variable# What You Should Know For Lecture: Week 5

Wednesday:

Announcements-

Homework

Any exercises not completed in class will be due Monday at 11 pm. There will be a HW 5 assignment due during discussion week6.

Python Scripts

Before we learn about dictionaries let’s learn about python scripts.

The anatomy of a python script is very similar to that of a bash script. Python scripts typically have shebangs. This tells the user and the operating system that this is a python script.

#!/usr/bin/env python2

After the shebang, we give python a command. Let’s do the following:

Make a new file and call it Week_5_Wed.py. Inside the file type the following:

#!/usr/bin/env python2
# usage: python Week_5_Wed.py
# this file prints: Week5 is here!

print 'Week5 is here!'

Congratulations you just made a python script! Notice the .py file extension. This indicates that this file is a python script.

To execute this code you need to do the following: 1) load python, and 2) type

$ python Week_5_Wed.py
Week5 is here!

Notice you did not need to open python to run this script. You are still in bash.

Let’s take this a step further and write a shell script that executed this python script!!!

Make a bash file called Week_5_Wed.sh

Inside the file type the following:

#!/bin/bash
# usage: sh Week_5_Wed.sh
# this executes the program Week_5_Wed.py
# Step 1) load python <- it is possible that python will already be loaded and this will have no effect.  However if python is not loaded you can load it in the script!
# Step 2) run python script

source /u/local/Modules/default/init/bash   # this line tells Hoffman2 where to look for the python module, without this line python will not load
module load python

python Week_5_Wed.py

You can convince yourself that python is being loaded in the program by unloading it on Hoffman2.

$ module unload python

The 'python/2.6' module is being unloaded

Then run the shell script:

$ sh Week_5_Wed.sh
Week5 is here!

A note about comments. In python you can use the # to make comments just as you did in bash. You can also comment out blocks of text using triple quotes """ before and after the block of text.

Make a bash file called Week_5_comment_block.py

Inside the file type the following:

#!/usr/bin/env python2
"""
usage: python Week_5_comment_block.py
we are practicing commenting out a block of text with triple quotes
this file prints: Week5 is here!
"""

print 'Week5 is here!'

Then run the script.

$ python Week_5_comment_block.py
Week5 is here!

Dictionaries

A data collection that is unordered, changeable and indexed. They are written with curly brackets, and have keys and values.

Let’s learn about sets while using a python script. Make a script called dictionary.py

#!/usr/bin/env python2


"""
usage: python Dictionary.py
We will make a dictionary and print the items in the keys and values
"""
petdict =   {
  "dog": "Fluffy",
  "cat": "Muffin",
  "turtle": "yurtle"
}

print(petdict)

Then run the script.

$ python Dictionary.py
{'turtle': 'yurtle', 'dog': 'Fluffy', 'cat': 'Muffin'}

Dictionaries have keys and values. For example ‘turtle’ is a key and yurtle is a value. We can access the value of a key using the following:

x = petdict[“turtle”]

Let’s add the following to Dictionary.py:

# access the value of the key turtle
x = petdict["turtle"]
print x

If we run the script again we get the following:

$ python Dictionary.py
{'turtle': 'yurtle', 'dog': 'Fluffy', 'cat': 'Muffin'}
yurtle

We can get the same result using .get() and we can test that this is true using an if statement:

Add the following to Dictionary.py:

# access the value of the key turtle using .get()
y = petdict.get("turtle")
print y

# we can test that the values are equivalent

if y == x:
  print 'true'
else:
  print 'false'