Like most kids, mine spend a lot of time on the Internet. As far as I can tell, it’s the thing to do these days. Among this latest generation, computer geeks and gurus seem to be held in the same sort of esteem that my generation once held rock stars. When kids disappear into their rooms, the chances are good that they are hacking on computers, not mastering guitar riffs. It’s probably healthier than some of the diversions of my own misspent youth, but that’s a topic for another kind of book.
If you have teenage kids and computers, or know someone who does, you probably know that it’s not a bad idea to keep tabs on what those kids do on the Web. Type your favorite four-letter word in almost any web search engine and you’ll understand the concern—it’s much better stuff than I could get during my teenage career. To sidestep the issue, only a few of the machines in my house have Internet feeds.
While they’re on one of these machines, my kids download lots of games. To avoid infecting our Very Important Computers with viruses from public-domain games, though, my kids usually have to download games on a computer with an Internet feed and transfer them to their own computers to install. The problem is that game files are not small; they are usually much too big to fit on a floppy (and burning a CD takes away valuable game-playing time).
If all the machines in my house ran Linux, this would be a nonissue. There are standard command-line programs on Unix for chopping a file into pieces small enough to fit on a floppy (split), and others for putting the pieces back together to re-create the original file (cat). Because we have all sorts of different machines in the house, though, we needed a more portable solution.[*]
Since all the computers in my house run Python, a simple portable Python script came to the rescue. The Python program in Example 6-1 distributes a single file’s contents among a set of part files and stores those part files in a directory.
Example 6-1. PP3ESystemFiletoolssplit.py
#!/usr/bin/python ########################################################################## # split a file into a set of parts; join.py puts them back together; # this is a customizable version of the standard Unix split command-line # utility; because it is written in Python, it also works on Windows and # can be easily modified; because it exports a function, its logic can # also be imported and reused in other applications; ########################################################################## import sys, os kilobytes = 1024 megabytes = kilobytes * 1000 chunksize = int(1.4 * megabytes) # default: roughly a floppy def split(fromfile, todir, chunksize=chunksize): if not os.path.exists(todir): # caller handles errors os.mkdir(todir) # make dir, read/write parts else: for fname in os.listdir(todir): # delete any existing files os.remove(os.path.join(todir, fname)) partnum = 0 input = open(fromfile, 'rb') # use binary mode on Windows while 1: # eof=empty string from read chunk = input.read(chunksize) # get next part <= chunksize if not chunk: break partnum = partnum+1 filename = os.path.join(todir, ('part%04d' % partnum)) fileobj = open(filename, 'wb') fileobj.write(chunk) fileobj.close() # or simply open().write( ) input.close( ) assert partnum <= 9999 # join sort fails if 5 digits return partnum if _ _name_ _ == '_ _main_ _': if len(sys.argv) == 2 and sys.argv[1] == '-help': print 'Use: split.py [file-to-split target-dir [chunksize]]' else: if len(sys.argv) < 3: interactive = 1 fromfile = raw_input('File to be split? ') # input if clicked todir = raw_input('Directory to store part files? ') else: interactive = 0 fromfile, todir = sys.argv[1:3] # args in cmdline if len(sys.argv) == 4: chunksize = int(sys.argv[3]) absfrom, absto = map(os.path.abspath, [fromfile, todir]) print 'Splitting', absfrom, 'to', absto, 'by', chunksize try: parts = split(fromfile, todir, chunksize) except: print 'Error during split:' print sys.exc_info()[0], sys.exc_info( )[1] else: print 'Split finished:', parts, 'parts are in', absto if interactive: raw_input('Press Enter key') # pause if clicked
By default, this script splits the input file into chunks that are roughly the size of a floppy disk—perfect for moving big files between electronically isolated machines. Most importantly, because this is all portable Python code, this script will run on just about any machine, even ones without their own file splitter. All it requires is an installed Python. Here it is at work splitting the Python 1.5.2 self-installer executable on Windows:
C: emp>echo %X%
shorthand shell variable
C:PP3rdEdexamplesPP3E C: emp>ls -l py152.exe
-rwxrwxrwa 1 0 0 5028339 Apr 16 1999 py152.exe C: emp>python %X%SystemFiletoolssplit.py -help
Use: split.py [file-to-split target-dir [chunksize]] C: emp>python %X%SystemFiletoolssplit.py py152.exe pysplit
Splitting C: emppy152.exe to C: emppysplit by 1433600 Split finished: 4 parts are in C: emppysplit C: emp>ls -l pysplit
total 9821 -rwxrwxrwa 1 0 0 1433600 Sep 12 06:03 part0001 -rwxrwxrwa 1 0 0 1433600 Sep 12 06:03 part0002 -rwxrwxrwa 1 0 0 1433600 Sep 12 06:03 part0003 -rwxrwxrwa 1 0 0 727539 Sep 12 06:03 part0004
Each of these four generated part files represents one binary
chunk of the file py152.exe—aa chunk small
enough to fit comfortably on a floppy disk. In fact, if you add the
sizes of the generated part files given by the ls
command, you’ll come up with 5,028,339
bytes—exactly the same as the original file’s size. Before we see
how to put these files back together again, let’s explore a few of
the splitter script’s finer points.
