The main script created during Ambre Bertucci's talk "Learn Python automation by recreating Git Commit from scratch" at EuroPython 2021.

my_commit.py

from hashlib import sha1
from textwrap import dedent
from pathlib import Path
import sys
import zlib

OBJECT_PATH = Path(".git/objects")

EXCLUDED_FILES = {".git", "__pycache__", "my_commit.py"}

AUTHOR_IDENTITY = "Example User <example@localhost>"
COMMIT_TIMESTAMP = "1483920000 +0000"

def write_object(type_: str, content: bytes) -> str:
    """
    Write down the object with the provided content and type to
    the Git database and returns its hash
    """
    store = b"%s %d\0%s" % (type_.encode("utf8"), len(content), content)
    hash_ = sha1(store).hexdigest()
    compressed = zlib.compress(store)

    folder = OBJECT_PATH / hash_[:2]

    # Make sure that the folder exists
    if not folder.exists():
        folder.mkdir()

    # Write the file
    with open(folder / hash_[2:], "wb") as file:
        file.write(compressed)

    return hash_
 
def write_blob(path: Path) -> str:
    """Write the file to the database as a blob and return its SHA."""
    with open(path, "rb") as file:
        return write_object("blob", file.read())
        
def write_tree(path: Path) -> str:
    """Write the tree to the database and return its SHA."""
    lines = []
    
    children = list(path.iterdir())
    children.sort()

    for child in children:
        # Skip some problematic files.
        if child.name in EXCLUDED_FILES:
            continue

        # Register the file or folder to the database and add it to this tree
        if child.is_dir():
            hash_ = write_tree(child)
            mode = b"40000"
        else:
            hash_ = write_blob(child)
            mode = b"100644"

        # Create the line for this object
        hash_bytes = bytes.fromhex(hash_)
        line = b"%s %s\0%s" % (mode, child.name.encode("utf8"), hash_bytes)

        lines.append(line)
        
    content = b"".join(lines)
    return write_object("tree", content)

def write_commit(message: str) -> str:
    """Write the commit to the database and return its SHA."""
    tree = write_tree(Path("."))
    commit = dedent(f"""\
        tree {tree}
        author {AUTHOR_IDENTITY} {COMMIT_TIMESTAMP}
        committer {AUTHOR_IDENTITY} {COMMIT_TIMESTAMP}

        {message}
        """).encode("utf8")

    return write_object("commit", commit)

if __name__ == "__main__":
    if len(sys.argv) == 1:
        print("my_commit.py: no message provided.")
        exit(1)

    message = " ".join(sys.argv[1:])
    hash_ = write_commit(message)

    print(f"Created commit {hash_} !")