Used for better understanding of Base64 encoding.

Encoder.py

def solution(encoding, message):
    class BaseEncoder():
        BASE64_CHARS = 'ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/'
    
        UTF_8_BYTE_SIZE, B64_BYTE_SIZE, BINARY_BASE = 8, 6, 2
    
        @staticmethod
        def encode(message, in_byte_size, out_byte_size, char_set):
            enc_msg, padding = '', ''
            # Get the ascii code and convert that to binary.  ord('A') = 65;  bin(65) = 0b1000001
            # Pad right with zero since ASCII is 7-bit and we want 8-bit bytes before chunking to 6-bit bytes.
            message_in_binary = "".join(
                format(ord(x), 'b').zfill(in_byte_size) for x in message)

            for i in range(0, len(message_in_binary), out_byte_size):
                chunk = message_in_binary[i:i + out_byte_size]

                if len(chunk) < out_byte_size:
                    diff = out_byte_size - len(chunk)
                    chunk += '0' * diff  # or chunk.zfill(out_byte_size)
                    padding = '=' * (diff // 2)
    
                enc_msg += char_set[int(chunk, BaseEncoder.BINARY_BASE)]
            return enc_msg + padding
    
        @staticmethod
        def decode(message, in_byte_size, out_byte_size, char_set):
            # Get dictionary of character to index of the base64 table.
            message = message.replace('=', '')
            dec_msg = ''
            d_char_to_index = {c: i for i, c in enumerate(char_set)}
    
            # Get binary representation of the b64 character codes and ensure padded 6-bit chunks.
            message_in_binary = ''.join([
                format(d_char_to_index[c], 'b').zfill(in_byte_size)
                for c in message
            ])
            for i in range(0, len(message_in_binary), out_byte_size):
                chunk = message_in_binary[i:i + out_byte_size].zfill(out_byte_size)

                if chunk != '0' * out_byte_size:
                    # Handle the last padded string.  Probably at end of string anyways.
                    dec_msg += chr(int(chunk, BaseEncoder.BINARY_BASE))
    
            return dec_msg
  

    alt_base64_chars = BaseEncoder.BASE64_CHARS[:-2] + encoding

    return BaseEncoder.decode(message, BaseEncoder.B64_BYTE_SIZE, BaseEncoder.UTF_8_BYTE_SIZE,
                  alt_base64_chars)