$ python backup_ver1.py
Successful backup to /mnt/e/backup/20041208073244.zip
				
				

Now, we are in the testing phase where we test that our program works properly. If it doesn't behave as expected, then we have to debug our program i.e. remove the bugs (errors) from the program.

You will notice how we have converted our design into code in a step-by-step manner.

We make use of the os and time modules and so we import them. Then, we specify the files and directories to be backed up in the source list. The target directory is where store all the backup files and this is specified in the target_dir variable. The name of the zip archive that we are going to create is the current date and time which we fetch using the time.strftime() function. It will also have the .zip extension and will be stored in the target_dir directory.

The time.strftime() function takes a specification such as the one we have used in the above program. The %Y specification will be replaced by the year without the cetury. The %m specification will be replaced by the month as a decimal number between 01 and 12 and so on. The complete list of such specifications can be found in the [Python Reference Manual] that comes with your Python distribution. Notice that this is similar to (but not same as) the specification used in print statement (using the % followed by tuple).

We create the name of the target zip file using the addition operator which concatenates the strings i.e. it joins the two strings together and returns a new one. Then, we create a string zip_command which contains the command that we are going to execute. You can check if this command works by running it on the shell (Linux terminal or DOS prompt).

The zip command that we are using has some options and parameters passed. The -q option is used to indicate that the zip command should work quietly. The -r option specifies that the zip command should work recursively for directories i.e. it should include subdirectories and files within the subdirectories as well. The two options are combined and specified in a shorter way as -qr. The options are followed by the name of the zip archive to create followed by the list of files and directories to backup. We convert the source list into a string using the join method of strings which we have already seen how to use.

Then, we finally run the command using the os.system function which runs the command as if it was run from the system i.e. in the shell - it returns 0 if the command was successfully, else it returns an error number.

Depending on the outcome of the command, we print the appropriate message that the backup has failed or succeeded and that's it, we have created a script to take a backup of our important files!

Now that we have a working backup script, we can use it whenever we want to take a backup of the files. Linux/Unix users are advised to use the executable method as discussed earlier so that they can run the backup script anytime anywhere. This is called the operation phase or the deployment phase of the software.

The above program works properly, but (usually) first programs do not work exactly as you expect. For example, there might be problems if you have not designed the program properly or if you have made a mistake in typing the code, etc. Appropriately, you will have to go back to the design phase or you will have to debug your program.

				
$ python backup_ver2.py
Successfully created directory /mnt/e/backup/20041208
Successful backup to /mnt/e/backup/20041208/080020.zip

$ python backup_ver2.py
Successful backup to /mnt/e/backup/20041208/080428.zip

				
				

Most of the program remains the same. The changes is that we check if there is a directory with the current day as name inside the main backup directory using the os.exists function. If it doesn't exist, we create it using the os.mkdir function.

Notice the use of os.sep variable - this gives the directory separator according to your operating system i.e. it will be '/' in Linux, Unix, it will be '\\' in Windows and ':' in Mac OS. Using os.sep instead of these characters directly will make our program portable and work across these systems.

				
$ python backup_ver3.py
File "backup_ver3.py", line 25
target = today + os.sep + now + '_' +
					^
SyntaxError: invalid syntax
					
				

This program does not work!. Python says there is a syntax error which means that the script does not satisfy the structure that Python expects to see. When we observe the error given by Python, it also tells us the place where it detected the error as well. So we start debugging our program from that line.

On careful observation, we see that the single logical line has been split into two physical lines but we have not specified that these two physical lines belong together. Basically, Python has found the addition operator (+) without any operand in that logical line and hence it doesn't know how to continue. Remember that we can specify that the logical line continues in the next physical line by the use of a backslash at the end of the physical line. So, we make this correction to our program. This is called bug fixing.

				
$ python backup_ver4.py
Enter a comment --> added new examples
Successful backup to /mnt/e/backup/20041208/082156_added_new_examples.zip

$ python backup_ver4.py
Enter a comment -->
Successful backup to /mnt/e/backup/20041208/082316.zip
				
				

This program now works! Let us go through the actual enhancements that we had made in version 3. We take in the user's comments using the raw_input function and then check if the user actually entered something by finding out the length of the input using the len function. If the user has just pressed enter for some reason (maybe it was just a routine backup or no special changes were made), then we proceed as we have done before.

However, if a comment was supplied, then this is attached to the name of the zip archive just before the .zip extension. Notice that we are replacing spaces in the comment with underscores - this is because managing such filenames are much easier.