The cipher originally created by Blaise de Vigenère. Provides higher security than the Bellaso variant.
A short secret key is used to encrypt only the first few characters of the plaintext. From there, the keystream continues with the plaintext itself, shifted by the key length, rather than repeating the short key.
Since the keystream is never a short repeating sequence, the Kasiski test cannot be used to find a key length, making this cipher more secure than the repeating-key (Bellaso) variant.
You may find an implementation of Autokey cipher in sahithyandev/ciphers.
Attacking
Still vulnerable to statistical attacks, since the key length is typically much shorter than the message, so most of the keystream is plaintext of the same language, whose letter frequencies are known.