Make a dated ZIP backup and verify what was archived

01Define what the backup should contain

The script will archive regular files from one source folder, including files in nested subfolders. Paths inside the ZIP are stored relative to the source folder, so a file such as reports/week1.txt stays reports/week1.txt instead of becoming an absolute filesystem path.

The archive name contains the current local calendar date in YYYY-MM-DD form. If the example is run on 2026-09-20, the expected filename is backup_demo_2026-09-20.zip. Running it again on the same day with the same destination causes the script to stop instead of replacing the previous backup.

02Create a synthetic source folder

The following dataset is synthetic and was written specifically for this article. Save the setup script as create_backup_demo.py. It creates four files with exact byte contents, including one file in a nested reports folder.

from pathlib import Path

SOURCE = Path("outputs") / "backup_demo"
FILES = {
    "notes.txt": b"alpha\n",
    "reports/week1.txt": b"beta\n",
    "reports/week2.txt": b"gamma\n",
    "settings.ini": b"mode=test\n",
}

SOURCE.parent.mkdir(parents=True, exist_ok=True)
SOURCE.mkdir()  # Stop if the synthetic source already exists.

for relative_name, content in FILES.items():
    target = SOURCE / relative_name
    target.parent.mkdir(parents=True, exist_ok=True)
    with target.open("xb") as stream:
        stream.write(content)

python create_backup_demo.py

The source contains 4 regular files totaling 27 bytes of uncompressed content: 6 + 5 + 6 + 10. The size of the final ZIP file itself cannot be predicted from that total because ZIP compression and archive metadata add their own overhead.

03Prepare the dated backup run

1. Save the main script as dated_zip_backup.py beside the setup script.
2. Keep SOURCE set to outputs/backup_demo for this example.
3. Keep BACKUP_DIR set to outputs/backups so the archive is outside the source folder.
4. Run the script from the same working directory.

python dated_zip_backup.py

The script checks that the source exists and that the backup directory is not inside the source directory. The backup parent folder may already exist. The dated ZIP file itself must not exist.

04Create the ZIP and verify every member

The script first builds a sorted list of source files. It creates the archive with exclusive file creation, writes each file with ZIP_DEFLATED compression, and then reopens the archive for verification. Verification compares the intended member list, checks CRC integrity with testzip, and compares source and archived SHA-256 hashes.

from datetime import date
import hashlib
from pathlib import Path
from zipfile import ZIP_DEFLATED, ZipFile

SOURCE = Path("outputs") / "backup_demo"
BACKUP_DIR = Path("outputs") / "backups"
CHUNK_SIZE = 1024 * 1024


def hash_file(path: Path) -> str:
    digest = hashlib.sha256()
    with path.open("rb") as stream:
        while chunk := stream.read(CHUNK_SIZE):
            digest.update(chunk)
    return digest.hexdigest()


def hash_member(archive: ZipFile, member_name: str) -> str:
    digest = hashlib.sha256()
    with archive.open(member_name, "r") as stream:
        while chunk := stream.read(CHUNK_SIZE):
            digest.update(chunk)
    return digest.hexdigest()


def main() -> None:
    source_root = SOURCE.resolve(strict=True)
    if not source_root.is_dir():
        raise ValueError("SOURCE must be a directory.")

    backup_root = BACKUP_DIR.resolve()
    if backup_root.is_relative_to(source_root):
        raise ValueError("BACKUP_DIR must be outside SOURCE.")

    files = sorted(
        path for path in source_root.rglob("*")
        if path.is_file() and not path.is_symlink()
    )
    if not files:
        raise ValueError("SOURCE contains no regular files to back up.")

    BACKUP_DIR.mkdir(parents=True, exist_ok=True)
    archive_name = f"{source_root.name}_{date.today().isoformat()}.zip"
    archive_path = BACKUP_DIR / archive_name
    if archive_path.exists():
        raise FileExistsError(f"Backup already exists: {archive_path}")

    expected = {}
    with archive_path.open("xb") as raw:
        with ZipFile(raw, "w", compression=ZIP_DEFLATED) as archive:
            for path in files:
                relative = path.relative_to(source_root).as_posix()
                expected[relative] = (path.stat().st_size, hash_file(path))
                archive.write(path, arcname=relative)

    with ZipFile(archive_path, "r") as archive:
        bad_member = archive.testzip()
        if bad_member is not None:
            raise RuntimeError(f"CRC check failed: {bad_member}")

        members = [info for info in archive.infolist() if not info.is_dir()]
        actual_names = [info.filename for info in members]
        expected_names = list(expected)
        if actual_names != expected_names:
            raise RuntimeError("Archive member list does not match the source list.")

