Why is the Python calculated "hashlib.sha1" different from "git hash-object" for a file?

I'm trying to calculate the SHA-1 value of a file.

I've fabricated this script:

def hashfile(filepath):
    sha1 = hashlib.sha1()
    f = open(filepath, 'rb')
    try:
        sha1.update(f.read())
    finally:
        f.close()
    return sha1.hexdigest()

For a specific file I get this hash value:
8c3e109ff260f7b11087974ef7bcdbdc69a0a3b9
But when i calculate the value with git hash_object, then I get this value: d339346ca154f6ed9e92205c3c5c38112e761eb7

How come they differ? Am I doing something wrong, or can I just ignore the difference?