Read Large Files In Fixed Size Chunks

import os
from typing import BinaryIO, Iterator, Tuple


class ChunkedReadError(ValueError):
    """Raised when chunk parameters are invalid."""


def iter_chunks(path: str, chunk_size: int = 65536, offset: int = 0) -> Iterator[bytes]:
    """Yield the file at path in fixed-size byte chunks starting at offset."""
    if chunk_size < 1:
        raise ChunkedReadError("chunk_size must be >= 1")
    if offset < 0:
        raise ChunkedReadError("offset must be >= 0")
    if not os.path.isfile(path):
        raise FileNotFoundError(f"file not found: {path}")
    if offset > os.path.getsize(path):
        raise ChunkedReadError("offset is beyond the end of the file")

    with open(path, "rb") as handle:
        handle.seek(offset)
        while True:
            chunk = handle.read(chunk_size)
            if not chunk:
                return
            yield chunk


def read_all_chunks(path: str, chunk_size: int = 65536) -> Tuple[bytes, int]:
    """Return the whole file content and the total number of bytes read."""
    parts = []
    total = 0
    for chunk in iter_chunks(path, chunk_size=chunk_size):
        parts.append(chunk)
        total += len(chunk)
    return b"".join(parts), total