PNG Text Polyglot generator, insert tEXt chunk into PNG using Python (made for micropython/circuitpython, so it conserves memory and has minimal dependencies)

polyglot.py

# Minimum index is 1, the IHDR block MUST be first for a valid PNG
# this assumes the skipped blocks aren't very large, so it reads the whole block into memory at once, beware of setting index to more than 1
# The script is also read into memory so don't make it too large
def insert_text_chunk(filein, fileout, text, index = 1):
    from binascii import crc32
    from struct import pack, unpack
    with open(filein, 'rb') as fin, open(fileout, 'wb') as fout:
        fout.write(fin.read(8)) # PNG Header

        # skip blocks
        for _ in range(index):
            fout.write(block_len := fin.read(4))
            block_len = unpack(">I", block_len)[0] # Len of IHDR BLOCK
            fout.write(fin.read(block_len + 4 + 4)) # + for CRC and Type

        #insert text block
                #   type, label null, newline to start script, script, newline to cleanly finish just cuz
        script_block = b'tEXt\0\n' + open('script.ps1','rb').read() + b'\n'
        block_len = len(script_block) - 4 # Len doesn't include type but CRC does because of course bloody hell
        crc = crc32(script_block)

        fout.write(pack(">I", block_len))
        fout.write(script_block)
        fout.write(pack(">I", crc))

        # copy rest of the file in chunks of 100 to reduce memory consumption
        while fout.write(fin.read(100)):
            pass

if __name__ == '__main__':
    import sys
    insert_text_chunk(sys.argv[1],sys.argv[2],  '\0\n' + open(sys.argv[3]).read(), 1)

Also 010 editor's script really helped me debug and understand the PNG file format while making this