Deep Merge Nested Dictionaries for Configuration Management

def deep_merge(dict1, dict2):
    """
    Recursively merges dict2 into dict1.
    Modifies dict1 in place.
    """
    for key, value in dict2.items():
        if key in dict1 and isinstance(dict1[key], dict) and isinstance(value, dict):
            deep_merge(dict1[key], value)
        else:
            dict1[key] = value
    return dict1

# Default configuration
default_config = {
    'database': {
        'host': 'localhost',
        'port': 5432,
        'user': 'admin'
    },
    'logging': {
        'level': 'INFO',
        'file': '/var/log/app.log'
    },
    'server': {
        'port': 8000
    }
}

# User-specific or environment-specific overrides
user_config = {
    'database': {
        'host': 'db.example.com',
        'port': 1234 # Override port
    },
    'logging': {
        'level': 'DEBUG' # Override log level
    },
    'server': {
        'threads': 4 # Add new key
    }
}

# Merge user_config into default_config
merged_config = deep_merge(default_config.copy(), user_config)

print(f"Default config: {default_config}")
print(f"User config: {user_config}")
print(f"Merged config: {merged_config}")

# Expected merged_config:
# {
#     'database': {'host': 'db.example.com', 'port': 1234, 'user': 'admin'},
#     'logging': {'level': 'DEBUG', 'file': '/var/log/app.log'},
#     'server': {'port': 8000, 'threads': 4}
# }