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PRACTICAL PYTHON SCRIPTING - CHALLENGE 2 SOLUTION
Chapter 2: Configuration & Environment Variables
Challenge: Type Conversion and Validation
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PROBLEM:
Create a script that:
1. Loads a .env file
2. Converts port from string to integer
3. Converts debug flag from string to boolean
4. Validates that required variables exist

SOLUTION:
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from dotenv import load_dotenv
import os
import sys
from pathlib import Path

# Load the .env file
env_file = Path(__file__).parent / ".env"
load_dotenv(env_file)

print("=" * 60)
print("CONFIGURATION VALIDATION")
print("=" * 60 + "\n")

# Step 1: Define required variables
REQUIRED_VARS = ['DB_HOST', 'DB_USER', 'DB_PASS', 'DB_NAME']
OPTIONAL_VARS = ['DB_PORT', 'DB_CHARSET', 'DEBUG', 'LOG_LEVEL']

# Step 2: Check required variables exist
print("🔍 Checking required variables...\n")
missing_vars = []

for var in REQUIRED_VARS:
    if var in os.environ:
        print(f"  ✅ {var}: Found")
    else:
        print(f"  ❌ {var}: MISSING!")
        missing_vars.append(var)

if missing_vars:
    print(f"\n❌ ERROR: Missing required variables: {', '.join(missing_vars)}")
    sys.exit(1)

print("\n✅ All required variables found!\n")

# Step 3: Type conversion and validation
print("🔄 Converting and validating types...\n")

# String (no conversion needed)
db_host = os.environ.get('DB_HOST')
print(f"  DB_HOST: '{db_host}' (string)")

db_user = os.environ.get('DB_USER')
print(f"  DB_USER: '{db_user}' (string)")

db_pass = os.environ.get('DB_PASS')
print(f"  DB_PASS: '{'*' * len(db_pass)}' (string, hidden)")

db_name = os.environ.get('DB_NAME')
print(f"  DB_NAME: '{db_name}' (string)")

# Integer conversion (port)
try:
    db_port = int(os.environ.get('DB_PORT', '3306'))
    print(f"  DB_PORT: {db_port} (integer)")
except ValueError:
    print(f"  ❌ DB_PORT: Invalid! Must be a number (got '{os.environ.get('DB_PORT')}')")
    sys.exit(1)

# String (charset)
db_charset = os.environ.get('DB_CHARSET', 'utf8mb4')
print(f"  DB_CHARSET: '{db_charset}' (string)")

# Boolean conversion (debug mode)
debug_str = os.environ.get('DEBUG', 'false').lower()
debug = debug_str in ('true', '1', 'yes', 'on')
print(f"  DEBUG: {debug} (boolean, converted from '{debug_str}')")

# String (log level)
log_level = os.environ.get('LOG_LEVEL', 'info').upper()
print(f"  LOG_LEVEL: '{log_level}' (string)")

print("\n✅ All conversions successful!\n")

# Step 4: Display final configuration summary
print("=" * 60)
print("FINAL CONFIGURATION")
print("=" * 60 + "\n")

print("Database Configuration:")
print(f"  Host: {db_host}:{db_port}")
print(f"  Database: {db_name}")
print(f"  User: {db_user}")
print(f"  Charset: {db_charset}")

print("\nApplication Configuration:")
print(f"  Debug Mode: {debug}")
print(f"  Log Level: {log_level}")

if debug:
    print("\n⚠️ DEBUG MODE IS ENABLED - Do not use in production!")

print("\n" + "=" * 60)
print("✅ Configuration validated and ready!")
print("=" * 60)

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HOW IT WORKS:
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1. LOAD CONFIGURATION
   load_dotenv(env_file)
   - Load all variables from .env into os.environ

2. CHECK REQUIRED VARIABLES
   for var in REQUIRED_VARS:
       if var in os.environ:  # Check if variable exists

   - Loop through required variables
   - Check if each one exists in os.environ
   - Store missing ones for error reporting
   - Exit if any are missing

3. STRING CONVERSION (NO CONVERSION NEEDED)
   db_host = os.environ.get('DB_HOST')
   - Strings are the default type
   - No conversion necessary
   - .get() returns None if not found

4. INTEGER CONVERSION
   db_port = int(os.environ.get('DB_PORT', '3306'))
   - int() converts string to integer
   - Provide a default ('3306') if not found
   - int() raises ValueError if the string isn't a number
   - Use try/except to catch conversion errors

5. BOOLEAN CONVERSION
   debug_str = os.environ.get('DEBUG', 'false').lower()
   debug = debug_str in ('true', '1', 'yes', 'on')

   - .lower() converts to lowercase for comparison
   - Check if the string matches true values
   - Handles variations: 'true', '1', 'yes', 'on'
   - Anything else is False

6. ERROR HANDLING WITH sys.exit(1)
   - Exit immediately if validation fails
   - sys.exit(1) means "exit with error"
   - 0 = success, non-zero = error

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UNDERSTANDING TYPE CONVERSION:
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String → Integer:
int("3306")           # Returns 3306
int("30.5")          # ValueError! (has decimal point)
int("abc")           # ValueError! (not a number)
int("3306", base=10) # Explicit base (base 10 = decimal)

String → Float:
float("30.5")        # Returns 30.5
float("30")          # Returns 30.0
float("abc")         # ValueError!

