Sunday, May 31, 2009

Johan Louwers Personal Blog.: Python dictionary variables

Reading tips: another Python blog:
Johan Louwers Personal Blog.: Python dictionary variables

Thursday, May 28, 2009

A simple console program

Say you want to make your own console program. This program should be able of recording data you enter to a text file, and extracting data from the text file to analyze them. One such program can be a tool to register values of different variables you are measuring, such as the number of visitors you have had in your office today. Say you count office visits, and you want to store this in the file under the name "office". Then, you also count how many times during the day your phone rings. This variable is called "phone". You create a program that pops up this menu:

1. Register KPI
2. Create KPI report
3. Quit

When you choose 1, you are prompted for name an value of variable.

The menu is displayed when a function called mainmenu is called. Such a menu can be written as follows in Python:

def mainmenu():
print '1: Register KPI'
print '2: Create KPI report'
print '3: Quit'
menuChoice = raw_input('>> ')

Depending on the input the user gives, we cann call different functions (still within mainmenu):

if menuChoice == '1':
elif menuChoice == '2':
elif menuChoice == '3':
print 'Goodbye'
print 'Not a valid menu item. Pleast try again'

There you go, a working menu. Next time: we show what the functions regKPI and createReport look like :-)

Tuesday, May 26, 2009

Console programs

There are many programs that run in console environments. They are more often seen on Linux than other systems, but that doesn't mean they are not useful on Windows. My *nix favorite console programs are

  • alpine: e-mail client for the console (easy to use and with a ton of features)
  • midnight commander: file organizer
  • vim: text editor
  • gnuplot: open source plotting program
These are quite complicated also when it comes to user interfaces, and are very usable programs.

If you are a little less ambitious, making simple console programs with "menus" is quite simple. A database handling program may have the following menu:



Easy enought to use, eh? You want to read the help file, hit 3 on your keyboard followed by enter. Next post will show how to build a quite useful program using a console interface of this type.... (not uncommon in apps from the 1970's and 1980's)!

Sunday, May 24, 2009

Avoid the *.py extension when calling scripts under Windows

If you'd like to make your scripts run by typing

myscript args

instead of args

you can wrap the python script inside a *.bat file. Say you want to run and it takes two arguments, make the following batch DOS file: %1 %2

The percentage+number things play the role of sys.argv in Python such that command line arguemnts are piped through to the Python script.

Command line tools take command line arguments

You all know them and use them; command line tools with arguments. Especially on linux, this is the way things go, but also in Windows. Think about the dos command dir, and its attributes;

  • dir /p: list directory one screen full at the time
  • dir /w: list directory in columns,
  • ....
On linux you have ls with a bunch of options, for example

ls -al

to show files with owners and permissions. How do we make tools that can take command line arguments in Python?

Fortunately, as the site says, Python comes with batteries included. Command line arguments are stored in a list that can be accesed from the sys module. The stdin (command line input) arguments are found using sys.argv. If you use this line:

from sys import argv as inputargs

inputargs will be a list containing text strings with your input arguments. Then, if you have some script like this

from sys import argv as inputargs
myString = 'Hello '+inputargs[1]+', this is '+inputargs[2]+' speaking!'
print myString

The result of running this command:

./ guest "the owner"

will be:

Hello guest, this is the owner speaking!

Nice to know! We'll put it to use in the next post! Happy hacking :-)

Saturday, May 23, 2009

Writing time data to file

In the last post I wrote about a small command line utility for registering hours spent on different tasks. I never got to showing how to write to text files in Python, so let's get down to it now (what's better to do a Saturday evening than hacking away in Python?)!

First, to open a file for editing, you use the following syntax in Python:


where attribute is one of the following:
  • a - append: appends new text to the end of the file
  • w - write: write to file, will overwrite contents already there
  • r - read: does only give read access to the file
For our purpose we will use the append attrribute (a). What you create in this way is a file object, that has certain methods available to it depending on the attribute given to the constructor open. The one we will be using is, surprisingly enough, write:


When you create a file object, the entire file is loaded into your computers working memory (RAM). When you are done working on the file, you should close it to free memory and avoid unforeseen computer trolls to pop out and eat you. You do this by issueing the close method to your file object;


If you add the three code lines written here to the script outlined in the last post, you will have a working time registering tool!

Friday, May 22, 2009

Time registering command line tool in Python

Often, I want to write a one-liner to a file to register information. I do my time tracking this way, for example. First, you need Python to get data from the command line. Say you want to track number of hours spent on a project at work and get this into a tab-delimited file that you can open in a spreadsheet later (Excel, Calc, etc). You want a utility where you simply write the following at the command prompt: 

regtime projectname hours

and it should automatically make the following line in your "spreadsheet":

date projectname hours

In order to do this, we need to get the current date in Python. Luckily, there is a nice module called time that allows us to do that in a simple fashion. First, import it as follows: 

from  time import strftime as getdate

What this does is that it gets the current local time used on the computer and writes a text string with the date:


We also need to get the command line arguments projectname and hours into Python.
For this we need the list sys.argv.

from sys import argv as arguments
projectname=argv[1]; hours=argv[2];

Ok, lets summarize with a script that takes the two command line arguments and writes the "spreadsheet" line to the screen:

from sys import argv as cla 
from time import strftime as getDate 
myString = getDate('%Y-%m-%d')+'\t'+cla[1]+'\t'+cla[2]+'\n' 
print myString

The result of running this script with arguments "Command line tool" and 1 is shown below:

2009-05-22      Command line tool       1

Next post will show how to write this at the end of a text file. 

Some comments on the above code:
a) the character \t means "insert tab in text string"
b) the character \n means "insert line break"
c) the string '%Y-%m-%d' formats the date to ISO standard
d) adding string elements works in the obvious way: 'foo'+'bar'='foobar'

Write your own command line tools!

As a multi-platform computer user, I always miss some things when going from one platform to antoher. Especially when I go to Windows, I miss some command line tools. One remedy would be Cygwin, but for some reason I don't get the idea of Cygwin. I'd rather use Linux directly then, and I don't need complicated tools. I just like to have some simple possibilities. This blog will contain posts about writing small command line tools in Python that work on Windows (XP), Macs and Linux computers, all using the same scripts, or with very minor modifications. This is kinda my hobby, but it has also made my work day more fun.