#!/bin/env python
"""
Retrieve number and subjects of unread mail from Gmail.

External dependencies:
    Set a GMAIL_LOGIN environment variable as a json string with Gmail access credentials.
    An example in Zsh:
        export GMAIL_LOGIN='{"username":"you@gmail.com", "password":"app_password"}'

Usage example:
    In Zsh:
        $ chmod +x check_mail.py
        $ ./check_mail.py

        Or drop the .py and copy the script to $HOME/.local/bin, then:
        $ check_mail
"""

import os
import json
import imaplib
from socket import gaierror
from typing import Generator
from email import policy, message_from_bytes


def check_unread_email(
    username: str, password: str, gmail_handle: imaplib.IMAP4_SSL
) -> Generator[str, None, None]:
    """Check if there's unread mail in Gmail

    Args:
        username: Gmail email address.
        password: Google app password https://support.google.com/accounts/answer/185833
        gmail_handle: An IMAP SSL object connected to Gmail.

    Returns:
        A generator of string(s).

    Side effect:
        Prints a message if there's no unread mail.

    Raises:
        None
    """

    try:
        gmail_handle.login(username, password)
        gmail_handle.select("INBOX")

        _, response = gmail_handle.search(None, "(UNSEEN)")

        if unread := response[0].split():
            for email_number in reversed(unread):
                status, email_data = gmail_handle.fetch(
                    email_number, "(BODY.PEEK[HEADER.FIELDS (subject)])"
                )
                if status == "OK":
                    yield message_from_bytes(
                        email_data[0][1], policy=policy.default  # pyright: ignore
                    )["subject"].strip().replace("\r\n", "")
        else:
            print("Inbox zero!")
            exit(0)

    except imaplib.IMAP4.error as e:
        print(e)

    finally:
        gmail_handle.close()
        gmail_handle.logout()


if __name__ == "__main__":
    try:
        for num, subject in enumerate(
            iterable=check_unread_email(
                gmail_handle=imaplib.IMAP4_SSL("imap.gmail.com"),
                **json.loads(os.getenv("GMAIL_LOGIN")),  # pyright: ignore
            ),
            start=1,
        ):
            print(f"{num}. {subject}")

    except (gaierror, ConnectionResetError):
        print("Could not connect to Gmail")