This script is designed to input its parameters in either interactive or command-line mode; it checks the number of command-line arguments to find out the mode in which it is being used. In command-line mode, you list the file to be split and the output directory on the command line, and you can optionally override the default part file size with a third command-line argument.
In interactive mode, the script asks for a filename and
output directory at the console window with raw_input
and pauses for a key press at
the end before exiting. This mode is nice when the program file is
started by clicking on its icon; on Windows, parameters are typed
into a pop-up DOS box that doesn’t automatically disappear. The
script also shows the absolute paths of its parameters (by running
them through os.path.abspath
)
because they may not be obvious in interactive mode. We’ll see
examples of other split modes at work in a moment.
This code is careful to open both input and output
files in binary mode (rb
,
wb
), because it needs to
portably handle things like executables and audio files, not just
text. In Chapter 4, we
learned that on Windows, text-mode files automatically map
end-of-line sequences to
on input and map
to
on output. For true binary data, we
really don’t want any
characters in the data to go away when read, and we don’t want any
superfluous
characters to be
added on output. Binary-mode files suppress this
mapping when the script is run on
Windows and so avoid data corruption.
This script also goes out of its way to manually close its files. For instance:
fileobj = open(partname, 'wb') fileobj.write(chunk) fileobj.close( )
As we also saw in Chapter 4, these three lines can usually be replaced with this single line:
open(partname, 'wb').write(chunk)
This shorter form relies on the fact that the current Python
implementation automatically closes files for you when file
objects are reclaimed (i.e., when they are garbage collected,
because there are no more references to the file object). In this
line, the file object would be reclaimed immediately, because the
open
result is temporary in an
expression and is never referenced by a longer-lived name.
Similarly, the input
file is
reclaimed when the split
function exits.
As I was writing this chapter, though, there was some
possibility that this automatic-close behavior may go away in the
future. Moreover, the Jython Java-based Python implementation does
not reclaim unreferenced objects as immediately as the standard
Python. If you care about the Java port, your script may
potentially create many files in a short amount of time, and it
may run on a machine that has a limit on the number of open files
per program and then close manually. The close
calls in this script have never
been necessary for my purposes, but because the split
function in this module is
intended to be a general-purpose tool, it accommodates such
worst-case scenarios.
Back to moving big files around the house: after downloading a big game program file, my kids generally run the previous splitter script by clicking on its name in Windows Explorer and typing filenames. After a split, they simply copy each part file onto its own floppy, walk the floppies upstairs, and re-create the split output directory on their target computer by copying files off the floppies. Finally, the script in Example 6-2 is clicked or otherwise run to put the parts back together.
