<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def change_password(self, user, password, send_email=None):
 """ Service method to change a user's password. Sends signal `password_changed`. :param user: The :class:`User`'s password to change. :param password: The new password. :param send_email: Whether or not to override the config option ``SECURITY_SEND_PASSWORD_CHANGED_EMAIL`` and force either sending or not sending an email. """

        user.password = password
        self.user_manager.save(user)
        if send_email or (app.config.SECURITY_SEND_PASSWORD_CHANGED_EMAIL
                          and send_email is None):
            self.send_mail(
                _('flask_unchained.bundles.security:email_subject.password_changed_notice'),
                to=user.email,
                template='security/email/password_changed_notice.html',
                user=user)
        password_changed.send(app._get_current_object(), user=user)