Lectures on Computational Economics

John Stachurski

Script Files

Until now we have just entered commands into interpreter

To write a longer program need to write a program file or script

• Text file with Python commands that is run through the interpreter
• Extension is 'py', as in 'filename.py'

We will look at a few ways of writing and running scripts

Note: The method of running scripts is very platform specific

• This lecture is based on Python within Linux
  • But still worth a quick read if you're using Windows or Mac
• If you are using Windows:
  • For Python(x,y), see the documentation or Google for help
  • These videos might be useful

Writing a Program

Normally, you will use a text editor like Emacs, Vim, Geany, or Notepad++

Here is a demo with Geany (because it's simple)

Let's open Geany:

Now I write my script/program in Geany

And save it as test.py in my home directory, called /home/john

(Will look different on Windows/Mac)

Notice that Geany now knows its a Python file because of the .py extension

Geany applies syntax highlighting

We've now created a file containing Python commands, stored on the hard disk

The next step is to run it

Running Scripts from the Command Line

One method is to run it from the command line (or shell in UNIX/Linux)

• Windows users can probably skip this section

Here I'm running it on Ubuntu Linux, through the shell

Better to use the -i flag to stay in Python when it finishes

• So variables are not lost

Generally, the command line method is not the best one for scientific programming

Let's look at some other ways

Running Scripts with IDLE

A very easy way to run scripts is with IDLE

• Fine to start, probably came together with your Python installation
  • If you can't find it, try a search of your hard disk
• But not very high quality
  • Eventually you should learn to use IPython (see below)

After starting IDLE, the first window is the Python shell (interpreter)

Go to the file menu

Yes, I know the fonts are ugly...

Select "New Window" to get a window like this:

This window serves as a text editor where we can write our program:

• Make sure you reproduce it exactly

Before running the file we need to save it

Select "Save As" from the "File" menu

You should get a dialogue box like this (Might look different on Windows/Mac)

Make sure you save it somewhere sensible, where you can find it again

Now that it's saved we can run it

Select "Run Module" from the "Run" menu

• Or just press F5

The output is sent to the Python Shell

Running Scripts with IPython

After you get comfortable, switch to IPython

• An enhanced interface to the Python interpreter
• Requires text editor to write the file (plus IPython to run it)
• Needs to be installed separately

Videos on installing and using IPython can be found here

Here's a very quick tutorial using screenshots from my Linux machine

First we fire up IPython

The next step is to run our file test.py in /home/john

The output is displayed in IPython

The only problem you might have is that test.py is not in IPython's current working directory

Error message looks like this

Let's check what directory we are in

'pwd' stands for "present working directory"

• This is a UNIX command
• Might be different according to your OS
• Might need to prepend with % as in '%pwd'
  • Depends on whether 'automagic' is on or off

We are in /tmp, and the file is in /home/john

So I change to /home/john

'cd' stands for "change directory"

• See comments for 'pwd'

Now it runs okay

Things will be different with Windows or Mac

• I leave the details to you

Problems

Write a program which

• Asks the user for a sequence of integers, separated by commas
  • For example, the user enters 1, 17, -3
• Prints out a list with the integers sorted from smallest to largest
  • In this example, prints [-3, 1, 17]
• Hint: X.sort() sorts a list of integers from smallest to largest

Solution:

X = raw_input("Enter a list of integers separated by commas: ")
X = X.split(',')
X = [int(x) for x in X]
X.sort()
print X

Copyright John Stachurski 2010