Example 6-2. PP3ESystemFiletoolsjoin.py
#!/usr/bin/python ########################################################################## # join all part files in a dir created by split.py, to re-create file. # This is roughly like a 'cat fromdir/* > tofile' command on unix, but is # more portable and configurable, and exports the join operation as a # reusable function. Relies on sort order of filenames: must be same # length. Could extend split/join to pop up Tkinter file selectors. ########################################################################## import os, sys readsize = 1024 def join(fromdir, tofile): output = open(tofile, 'wb') parts = os.listdir(fromdir) parts.sort( ) for filename in parts: filepath = os.path.join(fromdir, filename) fileobj = open(filepath, 'rb') while 1: filebytes = fileobj.read(readsize) if not filebytes: break output.write(filebytes) fileobj.close( ) output.close( ) if _ _name_ _ == '_ _main_ _': if len(sys.argv) == 2 and sys.argv[1] == '-help': print 'Use: join.py [from-dir-name to-file-name]' else: if len(sys.argv) != 3: interactive = 1 fromdir = raw_input('Directory containing part files? ') tofile = raw_input('Name of file to be recreated? ') else: interactive = 0 fromdir, tofile = sys.argv[1:] absfrom, absto = map(os.path.abspath, [fromdir, tofile]) print 'Joining', absfrom, 'to make', absto try: join(fromdir, tofile) except: print 'Error joining files:' print sys.exc_info()[0], sys.exc_info( )[1] else: print 'Join complete: see', absto if interactive: raw_input('Press Enter key') # pause if clicked
After running the join
script, my kids still may need to run something like zip
, gzip
, or tar
to unpack an archive file, unless it’s
shipped as an executable;[*] but at least they’re much closer to seeing the
Starship Enterprise spring into action. Here is a join in progress
on Windows, combining the split files we made a moment ago:
C: emp>python %X%SystemFiletoolsjoin.py -help
Use: join.py [from-dir-name to-file-name] C: emp>python %X%SystemFiletoolsjoin.py pysplit mypy152.exe
Joining C: emppysplit to make C: empmypy152.exe Join complete: see C: empmypy152.exe C: emp>ls -l mypy152.exe py152.exe
-rwxrwxrwa 1 0 0 5028339 Sep 12 06:05 mypy152.exe -rwxrwxrwa 1 0 0 5028339 Apr 16 1999 py152.exe C: emp>fc /b mypy152.exe py152.exe
Comparing files mypy152.exe and py152.exe FC: no differences encountered
The join script simply uses os.listdir
to collect all the part files
in a directory created by split
,
and sorts the filename list to put the parts back together in the
correct order. We get back an exact byte-for-byte copy of the
original file (proved by the DOS fc
command in the code; use cmp
on Unix).
Some of this process is still manual, of course (I haven’t
quite figured out how to script the “walk the floppies upstairs” bit
yet), but the split
and join
scripts make it both quick and simple
to move big files around. Because this script is also portable
Python code, it runs on any platform to which we care to move split
files. For instance, my kids typically download both Windows and
Linux games; since this script runs on either platform, they’re
covered.
Before we move on, there are a couple of details
worth underscoring in the join
script’s code. First of all, notice that this script deals with
files in binary mode but also reads each part file in blocks of 1
KB each. In fact, the readsize
setting here (the size of each block read from an input part file)
has no relation to chunksize
in
split.py (the total size of each output part
file). As we learned in Chapter
4, this script could instead read each part file all at
once:
filebytes = open(filepath, 'rb').read( ) output.write(filebytes)
The downside to this scheme is that it really does load all
of a file into memory at once. For example, reading a 1.4 MB part
file into memory all at once with the file object read
method generates a 1.4 MB string in
memory to hold the file’s bytes. Since split
allows users to specify even
larger chunk sizes, the join
script plans for the worst and reads in terms of limited-size
blocks. To be completely robust, the split
script could read its input data
in smaller chunks too, but this hasn’t become a concern in
practice (recall that as your program runs, Python automatically
reclaims strings that are no longer referenced, so this isn’t as
wasteful as it might seem).
If you study this script’s code closely, you may also notice
that the join
scheme it uses
relies completely on the sort order of filenames in the parts
directory. Because it simply calls the list sort
method on the filenames list
returned by os.listdir
, it
implicitly requires that filenames have the same length and format
when created by split
. The
splitter uses zero-padding notation in a string formatting
expression ('part%04d'
) to make
sure that filenames all have the same number of digits at the end
(four), much like this list:
>>> list = ['xx008', 'xx010', 'xx006', 'xx009', 'xx011', 'xx111'] >>> list.sort( ) >>> list ['xx006', 'xx008', 'xx009', 'xx010', 'xx011', 'xx111']
When sorted, the leading zero characters in small numbers
guarantee that part files are ordered for joining correctly.
Without the leading zeros, join
would fail whenever there were more than nine part files, because
the first digit would dominate:
>>> list = ['xx8', 'xx10', 'xx6', 'xx9', 'xx11', 'xx111'] >>> list.sort( ) >>> list ['xx10', 'xx11', 'xx111', 'xx6', 'xx8', 'xx9']
Because the list sort
method accepts a comparison function as an argument, we could in
principle strip off digits in filenames and sort
numerically:
>>> list = ['xx8', 'xx10', 'xx6', 'xx9', 'xx11', 'xx111'] >>> list.sort(lambda x, y: cmp(int(x[2:]), int(y[2:]))) >>> list ['xx6', 'xx8', 'xx9', 'xx10', 'xx11', 'xx111']
But that still implies that all filenames must start with
the same length substring, so this doesn’t quite remove the
file-naming dependency between the split
and join
scripts. Because these scripts are
designed to be two steps of the same process, though, some
dependencies between them seem reasonable.
