https://medium.com/@hang.c/processing-large-s3-files-with-aws-lambda-2c5840ae5c91

get_object_bodylines.py

def get_object_bodylines(s3_object, offset):
    resp = s3_object.get(Range=f'bytes={offset}-')
    body: botocore.response.StreamingBody = resp['Body']
    return BodyLines(body)


class BodyLines:
    def __init__(self, body: botocore.response.StreamingBody, initial_offset=0):
        self.body = body
        self.offset = initial_offset

    def iter_lines(self, chunk_size=1024):
        """Return an iterator to yield lines from the raw stream.

        This is achieved by reading chunk of bytes (of size chunk_size) at a
        time from the raw stream, and then yielding lines from there.
        """
        pending = b''
        for chunk in self.body.iter_chunks(chunk_size):
            lines = (pending + chunk).splitlines(True)
            for line in lines[:-1]:
                self.offset += len(line)
                yield line.decode('utf-8')
            pending = lines[-1]
        if pending:
            self.offset += len(pending)
            yield pending.decode('utf-8')