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Recipe 2.29. Versioning Filenames
Credit: Robin Parmar, Martin Miller
Problem
You want to make a backup copy of a file, before you overwrite it,
with the standard convention of appending a three-digit version
number to the name of the old file.
Solution
We just need to code a function to perform the backup copy
appropriately:
def VersionFile(file_spec, vtype='copy'):
import os, shutil
if os.path.isfile(file_spec):
# check the 'vtype' parameter
if vtype not in ('copy', 'rename'):
raise ValueError, 'Unknown vtype %r' % (vtype,)
# Determine root filename so the extension doesn't get longer
n, e = os.path.splitext(file_spec)
# Is e a three-digits integer preceded by a dot?
if len(e) == 4 and e[1:].isdigit( ):
num = 1 + int(e[1:])
root = n
else:
num = 0
root = file_spec
# Find next available file version
for i in xrange(num, 1000):
new_file = '%s.%03d' % (root, i)
if not os.path.exists(new_file):
if vtype == 'copy':
shutil.copy(file_spec, new_file)
else:
os.rename(file_spec, new_file)
return True
raise RuntimeError, "Can't %s %r, all names taken"%(vtype,file_spec)
return False
if _ _name_ _ == '_ _main_ _':
import os
# create a dummy file 'test.txt'
tfn = 'test.txt'
open(tfn, 'w').close( )
# version it 3 times
print VersionFile(tfn)
# emits: True
print VersionFile(tfn)
# emits: True
print VersionFile(tfn)
# emits: True
# remove all test.txt* files we just made
for x in ('', '.000', '.001', '.002'):
os.unlink(tfn + x)
# show what happens when the file does not exist
print VersionFile(tfn)
# emits: False
print VersionFile(tfn)
# emits: False
Discussion
The purpose of the VersionFile function is to ensure
that an existing file is copied (or renamed, as indicated by the
optional second parameter) before you open it for writing or updating
and therefore modify it. It is polite to make such backups of files
before you mangle them (one functionality some people still pine for
from the good old VMS operating system, which performed it
automatically!). The actual copy or renaming is performed by
shutil.copy and os.rename,
respectively, so the only issue is which name to use as the target.A popular way to determine backups' names is
versioning (i.e., appending to the filename a gradually incrementing
number). This recipe determines the new name by first extracting the
filename's root (just in case you call it with an
already-versioned filename) and then successively appending to that
root the further extensions .000,
.001, and so on, until a name built in this
manner does not correspond to any existing file. Then, and only then,
is the name used as the target of a copy or renaming. Note that
VersionFile is limited to 1,000 versions, so you
should have an archive plan after that. The file must exist before it
is first versionedyou cannot back up what does not yet exist.
However, if the file doesn't exist, function
VersionFile simply returns False
(while it returns TRue if the file exists and has
been successfully versioned), so you don't need to
check before calling it!
See Also
Documentation for the os and
shutil modules in the Library
Reference and Python in a
Nutshell.
