Using Python's os Module for File Operations

From Clutter to Structure

A backup script can work correctly and still become difficult to use. If every archive is stored in one directory with a long timestamped filename, repeated backups can produce dozens of similarly named files. A hierarchical naming scheme improves this situation by dividing the backups first by date and then by time.

The Three-Level Backup Path

The main directory is the common home for all backups. Each date-based subdirectory groups backups made on one day. Each zip file inside that date directory represents a backup made at a particular time. This makes it easier to navigate to a day and then distinguish the backups created during that day.

Turning Time into Names

The time.strftime function returns a formatted string representing the current date or time. In this backup design, one format produces the date directory name and another produces the zip filename. The format code %Y%m%d produces a year-month-day string such as 20240115, while %H%M%S produces a 24-hour time string such as 143022.

import time

date_name = time.strftime('%Y%m%d')
time_name = time.strftime('%H%M%S')

print(date_name)
print(time_name + '.zip')

Checking Before Creating

Before saving an archive into the date-based directory, the script must determine whether that directory already exists. os.path.exists performs the check. If the directory exists, the script can use it. If it does not exist, os.mkdir creates it. This check matters when the script runs repeatedly, because a second backup on the same day should use the existing date directory rather than attempt to create it again.

import os import time backup_dir = '/Users/swa/backup' date_name = time.strftime('%Y%m%d') date_dir = backup_dir + '/' + date_name if not os.path.exists(date_dir): os.mkdir(date_dir)

Building the Archive Command

After the directory and names are ready, the script constructs a system command for creating the compressed archive. The command contains the target zip path and the source paths to be archived. The resulting command is passed to os.system, which executes it as an operating-system command.

target = '/Users/swa/backup/20240115/143022.zip'
source = '/Users/swa/notes'
command = 'zip -r ' + target + ' ' + source
result = os.system(command)

Tracing a Second Backup

Imagine that the script runs on January 15, 2024, at 2:30:22 PM and then runs again on the same day at 6:05:00 PM. The date string remains the same, so both backups belong in the 20240115 directory. The time string changes, so the second backup receives a different time-stamped zip filename.

Mistakes Beginners Make

• Creating the date directory every time

  A repeated run on the same day may encounter a directory that was already created by an earlier run.

  Fix: Use os.path.exists first and call os.mkdir only when the directory is absent.

• Putting the full timestamp into one flat filename

  Repeated backups can make the directory cluttered with similarly named files, making a particular day harder to find.

  Fix: Use the date portion for a subdirectory and the time portion for the zip filename.

• Ignoring the result from os.system

  The script loses the supplied success or failure signal for the archive operation.

  Fix: Check whether the returned value is 0 for success or non-zero for failure.

• Mixing up the date and time formats

  The intended hierarchy uses the date to group backups and the time to distinguish archives within that group.

  Fix: Use %Y%m%d for the date directory and %H%M%S for the zip filename.

Practical Design Notes

import os
import time

backup_dir = '/Users/swa/backup'
source = '/Users/swa/notes'
date_name = time.strftime('%Y%m%d')
time_name = time.strftime('%H%M%S')
date_dir = backup_dir + '/' + date_name
target = date_dir + '/' + time_name + '.zip'

if not os.path.exists(date_dir):
    os.mkdir(date_dir)

command = 'zip -r ' + target + ' ' + source
status = os.system(command)

if status == 0:
    print('Backup completed')
else:
    print('Backup failed')

Practice the Trace

Key Takeaways

1. A hierarchical backup path uses a main directory, a date-based subdirectory, and a time-stamped zip file.
2. time.strftime('%Y%m%d') supplies the date directory name, while time.strftime('%H%M%S') supplies the time-based archive name.
3. os.path.exists checks whether the date directory is already present.
4. os.mkdir creates the date directory only when the check shows that it is absent.
5. os.system runs the zip command, and its return value is 0 for success and non-zero for failure.