String → Boolean:
Option 1 (Simple):
bool("true")         # Returns True (any non-empty string is True!)
bool("")             # Returns False (empty string is False)
bool("false")        # Returns True (even the string "false"!)

Option 2 (Correct - what we use):
debug = "true".lower() in ('true', '1', 'yes', 'on')

Option 3 (Using function):
def to_bool(s):
    return s.lower() in ('true', '1', 'yes', 'on')

debug = to_bool(os.environ.get('DEBUG', 'false'))

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VALIDATION PATTERNS:
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Pattern 1: Check if variable exists
if var in os.environ:
    # Variable exists
else:
    # Variable doesn't exist

Pattern 2: Get with default
value = os.environ.get('VAR', 'default_value')
# Uses 'default_value' if VAR not found

Pattern 3: Type conversion with error handling
try:
    port = int(os.environ.get('DB_PORT', '3306'))
except ValueError:
    print(f"ERROR: DB_PORT must be a number")
    sys.exit(1)

Pattern 4: Validate range
port = int(os.environ.get('DB_PORT', '3306'))
if port < 1 or port > 65535:
    print("ERROR: Port must be between 1 and 65535")
    sys.exit(1)

Pattern 5: Validate choices
log_level = os.environ.get('LOG_LEVEL', 'info').lower()
if log_level not in ('debug', 'info', 'warning', 'error'):
    print(f"ERROR: LOG_LEVEL must be one of: debug, info, warning, error")
    sys.exit(1)

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TESTING THE SOLUTION:
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1. First, create a .env file with test values:
   cat > .env << 'EOF'
DB_HOST=localhost
DB_NAME=testdb
DB_USER=testuser
DB_PASS=testpass123
DB_PORT=3306
DB_CHARSET=utf8mb4
DEBUG=true
LOG_LEVEL=debug
EOF

2. Run the script:
   python config_validator.py

Expected output:
============================================================
CONFIGURATION VALIDATION
============================================================

🔍 Checking required variables...

  ✅ DB_HOST: Found
  ✅ DB_USER: Found
  ✅ DB_PASS: Found
  ✅ DB_NAME: Found

✅ All required variables found!

🔄 Converting and validating types...

  DB_HOST: 'localhost' (string)
  DB_USER: 'testuser' (string)
  DB_PASS: '***********' (string, hidden)
  DB_NAME: 'testdb' (string)
  DB_PORT: 3306 (integer)
  DB_CHARSET: 'utf8mb4' (string)
  DEBUG: True (boolean, converted from 'true')
  LOG_LEVEL: 'DEBUG' (string)

✅ All conversions successful!

============================================================
FINAL CONFIGURATION
============================================================

Database Configuration:
  Host: localhost:3306
  Database: testdb
  User: testuser
  Charset: utf8mb4

Application Configuration:
  Debug Mode: True
  Log Level: DEBUG

⚠️ DEBUG MODE IS ENABLED - Do not use in production!

============================================================
✅ Configuration validated and ready!
============================================================

3. Test with missing variable:
   Remove DB_PASS from .env and run again:

   Output:
   🔍 Checking required variables...

     ✅ DB_HOST: Found
     ✅ DB_USER: Found
     ❌ DB_PASS: MISSING!
     ✅ DB_NAME: Found

   ❌ ERROR: Missing required variables: DB_PASS

4. Test with invalid port:
   Change DB_PORT=abc and run:

   Output:
   🔄 Converting and validating types...

     ...
     ❌ DB_PORT: Invalid! Must be a number (got 'abc')

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REAL-WORLD USAGE:
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In production code, you'd create a config function:

def load_and_validate_config():
    from dotenv import load_dotenv
    import os
    import sys

    load_dotenv()

    config = {
        'db_host': os.environ.get('DB_HOST'),
        'db_name': os.environ.get('DB_NAME'),
        'db_user': os.environ.get('DB_USER'),
        'db_pass': os.environ.get('DB_PASS'),
        'db_port': int(os.environ.get('DB_PORT', '3306')),
        'debug': os.environ.get('DEBUG', 'false').lower() == 'true',
    }

    # Validate required fields
    required = ['db_host', 'db_name', 'db_user', 'db_pass']
    missing = [k for k in required if not config[k]]

    if missing:
        print(f"Missing config: {missing}")
        sys.exit(1)

    return config

# In your main code:
if __name__ == '__main__':
    config = load_and_validate_config()
    db = connect_to_database(config)

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KEY TAKEAWAYS:
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✅ Environment variables are always strings
✅ Use int() to convert to integers
✅ Use float() to convert to floats
✅ Use string comparison for booleans (not bool())
✅ Always use try/except for type conversion
✅ Validate that required variables exist
✅ Use sys.exit(1) to signal errors
✅ Hide sensitive values in output
✅ Warn if debug mode is enabled in production

Type Conversion Checklist:
- ✅ Strings: no conversion needed
- ✅ Integers: use int()
- ✅ Floats: use float()
- ✅ Booleans: compare to true values
- ✅ All conversions: use try/except
- ✅ All conversions: provide defaults
- ✅ Always validate required variables

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