        for info in members:
            expected_size, expected_hash = expected[info.filename]
            if info.file_size != expected_size:
                raise RuntimeError(f"Size mismatch: {info.filename}")
            if hash_member(archive, info.filename) != expected_hash:
                raise RuntimeError(f"Hash mismatch: {info.filename}")

    total_bytes = sum(size for size, _ in expected.values())
    print(f"Verified {len(expected)} files totaling {total_bytes} bytes.")
    print(f"Archive: {archive_path.as_posix()}")


if __name__ == "__main__":
    main()

The code skips symbolic links and archives regular files only. The expected hashes are calculated from the source immediately before each file is added. For a normal local folder that remains unchanged during the run, this provides a useful content-level comparison after the archive is created.

05Check the expected archive by hand

If the script is run on 2026-09-20, the expected archive path is outputs/backups/backup_demo_2026-09-20.zip. The ZIP should contain exactly the following four member paths.

The nested reports folder is represented by the member paths themselves. The script does not need a separate directory entry for reports because the files already carry that relative path.

The expected console output below is derived manually for a run on 2026-09-20. It is not a captured execution log.

Verified 4 files totaling 27 bytes.
Archive: outputs/backups/backup_demo_2026-09-20.zip

06Perform a few independent checks

1. Open the ZIP with an archive viewer and confirm that there are exactly 4 files with the expected relative paths.
2. Extract the archive into a separate temporary location and compare the text contents with the 4 source files.
3. Confirm that the original files still exist in outputs/backup_demo after the backup completes.
4. Run the backup script again on the same date without changing the destination. It should stop with FileExistsError instead of replacing the existing archive.
5. Change a copied source file in a separate test and create a new backup on a different archive name if you want to confirm that verification follows the new content.

Do not use the archive file size as the primary verification check. Compression means a valid archive can be smaller or larger than the uncompressed source total, depending on the data and metadata.

07Recognize common errors

If an exception occurs after the ZIP file has been created, a partial or unverified archive can remain in the backup folder. The script does not automatically delete it because automatic cleanup can hide evidence useful for diagnosing the failure.

08Understand what this backup does not guarantee

This is a file-copy archive workflow, not a transactional filesystem snapshot. If another application modifies a file while it is being hashed or archived, the result can become inconsistent. For important data, stop writes to the source or use a snapshot-capable backup system.

The example does not preserve every filesystem property. Permissions, ownership, access-control lists, extended attributes, alternate data streams, symbolic links, and application-specific metadata may require a different backup tool. ZIP is useful for portable file collections, but it is not a complete image of a filesystem.

A verified archive is still only one copy. A real backup plan should consider multiple copies, storage failure, accidental deletion, ransomware, retention periods, encryption, and whether recovery has actually been tested. Verification here shows that this archive matched the source content observed during this run; it does not establish a complete disaster-recovery strategy.

Execution and verification record

2026-09-21 · ChatGPT Python tool · Python 3.13.5 · standard library datetime/hashlib/pathlib/zipfile

• The synthetic source contained exactly 4 regular files totaling 27 bytes: notes.txt=6, reports/week1.txt=5, reports/week2.txt=6, settings.ini=10.
• The actual run printed 'Verified 4 files totaling 27 bytes.' and 'Archive: outputs/backups/backup_demo_2026-09-21.zip'.
• The article's 2026-09-20 filename and console example are explicitly conditional on running the script on 2026-09-20; the runtime correctly used the actual local date, 2026-09-21.
• The ZIP contained exactly notes.txt, reports/week1.txt, reports/week2.txt, and settings.ini in the expected order.
• ZIP member uncompressed sizes were exactly 6, 5, 6, and 10 bytes.
• ZipFile.testzip() returned no bad member.
• SHA-256 values of every archived member matched the corresponding source file.
• All source-file SHA-256 hashes were unchanged after archive creation.
• Running the script again with the same dated archive present raised FileExistsError instead of replacing it.

References

The explanations and examples were written for this site. See the official sources below for the related behavior and concepts.

• Official Python 3.12 documentation · zipfile ↗ (New tab)
• Official Python 3.12 documentation · hashlib ↗ (New tab)
• Official Python 3.12 documentation · pathlib ↗ (New tab)
• Official Python 3.12 documentation · datetime ↗ (New tab)