A simple backup-pruning Python script

prune_backups.py

#!/usr/bin/env python3
from math import ceil
from pathlib import Path
from datetime import datetime

"""
A simple backup-pruning Python script
    created by Amunak;
    feel free to re-use and modify

Note that it uses a single pass for selecting the files and very simple logic to do so.
The files kept are CUMULATIVE, which means that if you select that you want last 20 files and last 4 weeks,
you will always retain the last 20 files and then 4 weeks ON TOP of that (same with months). This leads to often having
multiple files per week/month one after another, which is a limitation of the script (though not a big one IMO).
Additionally one file per year is kept.

Note that the files are simply sorted latest to oldest and the logic is applied from there, which means that for the past X
months you are guaranteed to get at least one file per month, but unlike usual pruning solutions you will get the LAST file
in that month as opposed to the first one. It might seem a bit ugly, but it keeps this script simple and doesn't matter in the end.
"""

# Arm the script? (True deletes files, False is a dry run)
arm = False

# The target directory
target_dir = '/var/backup/mysql'
# Glob pattern for matching files (so that you select only what you want)
glob_pattern = 'mysql-*.sql'
# Pattern for extracting date out of the filenames (gets passed to datetime.strptime)
date_pattern = 'mysql-%Y-%m-%d.sql'

# How many of each files to keep
keep_last = 10
keep_weeks = 4
keep_months = 6

def week_of_month(dt):
    first_day = dt.replace(day=1)

    dom = dt.day
    adjusted_dom = dom + first_day.weekday()

    return int(ceil(adjusted_dom/7.0))


def get_files(target):
    path = Path(target)
    files = [file for file in path.glob(glob_pattern) if file.is_file()]
    files.sort(reverse=True)

    return files


def __main__():
    week = 0
    weeks = keep_weeks
    month = 0
    months = keep_months
    year = 0
    count = 0
    files_to_remove = []
    for file in get_files(target_dir):
        try:
            date = datetime.strptime(file.name, date_pattern)
        except ValueError:
            print(f'Failed reading date from "{file.name}", skipping')
            continue

        count += 1
        print(f'File #{count} "{file.name}": ', end='')

        remove = True

        if count <= keep_last:
            print(f'{count} out of last {keep_last} that are always kept')
            remove = False
        elif week != week_of_month(date) and weeks > 0:
            week = week_of_month(date)
            print(f'First of week {week} (keeping week {weeks} out of {keep_weeks})')
            weeks -= 1
            remove = False
        elif month != date.month and months > 0:
            month = date.month
            print(f'First of month {month} (keeping month {months} out of {keep_months})')
            months -= 1
            remove = False
        elif year != date.year:
            year = date.year
            print(f'First of year {year}')
            remove = False

        if remove:
            print('REMOVING')
            files_to_remove.append(file)

    if arm:
        for file in files_to_remove:
            file.unlink()


if __name__ == '__main__':
    __main__()