Let’s run a few more experiments with these Python system
utilities to demonstrate other usage modes. When run without full command-line arguments,
both split
and join
are smart enough to input their
parameters interactively. Here they are
chopping and gluing the Python self-installer file on Windows again,
with parameters typed in the DOS console window:
C: emp>python %X%SystemFiletoolssplit.py
File to be split?py152.exe
Directory to store part files?splitout
Splitting C: emppy152.exe to C: empsplitout by 1433600 Split finished: 4 parts are in C: empsplitout Press Enter key C: emp>python %X%SystemFiletoolsjoin.py
Directory containing part files?splitout
Name of file to be recreated?newpy152.exe
Joining C: empsplitout to make C: emp ewpy152.exe Join complete: see C: emp ewpy152.exe Press Enter key C: emp>fc /B py152.exe newpy152.exe
Comparing files py152.exe and newpy152.exe FC: no differences encountered
When these program files are double-clicked in a file explorer GUI, they work the same way (there are usually no command-line arguments when they are launched this way). In this mode, absolute path displays help clarify where files really are. Remember, the current working directory is the script’s home directory when clicked like this, so the name tempsplit actually maps to a source code directory; type a full path to make the split files show up somewhere else:
[in a pop-up DOS console box when split is clicked] File to be split?c: emppy152.exe
Directory to store part files?tempsplit
Splitting c: emppy152.exe to C:PP3rdEdexamplesPP3ESystemFiletools tempsplit by 1433600 Split finished: 4 parts are in C:PP3rdEdexamplesPP3ESystemFiletools tempsplit Press Enter key [in a pop-up DOS console box when join is clicked] Directory containing part files?tempsplit
Name of file to be recreated?c: empmorepy152.exe
Joining C:PP3rdEdexamplesPP3ESystemFiletools empsplit to make c: empmorepy152.exe Join complete: see c: empmorepy152.exe Press Enter key
Because these scripts package their core logic in functions, though, it’s just as easy to reuse their code by importing and calling from another Python component:
C: emp>python
>>>from PP3E.System.Filetools.split import split
>>>from PP3E.System.Filetools.join import join
>>> >>>numparts = split('py152.exe', 'calldir')
>>>numparts
4 >>>join('calldir', 'callpy152.exe')
>>> >>>import os
>>>os.system(r'fc /B py152.exe callpy152.exe')
Comparing files py152.exe and callpy152.exe FC: no differences encountered 0
A word about performance: all the split
and join
tests shown so far process a 5 MB
file, but they take at most one second of real wall-clock time to
finish on my Windows 98 300 and 650 MHz laptop computers—plenty fast
for just about any use I could imagine. (They run even faster after
Windows has cached information about the files involved, and they
would be even quicker on a more modern computer.) Both scripts run
just as fast for other reasonable part file sizes too; here is the
splitter chopping up the file into 500,000- and 50,000-byte
parts:
C: emp>python %X%SystemFiletoolssplit.py py152.exe tempsplit 500000
Splitting C: emppy152.exe to C: emp empsplit by 500000 Split finished: 11 parts are in C: emp empsplit C: emp>ls -l tempsplit
total 9826 -rwxrwxrwa 1 0 0 500000 Sep 12 06:29 part0001 -rwxrwxrwa 1 0 0 500000 Sep 12 06:29 part0002 -rwxrwxrwa 1 0 0 500000 Sep 12 06:29 part0003 -rwxrwxrwa 1 0 0 500000 Sep 12 06:29 part0004 -rwxrwxrwa 1 0 0 500000 Sep 12 06:29 part0005 -rwxrwxrwa 1 0 0 500000 Sep 12 06:29 part0006 -rwxrwxrwa 1 0 0 500000 Sep 12 06:29 part0007 -rwxrwxrwa 1 0 0 500000 Sep 12 06:29 part0008 -rwxrwxrwa 1 0 0 500000 Sep 12 06:29 part0009 -rwxrwxrwa 1 0 0 500000 Sep 12 06:29 part0010 -rwxrwxrwa 1 0 0 28339 Sep 12 06:29 part0011 C: emp>python %X%SystemFiletoolssplit.py py152.exe tempsplit 50000
Splitting C: emppy152.exe to C: emp empsplit by 50000 Split finished: 101 parts are in C: emp empsplit C: emp>ls tempsplit
