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Recipe 10.7. Backing Up Files
Credit: Anand Pillai, Tiago Henriques, Mario
Ruggier
Problem
You want to make
frequent backup copies of all files you have modified within a
directory tree, so that further changes won't
accidentally obliterate some of your editing.
Solution
Version-control systems, such as RCS, CVS, and SVN, are very powerful
and useful, but sometimes a simple script that you can easily edit
and customize can be even handier. The following script checks for
new files to back up in a tree that you specify. Run the script
periodically to keep your backup copies up to date.
import sys, os, shutil, filecmp
MAXVERSIONS=100
def backup(tree_top, bakdir_name='bakdir'):
for dir, subdirs, files in os.walk(tree_top):
# ensure each directory has a subdir called bakdir
backup_dir = os.path.join(dir, bakdir_name)
if not os.path.exists(newdir):
os.makedirs(newdir)
# stop any recursing into the backup directories
subdirs[:] = [d for d in subdirs if d != bakdir_name]
for file in files:
filepath = os.path.join(dir, file)
destpath = os.path.join(backup_dir, file)
# check existence of previous versions
for index in xrange(MAXVERSIONS):
backup = '%s.%2.2d' % (destpath, index)
if not os.path.exists(backup): break
if index > 0:
# no need to backup if file and last version are identical
old_backup = '%s.%2.2d' % (destpath, index-1)
try:
if os.path.isfile(old_backup
) and filecmp.cmp(abspath, old_backup, shallow=False):
continue
except OSError:
pass
try:
shutil.copy(filepath, backup)
except OSError:
pass
if _ _name_ _ == '_ _main_ _':
# run backup on the specified directory (default: the current directory)
try: tree_top = sys.argv[1]
except IndexError: tree_top = '.'
backup(tree_top)
Discussion
Although version-control systems are more powerful, this script can
be useful in development work. I often customize it, for example, to
keep backups only of files with certain extensions (or, when
that's handier, of all files
except those with certain extensions); it
suffices to add an appropriate test at the very start of the
for file in files loop, such as:
name, ext = os.path.splitext(file)
if ext not in ('.py', '.txt', '.doc'): continue
This snippet
first uses function splitext from the standard
library module os.path to extract the file
extension (starting with a period) into local variable
ext, then conditionally executes statement
continue, which passes to the next leg of the
loop, unless the extension is one of a few that happen to be the ones
of interest in the current subtree.Other potentially useful variants
include backing files up to some other subtree
(potentially on a removable drive, which has some clear advantages
for backup purposes) rather than the current one, compressing the
files that are being backed up (look at standard library module
gzip for this purpose), and more refined ones yet.
However, rather than complicating function backup by
offering all of these variants as options, I prefer to copy the
entire script to the root of each of the various subtrees of
interest, and customize it with a little simple editing. While this
strategy would be a very bad one for any kind of complicated, highly
reusable production-level code, it is reasonable for a simple,
straightforward system administration utility such as the one in this
recipe.Worthy of note in this recipe's implementation is
the use of function os.walk, a generator from the
standard Python library's module
os, which makes it very simple to iterate over all
or most of a filesystem subtree, with no need for such subtleties as
recursion or callbacks, just a straightforward for
statement. To avoid backing up the backups, this recipe uses one
advanced feature of os.walk: the second one of the
three values that os.walk yields at each step
through the loop is a list of subdirectories of the current
directory. We can modify this list in place,
removing some of the subdirectory names it contains. When we perform
such an in-place modification, os.walk does not
recurse through the subdirectories whose names we removed. The
following steps deal only with the subdirectories whose names are
left in. This subtle but useful feature of os.walk
is one good example of how a generator can receive information from
the code that's iterating on it, to affect details
of the iteration being performed.
See Also
Documentation of standard library modules os,
shutils, and gzip in the
Library Reference and Python in a
Nutshell.
