Hash a large file in Python

large-file-hash.py

import hashlib as hash

# Specify how many bytes of the file you want to open at a time
BLOCKSIZE = 65536

sha = hash.sha256()
with open('kali.iso', 'rb') as kali_file:
    file_buffer = kali_file.read(BLOCKSIZE)
    while len(file_buffer) > 0:
        sha.update(file_buffer)
        file_buffer = kali_file.read(BLOCKSIZE)
        
print sha.hexdigest()

Don't forget that an empty file_buffer (of length 0) is False-y, so you can just use while file_buffer: there without checking its length manually.

Also for CPython >= 3.8, you can use the "walrus operator":

with open(...) as kali_file:
    while (file_buffer := kali_file.read(...)):
        sha.update(file_buffer)

Just for the sake of making a function.

import hashlib
import pathlib

# Specify how many bytes of the file you want to open at a time
DEFAULT_BLOCK_SIZE = 65536


def compute_sha256_hash(path: pathlib.Path, buffer_size: int = DEFAULT_BLOCK_SIZE) -> str:
    """Compute the SHA256 hash of a potentially large file.

    Args:
        path (pathlib.Path): the path to the file.
        buffer_size (int, optional): the buffer size in bytes. Defaults to DEFAULT_BLOCK_SIZE.

    Returns:
        str: the sha256 hash of the file.
    """
    if isinstance(path, str):
        path = pathlib.Path(path)

    sha256_hash = hashlib.sha256()

    with path.open('rb') as file:
        while file_buffer := file.read(buffer_size):
            sha256_hash.update(file_buffer)

    return sha256_hash.hexdigest()


print(f"{compute_sha256_hash('kali.iso') = }")