Implementation of Base64 encoding in Python3

base64.py

#!/usr/bin/env python3

"""Script to encode & decode strings to and from Base64"""

import sys
import argparse

CHARS = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/"
PAD = "="
LINE_WIDTH = 76

b64_ord = dict(zip(CHARS, range(64)))
ord_b64 = dict(zip(range(64), CHARS))

parser = argparse.ArgumentParser(prog='Base64',
                                 description='Encode/decode strings to/from Base64.')
arg_grp = parser.add_mutually_exclusive_group(required=True)
arg_grp.add_argument("-d", action="store", type=str, help="decode <string>",
                     metavar="DECODE")
arg_grp.add_argument("-e", action="store", type=str, help="encode <string>",
                     metavar="ENCODE")

args = parser.parse_args()

# DECODE
if args.d:
    b64_data = args.d
    out = ""
    byte_arr = []

    for char in b64_data:
        if char == PAD:
            break
        if char == '\n':
            continue
        try:
            byte_arr.append(format(int(b64_ord[char]), '08b')[2:])
        except KeyError:
            print(f'Invalid Base64 character: {char}')
            sys.exit(2)

    byte_arr = ''.join(byte_arr)

    pos = 0
    for _ in byte_arr:
        byte = byte_arr[pos:pos+8]
        if byte:  # Empty string means we are done.
            out += chr(int(byte, 2))
        pos += 8

    print(out)

#ENCODE
if args.e:
    ascii_data = args.e
    b64_out = ""
    byte_arr = []

    for char in ascii_data:
        byte_arr.append(format(ord(char), '08b'))
    byte_arr = ''.join(byte_arr)
    pads_needed = len(byte_arr) % 3

    pos = 0
    line_len = 0
    for _ in byte_arr[:0 - pads_needed - 6]:
        byte = byte_arr[pos:pos+6]
        if byte:
            b64_out += ord_b64[(int(byte, 2))]
        else:
            break
        pos += 6
        if line_len == LINE_WIDTH:
            b64_out += '\n'
            line_len = -1
        line_len += 1

    b64_out += PAD * pads_needed

    print(b64_out)