like a setting or secret stored in the computer's system that programs can read.

beginner

How do you access environment variables in Python?

You use the os module and call os.getenv('VARIABLE_NAME') to get the value of an environment variable.

intermediate

Why should sensitive information like passwords be stored in environment variables?

Storing sensitive info in environment variables keeps secrets out of the code. This helps keep passwords safe and makes it easier to change them without changing the program.

beginner

What happens if you try to get an environment variable that does not exist using os.getenv?

If the environment variable does not exist, os.getenv returns None by default, or a default value if you provide one.

beginner

How can you set an environment variable in a Unix-like terminal before running a Python script?

You can set it by typing export VARIABLE_NAME=value in the terminal. Then when you run the Python script, it can read that variable.

Which Python module is commonly used to access environment variables?

A os

B sys

C math

D random

The os module provides functions like os.getenv to access environment variables.

What does os.getenv('API_KEY') return if 'API_KEY' is not set?

A An error is raised

B None

C Zero

D An empty string

If the environment variable is not set, os.getenv returns None by default.

Why is it better to store passwords in environment variables rather than hardcoding them?

A It allows passwords to be printed easily

B It makes the program run faster

C It uses less memory

D It keeps passwords secret and separate from code

Storing passwords in environment variables keeps them secret and makes it easier to change them without editing code.

How do you set an environment variable named 'TOKEN' to 'abc123' in a Unix terminal?

A export TOKEN=abc123

B TOKEN=abc123

C set TOKEN=abc123

D env TOKEN abc123

The correct command is export TOKEN=abc123 to set an environment variable in Unix shells.

Which of these is a safe way to provide a default value when reading an environment variable in Python?

A os.getenv('VAR', 'default')

B os.getenv('VAR') or 'default'

C Both B and C

D None of the above

Both os.getenv('VAR', 'default') and os.getenv('VAR') or 'default' safely provide a default value.

Explain how to read an environment variable in Python and why it is useful.

Think about how programs get settings from outside.

Describe how to set an environment variable in a terminal and how a Python script can use it.

Consider the steps before running the Python script.

Practice

1. What is the main purpose of using environment variables in a Python program?

easy

A. To print output on the screen

B. To write data to files

C. To create new Python functions

D. To store configuration settings outside the code

Solution

1. Step 1: Understand environment variables role

   Environment variables hold settings or secrets outside the program code.

2. Step 2: Identify correct purpose

   Using environment variables helps keep code flexible and secure by not hardcoding values.

3. Final Answer:

   To store configuration settings outside the code -> Option D

4. Quick Check:

   Environment variables = external settings [OK]

Hint: Environment variables hold settings outside code [OK]

Common Mistakes:

• Thinking environment variables store data inside the program
• Confusing environment variables with file operations
• Assuming environment variables create functions

2. Which of the following is the correct way to import the module needed to access environment variables in Python?

easy

A. import environment

B. import os

C. import sys

D. import env

Solution

1. Step 1: Recall Python module for environment variables

   The standard module to access environment variables is os.

2. Step 2: Check options for correct import

   Only import os is correct; others are invalid or unrelated.

3. Final Answer:

   import os -> Option B

4. Quick Check:

   Module for env vars = os [OK]

Hint: Use 'import os' to access environment variables [OK]

Common Mistakes:

• Using 'import environment' which does not exist
• Confusing 'sys' module with environment variables
• Trying to import 'env' which is not a standard module

3. What will be the output of this code if the environment variable USER is set to alice?

import os
name = os.getenv('USER', 'guest')
print(name)

medium

A. alice

B. USER

C. null

D. guest

Solution

1. Step 1: Understand os.getenv behavior

   os.getenv('USER', 'guest') returns the value of USER if set, else 'guest'.

2. Step 2: Apply given environment variable value

   Since USER is set to 'alice', the function returns 'alice'.

3. Final Answer:

   alice -> Option A

4. Quick Check:

   os.getenv returns env var value if set [OK]

Hint: os.getenv returns env var value or default [OK]

Common Mistakes:

• Assuming default is always returned
• Printing the variable name instead of its value
• Confusing null with default value

4. Identify the error in this code snippet that tries to read an environment variable:

import os
api_key = os.getenv('API_KEY')
print(api_key.upper())

Assume API_KEY is not set in the environment.

medium

A. AttributeError because api_key is null

B. SyntaxError due to missing parentheses

C. No error, code runs fine

D. NameError because os is not imported

Solution

1. Step 1: Check os.getenv return when variable missing

   When API_KEY is not set, os.getenv returns null.

2. Step 2: Understand method call on null

   Calling upper() on null causes an AttributeError because null has no such method.

3. Final Answer:

   AttributeError because api_key is null -> Option A

4. Quick Check:

   null.upper() causes AttributeError [OK]

Hint: Check for null before calling string methods [OK]

Common Mistakes:

• Assuming os.getenv returns empty string if missing
• Ignoring that null has no string methods
• Thinking code runs without error

5. You want to safely read an environment variable PORT as an integer with a default of 8080 if not set or invalid. Which code snippet correctly does this?

hard

A. port = int(os.getenv('PORT') or 8080)

B. port = os.getenv('PORT', 8080)

C. try:\n port = int(os.getenv('PORT'))\nexcept (TypeError, ValueError):\n port = 8080

D. port = int(os.getenv('PORT', 8080))

Solution

1. Step 1: Understand the problem requirements

   We must convert PORT to int, use 8080 if missing or invalid (non-integer).

2. Step 2: Analyze each option

   port = int(os.getenv('PORT', 8080)) fails if PORT is set but not an integer string (raises ValueError). port = os.getenv('PORT', 8080) does not convert to int. port = int(os.getenv('PORT') or 8080) uses or but fails if PORT is set to invalid string (ValueError). try:\n port = int(os.getenv('PORT'))\nexcept (TypeError, ValueError):\n port = 8080 uses try-except to handle missing or invalid values safely.

3. Final Answer:

   try:\n port = int(os.getenv('PORT'))\nexcept (TypeError, ValueError):\n port = 8080 -> Option C

4. Quick Check:

   Use try-except to safely convert env var [OK]

Hint: Use try-except to handle invalid env var conversions [OK]

Common Mistakes:

• Not handling invalid integer strings
• Assuming default works if env var is invalid
• Not converting string to int