that's actually really cool, i hadnt considered writing my own password manager but i feel like it'd be a fun and fairly useful project, did it take you particularly long to do? i'm interested in giving it a go :D
The heavy lifting is done by GPG in a subprocess, taking information on stdin or outputting the decrypted data on stdout. The rest is just generating the passwords, organizing the encrypted files, and perhaps interacting with the clipboard.
Have a look at https://www.passwordstore.org/ and also https://github.com/kmag/store_password_gpg