part0001 part0014 part0027 part0040 part0053 part0066 part0079 part0092 part0002 part0015 part0028 part0041 part0054 part0067 part0080 part0093 part0003 part0016 part0029 part0042 part0055 part0068 part0081 part0094 part0004 part0017 part0030 part0043 part0056 part0069 part0082 part0095 part0005 part0018 part0031 part0044 part0057 part0070 part0083 part0096 part0006 part0019 part0032 part0045 part0058 part0071 part0084 part0097 part0007 part0020 part0033 part0046 part0059 part0072 part0085 part0098 part0008 part0021 part0034 part0047 part0060 part0073 part0086 part0099 part0009 part0022 part0035 part0048 part0061 part0074 part0087 part0100 part0010 part0023 part0036 part0049 part0062 part0075 part0088 part0101 part0011 part0024 part0037 part0050 part0063 part0076 part0089 part0012 part0025 part0038 part0051 part0064 part0077 part0090 part0013 part0026 part0039 part0052 part0065 part0078 part0091
The split can take longer to finish, but only if the part file’s size is set small enough to generate thousands of part files; splitting into 1,006 parts works but runs slower (though machines today are quick enough that you probably won’t notice):
C: emp>python %X%SystemFiletoolssplit.py py152.exe tempsplit 5000
Splitting C: emppy152.exe to C: emp empsplit by 5000 Split finished: 1006 parts are in C: emp empsplit C: emp>python %X%SystemFiletoolsjoin.py tempsplit mypy152.exe
Joining C: emp empsplit to make C: emppy152.exe Join complete: see C: emppy152.exe C: emp>fc /B py152.exe mypy152.exe
Comparing files py152.exe and mypy152.exe FC: no differences encountered C: emp>ls -l tempsplit
...1,000 lines deleted... -rwxrwxrwa 1 0 0 5000 Sep 12 06:30 part1001 -rwxrwxrwa 1 0 0 5000 Sep 12 06:30 part1002 -rwxrwxrwa 1 0 0 5000 Sep 12 06:30 part1003 -rwxrwxrwa 1 0 0 5000 Sep 12 06:30 part1004 -rwxrwxrwa 1 0 0 5000 Sep 12 06:30 part1005 -rwxrwxrwa 1 0 0 3339 Sep 12 06:30 part1006
Finally, the splitter is also smart enough to create the output directory if it doesn’t yet exist and to clear out any old files there if it does exist. Because the joiner combines whatever files exist in the output directory, this is a nice ergonomic touch. If the output directory was not cleared before each split, it would be too easy to forget that a prior run’s files are still there. Given that my kids are running these scripts, they need to be as forgiving as possible; your user base may vary, but perhaps not by much.
C: emp>python %X%SystemFiletoolssplit.py py152.exe tempsplit 700000
Splitting C: emppy152.exe to C: emp empsplit by 700000
Split finished: 8 parts are in C: emp empsplit
C: emp>ls -l tempsplit
total 9827
-rwxrwxrwa 1 0 0 700000 Sep 12 06:32 part0001
-rwxrwxrwa 1 0 0 700000 Sep 12 06:32 part0002
-rwxrwxrwa 1 0 0 700000 Sep 12 06:32 part0003
...
...only new files here...
...
-rwxrwxrwa 1 0 0 700000 Sep 12 06:32 part0006
-rwxrwxrwa 1 0 0 700000 Sep 12 06:32 part0007
-rwxrwxrwa 1 0 0 128339 Sep 12 06:32 part0008
[*] As I’m writing the third edition of this book, I should probably note that some of this background story is now a bit dated. Some six years later, floppies have largely gone the way of the parallel port and the dinosaur. Moreover, burning a CD is no longer as painful as it once was, there are new options today such as large flash memory cards and wireless home networks, and the configuration of my home computers isn’t what it once was. For that matter, some of my kids are no longer kids (though they’ve retained backward compatibility with the past).
[*] It turns out that the zip
, gzip
, and tar
commands can all be replaced with
pure Python code today. The gzip
module in the Python standard
library provides tools for reading and writing compressed
gzip
files, usually named
with a .gz filename extension. It can serve
as an all-Python equivalent of the standard gzip
and gunzip
command-line utility programs.
This built-in module uses another module called zlib
that implements gzip
-compatible data compressions. In
recent Python releases, the zipfile
module can be imported to make
and use ZIP format archives (zip
is an archive and compression
format, gzip
is a compression
scheme), and the tarfile
module allows scripts to read and write tar archives. See the
Python library manual for details.
18.191.68.18