Problem Solving

We have explored various parts of the Auto language and now we will take a look at how all these parts fit together, by designing and writing a program which does something useful. The idea is to learn how to write a Auto script on your own.

The Problem

The problem we want to solve is:

I want a program which creates a backup of all my important files.

Although, this is a simple problem, there is not enough information for us to get started with the solution. A little more analysis is required. For example, how do we specify which files are to be backed up? How are they stored? Where are they stored?

After analyzing the problem properly, we design our program. We make a list of things about how our program should work. In this case, I have created the following list on how I want it to work. If you do the design, you may not come up with the same kind of analysis since every person has their own way of doing things, so that is perfectly okay.

• The files and directories to be backed up are specified in a list.
• The backup must be stored in a main backup directory.
• The files are backed up into a zip file.
• The name of the zip archive is the current date and time.
• We use the standard zip command available by default in any standard GNU/Linux or Unix distribution. Note that you can use any archiving command you want as long as it has a command line interface.

For Windows users

Windows users can install the zip command from the GnuWin32 project page and add C:\Program Files\GnuWin32\bin to your system PATH environment variable.

The Solution

As the design of our program is now reasonably stable, we can write the code which is an implementation of our solution.

Since Auto transpiles to Python via a2p, we can use Python's standard library modules such as os and time in our Auto code. The use keyword in Auto becomes import in the generated Python.

Save as backup_ver1.at:

The backup script - version 1

Output:

$ a2p backup_ver1.at && python backup_ver1.py
Zip command is:
zip -r /Users/swa/backup/20140328084844.zip /Users/swa/notes
Running:
  adding: Users/swa/notes/ (stored 0%)
  adding: Users/swa/notes/blah1.txt (stored 0%)
  adding: Users/swa/notes/blah2.txt (stored 0%)
  adding: Users/swa/notes/blah3.txt (stored 0%)
Successful backup to /Users/swa/backup/20140328084844.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.

If the above program does not work for you, copy the line printed after the Zip command is line in the output, paste it in the shell (on GNU/Linux and Mac OS X) / cmd (on Windows), see what the error is and try to fix it. Also check the zip command manual on what could be wrong. If this command succeeds, then the problem might be in the program itself, so check if it exactly matches the program written above.

How It Works

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 by first importing them with the use keyword. Then, we specify the files and directories to be backed up in the source list. The target directory is where we 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 generate using the time.strftime() function. It will also have the .zip extension and will be stored in the target_dir directory.

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

The time.strftime() function takes a format string such as the one we have used in the above program. The %Y format code will be replaced by the year with the century. The %m format code will be replaced by the month as a decimal number between 01 and 12 and so on. The complete list of such format codes can be found in the Python Reference Manual.

We create the name of the target zip file using the + 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 using an f-string (f"...") with $variable interpolation, which becomes {variable} in the Python output. The $ syntax is Auto's way of embedding expressions in strings, while Python uses {}.

The zip command that we are using has some options available, and one of these options is -r. The -r option specifies that the zip command should work recursively for directories, i.e. it should include all the subdirectories and files. 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.

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 successful, 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.

That's it, we have created a script to take a backup of our important files!

Note for Auto Programmers:

Auto uses ! for logical negation (not) instead of Python's not keyword. The a2p transpiler converts !expr to not expr automatically. Similarly, Auto's $var inside f-strings becomes {var} in Python.

Note to Windows Users

Instead of double backslash escape sequences, you can also use raw strings. For example, use "C:\\Documents" or r"C:\Documents". However, do not use "C:\Documents" since you end up using an unknown escape sequence \D.

Now that we have a working backup script, we can use it whenever we want to take a backup of the files. This is called the operation phase or the deployment phase of the software.

Second Version

The first version of our script works. However, we can make some refinements to it so that it can work better on a daily basis. This is called the maintenance phase of the software.

One of the refinements I felt was useful is a better file-naming mechanism - using the time as the name of the file within a directory with the current date as a directory within the main backup directory. The first advantage is that your backups are stored in a hierarchical manner and therefore it is much easier to manage. The second advantage is that the filenames are much shorter. The third advantage is that separate directories will help you check if you have made a backup for each day since the directory would be created only if you have made a backup for that day.

Save as backup_ver2.at:

The backup script - version 2 (daily backup naming)

Output:

$ a2p backup_ver2.at && python backup_ver2.py
Successfully created directory /Users/swa/backup/20140329
Zip command is:
zip -r /Users/swa/backup/20140329/073201.zip /Users/swa/notes
Running:
  adding: Users/swa/notes/ (stored 0%)
  adding: Users/swa/notes/blah1.txt (stored 0%)
  adding: Users/swa/notes/blah2.txt (stored 0%)
  adding: Users/swa/notes/blah3.txt (stored 0%)
Successful backup to /Users/swa/backup/20140329/073201.zip

How It Works

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

Third Version

The second version works fine when I do many backups, but when there are lots of backups, I am finding it hard to differentiate what the backups were for! For example, I might have made some major changes to a program or presentation, then I want to associate what those changes are with the name of the zip archive. This can be easily achieved by attaching a user-supplied comment to the name of the zip archive.

