Convert image to Python string

#!/usr/bin/env python3
# img2py: Convert a black and white image to a pyimage.
# Copyright (C) 2022  Jeffry Johnston
#
# This program is free software: you can redistribute it and/or modify
# it under the terms of the GNU General Public License as published by
# the Free Software Foundation, either version 3 of the License, or
# (at your option) any later version.
#
# This program is distributed in the hope that it will be useful,
# but WITHOUT ANY WARRANTY; without even the implied warranty of
# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
# GNU General Public License for more details.
#
# You should have received a copy of the GNU General Public License
# along with this program.  If not, see <https://www.gnu.org/licenses/>.

import sys
from PIL import Image

# Parse command line arguments
if len(sys.argv) != 3:
    sys.exit(f"Usage: {sys.argv[0]} B&W_IMAGE.png VAR_NAME")
filename = sys.argv[1]
var_name = sys.argv[2]

# Convert every 7 image pixels to one ASCII character
with Image.open(filename) as im:
    width, height = im.size
    value = 0
    bits = 0
    original_data: list[int] = [1, width, height]
    for y in range(height):
        for x in range(width):
            data = im.getpixel((x, y)) # type: ignore
            pixel = int(
                (data[0] if type(data) is tuple else data) == 0 # type: ignore
            )

            # This "backward" bit order was chosen for a few reasons:
            # 1) Trivial to decode with & and >>
            # 2) Any needed padding bits at the end of the image are automatic
            #    and their presence can be completely ignored by the decoder
            # 3) No special case when the last character is padded and repeated
            value |= pixel << bits
            bits += 1
            if bits >= 7:
                original_data.append(value)
                value = 0
                bits = 0
    if bits > 0:
        original_data.append(value)

# Determine which bytes are used
dist: dict[int, int] = {}
for i in range(128):
    dist[i] = False
for i in original_data:
    dist[i] = True
used = [i for i in dist if dist[i]]

# Add an offset to each byte in the hopes of avoiding restricted characters
for offset in range(128):
    invalid = False
    for i in [0, 10, 13, 34, 92]:
        if i in used:
            invalid = True
            break
    if not invalid:
        break
    for j in range(len(used)):
        used[j] = (used[j] + 1) % 128
else:
    sys.exit("Failed to find a valid offset")

# Convert data to a Python string
offset_data = "".join([chr((i + offset) % 128) for i in original_data])
var_str = f"{var_name}=b\"{offset_data}\""

# Write output
sys.stdout.buffer.write(var_str.encode("latin1"))