Use configuration constants

Always use predefined constants from homeassistant.const instead of string literals when accessing configuration data. For required configuration keys that must be present, use direct dictionary access (entry.data[CONF_KEY]) rather than .get() with defaults, as this provides better error handling and makes the required nature of the configuration explicit.

# Bad - using string literals and .get() for required keys
email = user_input.get("email")
password = user_input.get("password") 
api_key = entry.data.get("api_key", "")

# Good - using constants and direct access for required keys
from homeassistant.const import CONF_EMAIL, CONF_PASSWORD, CONF_API_KEY

email = user_input.get(CONF_EMAIL)
password = user_input.get(CONF_PASSWORD)
api_key = entry.data[CONF_API_KEY]  # Will raise KeyError if missing, which is appropriate

This approach improves code maintainability, prevents typos in configuration key names, and makes the required vs optional nature of configuration values clear through the access pattern used.