Warning: The following program does not work, so do not be alarmed, please follow along because there's a lesson in here.

Save as backup_ver3.at:

The backup script - version 3 (with user comment - BUGGY!)

Output:

$ a2p backup_ver3.at && python backup_ver3.py
  File "backup_ver3.py", line 39
    target = today + os.sep + now + '_' +
                                        ^
SyntaxError: invalid syntax

How This (does not) Work

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 correction of the program when we find errors is called bug fixing.

Note for Auto Programmers:

This is a great example of one of Auto's design advantages over Python. In Auto, statements end with } (for blocks) or with the natural line boundaries within a block, so line continuation issues like this one simply don't arise. Auto's block-based syntax with {} makes it clear where statements begin and end. The version 4 listing will show how we can fix this more elegantly in Auto using an if/else expression.

Fourth Version

Save as backup_ver4.at:

The backup script - version 4 (fixed with user comment)

Output:

$ a2p backup_ver4.at && python backup_ver4.py
Enter a comment --> added new examples
Zip command is:
zip -r /Users/swa/backup/20140329/074122_added_new_examples.zip /Users/swa/notes
Running:
  adding: Users/swa/notes/ (stored 0%)
  adding: Users/swa/notes/blah1.txt (stored 0%)
  adding: Users/swa/notes/blah2.txt (stored 0%)
  adding: Users/swa/notes/blah3.txt (stored 0%)
Successful backup to /Users/swa/backup/20140329/074122_added_new_examples.zip

How It Works

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 input function and then check if the user actually entered something by finding out the length of the input using the len function (in Auto, written as .len() on the string). If the user has just pressed enter without entering anything (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 filenames without spaces is much easier.

Notice how we fixed the bug from version 3. In the Auto code, we use an if/else expression that assigns directly to let target, which cleanly avoids the line continuation issue entirely. The a2p transpiler converts this to a standard Python if/else statement with a backslash continuation. This is an example of how Auto's syntax can help you avoid common Python pitfalls.

More Refinements

The fourth version is a satisfactorily working script for most users, but there is always room for improvement. For example, you can include a verbosity level for the zip command by specifying a -v option to make your program become more talkative or a -q option to make it quiet.

Another possible enhancement would be to allow extra files and directories to be passed to the script at the command line. We can get these names from the sys.argv list and we can add them to our source list using the extend method provided by the list class.

The most important refinement would be to not use the os.system way of creating archives and instead using the zipfile or tarfile built-in modules to create these archives. They are part of the standard library and available already for you to use without external dependencies on the zip program to be available on your computer.

However, I have been using the os.system way of creating a backup in the above examples purely for pedagogical purposes, so that the example is simple enough to be understood by everybody but real enough to be useful.

Can you try writing the fifth version that uses the zipfile module instead of the os.system call?

The Software Development Process

We have now gone through the various phases in the process of writing a software. These phases can be summarised as follows:

1. What (Analysis)
2. How (Design)
3. Do It (Implementation)
4. Test (Testing and Debugging)
5. Use (Operation or Deployment)
6. Maintain (Refinement)

A recommended way of writing programs is the procedure we have followed in creating the backup script: Do the analysis and design. Start implementing with a simple version. Test and debug it. Use it to ensure that it works as expected. Now, add any features that you want and continue to repeat the Do It-Test-Use cycle as many times as required.

Remember:

Software is grown, not built. -- Bill de hOra

Summary

We have seen how to create our own Auto programs/scripts and the various stages involved in writing such programs. You may find it useful to create your own program just like we did in this chapter so that you become comfortable with Auto as well as problem-solving.

Key takeaways for this chapter:

• use for Python modules -- Auto can use Python's standard library modules like os, time, and sys via the use keyword. The a2p transpiler converts these to import statements.
• os.system() -- runs a shell command and returns the exit code. A return value of 0 means success.
• os.sep -- the OS-specific path separator (/ on Unix, \\ on Windows).
• os.path.exists() and os.mkdir() -- check if a directory exists and create directories.
• time.strftime() -- format dates and times as strings.
• input() -- read user input from the console.
• .len() -- get the length of a string (Auto style; Python uses len()).
• .replace() -- replace substrings within a string.
• .join() -- join list elements into a string with a separator.
• f-strings with $var -- Auto's string interpolation syntax, transpiled to Python's f-string {var} syntax.
• if/else expressions -- Auto supports if/else as expressions that return values, helping avoid common bugs like missing line continuations.

Next, we will learn about data structures.