Backup Script in Python

Question

can some one please provide me with an explanation of the code especially the use of maxversions and statements following the line "for f in files:".

I want to understand what xrange(MAXVERSION) means? What is the use of indexing i.e

for index in xrange(MAXVERSIONS): backup = '%s.%2.2d' % (destpath, index)

The code:

!/usr/bin/env python

import sys,os, shutil, filecmp

MAXVERSIONS=100
BAKFOLDER = '.bak'

def backup_files(tree_top, bakdir_name=BAKFOLDER):

     top_dir = os.path.basename(tree_top)
    tree_top += os.sep

    for dir, subdirs, files in os.walk(tree_top):

        if os.path.isabs(bakdir_name):
            relpath = dir.replace(tree_top,'')
            backup_dir = os.path.join(bakdir_name, top_dir, relpath)
        else:
            backup_dir = os.path.join(dir, bakdir_name)

        if not os.path.exists(backup_dir):
            os.makedirs(backup_dir)

            subdirs[:] = [d for d in subdirs if d != bakdir_name]
        for f in files:
            filepath = os.path.join(dir, f)
            destpath = os.path.join(backup_dir, f)
                for index in xrange(MAXVERSIONS):
                backup = '%s.%2.2d' % (destpath, index)
                abspath = os.path.abspath(filepath)

                if index > 0:

                    old_backup = '%s.%2.2d' % (destpath, index-1)
                    if not os.path.exists(old_backup): break
                    abspath = os.path.abspath(old_backup)

                    try:
                        if os.path.isfile(abspath) and filecmp.cmp(abspath, filepath, shallow=False):
                            continue
                    except OSError:
                        pass

                try:
                    if not os.path.exists(backup):
                        print 'Copying %s to %s...' % (filepath, backup)
                        shutil.copy(filepath, backup)
                except (OSError, IOError), e:
                    pass

if __name__=="__main__":
    if len(sys.argv)<2:
        sys.exit("Usage: %s [directory] [backup directory]" % sys.argv[0])

    tree_top = os.path.abspath(os.path.expanduser(os.path.expandvars(sys.argv[1])))

    if len(sys.argv)>=3:
        bakfolder = os.path.abspath(os.path.expanduser(os.path.expandvars(sys.argv[2])))
    else:
        bakfolder = BAKFOLDER

    if os.path.isdir(tree_top):
        backup_files(tree_top, bakfolder)

No. Ask a specific question. What exactly are we supposed to do? Go through this entire program line by line? Use pdb for that and you can step through the code. — Falmarri
– Falmarri, Commented Feb 5, 2011 at 11:15
No offense, but no one is going to give you the full explanation of the code. You have to try it yourself first and ask only what is not clear. — user225312
– user225312, Commented Feb 5, 2011 at 11:16
@minnie: I'm sorry but that isn't much clearer. That whole block does quite a lot. Point out the areas of confusion (like "what does xrange(MAXVERSIONS) mean" etc.) and I'll vote to reopen. — Skurmedel
– Skurmedel, Commented Feb 5, 2011 at 12:19
Ok. Want to understand what does xrange(MAXVERSION)mean? What is the use of indexing i.e for index in xrange(MAXVERSIONS): backup = '%s.%2.2d' % (destpath, index) — minnie
– minnie, Commented Feb 5, 2011 at 12:25

Hugh Bothwell · Accepted Answer · 2011-02-08 17:11:00Z

3

The script tries to recursively copy the contents of a directory (defaults to current directory) to a backup directory (defaults to .bak in the current directory);

for each filename.ext, it creates a duplicate named filename.ext.00; if filename.ext.00 already exists, it creates filename.ext.01 instead, and so on.

xrange() is a generator which returns all numbers in 0..(MAXVERSION-1), so MAXVERSION controls how many version-suffixes to try, ie how many old versions of the file to keep.

answered Feb 8, 2011 at 17:11

Hugh Bothwell

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