import getpass
from typing import Optional, Tuple


class UserPassAuthenticator:

    def __init__(self, username: Optional[str] = None, password: Optional[str] = None) -> None:
        self._given_username = username
        self._given_password = password

        self._username = username
        self._password = password

    def get_credentials(self, reason: str) -> Tuple[str, str]:
        """
        Returns a tuple (username, password). Prompts user for username or
        password when necessary. Must be called with a "reason" that will be
        displayed in the credentials prompt.
        """

        if self._username is None and self._given_username is not None:
            self._username = self._given_username

        if self._password is None and self._given_password is not None:
            self._password = self._given_password

        if self._username is None or self._password is None:
            print(f"Enter credentials ({reason})")

        username: str
        if self._username is None:
            username = input("Username: ")
            self._username = username
        else:
            username = self._username

        password: str
        if self._password is None:
            password = getpass.getpass(prompt="Password: ")
            self._password = password
        else:
            password = self._password

        return (username, password)

    @property
    def username(self) -> str:
        """
        The username. Accessing this property may cause the authenticator to
        prompt the user.
        """

        (username, _) = self.get_credentials()
        return username

    @property
    def password(self) -> str:
        """
        The password. Accessing this property may cause the authenticator to
        prompt the user.
        """

        (_, path) = self.get_credentials()
        return password

    def invalidate_credentials(self) -> None:
        """
        Marks the credentials as invalid. If only a username was supplied in
        the constructor, assumes that the username is valid and only the
        password is invalid. If only a password was supplied in the
        constructor, assumes that the password is valid and only the username
        is invalid. Otherwise, assumes that username and password are both
        invalid.
        """

        self._username = None
        self._password = None

        if self._given_username is not None and self._given_password is not None:
            self._given_username = None
            self._given_password = None