Q:

ValueError: Invalid padding bytes when decrypting with AES256

I am trying to rewrite this solution using cryptography library, but in the process of decryption I get padding error i.e. ValueError: Invalid padding bytes. Here is the cipher class I have so far:
import os
import base64

from cryptography.hazmat.primitives import hashes
from cryptography.hazmat.primitives import padding
from cryptography.hazmat.backends import default_backend
from cryptography.hazmat.primitives.ciphers import modes
from cryptography.hazmat.primitives.ciphers import Cipher
from cryptography.hazmat.primitives.ciphers import algorithms
from cryptography.hazmat.primitives.kdf.pbkdf2 import PBKDF2HMAC

class AES256Cipher:

    def __init__(self, password, salt):
        self.password = password.encode('utf-8')
        self.salt = salt.encode('utf-8')

        self.backend = default_backend()
        self.key = self.pbkdf2()

    def encrypt(self, plaintext):
        iv = os.urandom(16)
        cipher = Cipher(
            algorithms.AES(self.key),
            modes.CBC(iv),
            backend=self.backend
        )

        encryptor = cipher.encryptor()
        padder = padding.PKCS7(256).padder()
        plaintext = plaintext.rstrip().encode('utf-8')
        padded = padder.update(plaintext) + padder.finalize()
        ciphertext = encryptor.update(padded) + encryptor.finalize()
        return base64.b64encode(ciphertext)

    def decrypt(self, ciphertext):
        ciphertext = base64.b64decode(ciphertext.rstrip())
        iv = ciphertext[:16]
        cipher = Cipher(
            algorithms.AES(self.key),
            modes.CBC(iv),
            backend=self.backend
        )

        decryptor = cipher.decryptor()
        unpadder = padding.PKCS7(256).unpadder()
        plaintext = decryptor.update(ciphertext) + decryptor.finalize()
        unpadded = unpadder.update(plaintext) + unpadder.finalize()
        return unpadded.decode('utf-8')

    def pbkdf2(self):
        kdf = PBKDF2HMAC(
            algorithm=hashes.SHA256(),
            length=32, salt=self.salt,
            iterations=100000,
            backend=self.backend
        )
        return kdf.derive(self.password)

The exception is raised when code reaches unpadded = unpadder.update(plaintext) + unpadder.finalize() in decrypt method. Why is the padding failing? E.g.:
password = 'dummy_password'
salt = 'IU^7862390rZI)&(*hi23q2rfbnO(*^$%#'
cipher = AES256Cipher(password, salt)

ct = cipher.encrypt('secret_string')
cipher.decrypt(ct)  # <-- ValueError: Invalid padding bytes.

A:

Your decrypt method is extracting the first 16 bytes of the cipher as the IV, but your encrypt method never put it there. The fix is to have encrypt put the IV in front of the ciphertext prior to base64 encoding it.
return base64.b64encode(iv + ciphertext)

On the decrypt side, you must properly remove it:
iv, ciphertext = ciphertext[:16], ciphertext[16:]

