Python1

Python for Scientists

Introduction

Getting Started on BlueCrystal Phase-2

After you have logged in, type the following at the command line:

module add languages/python-2.7.2.0
python

This should start up an interactive python session:

Python 2.7.2 (default, Aug 25 2011, 10:51:03) 
[GCC 4.3.3] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>>

where we can type commands at the >>> prompt.

With thanks to Simon Metson and Mike Wallace for much of the following material.

Python as a Calculator

Let's try a few commands out. If you type:

>>> print "Hello!"

you'll get:

Hello!

If you try:

>>> print 5 + 9

you'll get:

14

And here is a copy of a session containing a few more commands:

>>> five = 5
>>> neuf = 9
>>> print five + neuf
14
>>> def say_hello():
...     print "Hello, world!"
... # hit return here 
>>> say_hello()
Hello, world!

You can exit an interactive session but typing Ctrl-D.

Getting Help

Python has lots of useful documentation. For example, take a look at: http://docs.python.org/.

Making a Script

An interactive session can be fun and useful for trying things out. However--to save our fingers--we will typically want to execute a series of commands as a script, created using your favourite text editor. Here is the contents of an example script:

#!/bin/env python

print "Hello, from a python script!"

Ensure that your script is executable:

chmod u+x myscript.py

and now you can run it:

[ggdagw@bigblue4 ~]$ ./myscript.py 
Hello, from a python script!

Python and Whitespace

Python incorporates whitespace in it's syntax, so spacing is very important. For example:

Nuts and Bolts

Strings

For example, the eagle-eyed will have spotted in the previous examples that we could ask the length a character string--straight off the bat. No need to write a counting routine ourselves:

>>> print len(message)

We also take slices of our character string. In my case

>>> print message[:5]

elicits:

hello

Lists

An example of a list is:

>>> shopping = ['bread', 'marmalade', 'milk', 'tea']

and we can enquire about the length of that using the same function as before:

>>> len(shopping)

We can also take slices of a list, as we did with a string:

>>> shopping[0:2]

and even reset a portion of the list that way:

>>> shopping[0:2] = ['bagels', 'jam']
>>> shopping

A list object comes with a number of handy methods built in. For example we could type:

>>> shopping.append("butter")
>>> print shopping

and even:

>>> shopping.reverse()
>>> print shopping

or

>>> shopping.sort()
>>> print shopping