Simple backup class

backer.py

#!/usr/bin/env python3 

import os
import shutil

class Backer:
    """Implements a backup strategy for a file.  You might
       use this when preparing to overwrite an important configuration
       or log file that's not under version control."""

    def __init__(self, filename):
        """Create a new instance for a given file
        :param filename the full path to the file to be backed up.
        Backups will be created in the same directory."""
        self.base = os.path.basename(filename)
        self.orig = filename
        self.rpr = re.compile(f"{re.escape(self.base)}\\.?(\\d+)?")

    def backup(self, retain=5):
        """Backs up the file and rotates previous backups,
        retaining a specified number of previous backups.
        Copies the existing file to `filename.1`, and copies
        existing backups to the next higher backup number,
        retaining up to `retain` backup files, including the
        original.
        :param retain the number of backups (filename.N) to keep.
        """
        dirname = os.path.dirname(self.orig)
        keepers = [os.path.join(dirname, self.orig)]
        for i in range(1, retain + 1):
            keepers.append(os.path.join(dirname, f"{self.base}.{i}"))
        for i in range(len(keepers) - 2, -1, -1):
            if os.path.exists(keepers[i]):
                print(f"Copying {keepers[i]} to {keepers[i+1]}", file=sys.stderr)
                shutil.copyfile(keepers[i], keepers[i + 1])
        return keepers