Python - Level 2 material

Level 2 Python Topics

• Review Python Level 1 Topics
• Methods
• Importing Text files
• Navigating Lists
• Sanitising user input

Methods

In the Level 1 material we used a number of 'methods' already (like rantint() and range()). These were predefined by other people - but we can just as easily write our own to do what ever we want, is particular this comes in really handy when you want to do the same or similar things multiple times. Rather than copy pasting our code - we can just repeat a method with different inputs. 

Challenge: 

Construct a method that picks a random number between 2 numbers passed to the method but that DOESN'T include the numbers themselves.

Importing Text files

Importing text files and processing them is how I actually got into python, it's certainly some of the simplest functionality that allows you to do a huge amount!
As with everything we can also wrap it into a neat little method to use again later:

Lets say you're interested in global companies performance - you can get data from Wikipedia about their financial performance and turn it into a csv or txt file using Google spreadsheets (tutorial here), but in this just download this data as a csv.  

Navigating Lists

Now we've converted our text file to a list we can now navigate it's rows and columns easily

Challenge: 

Using this table from Wikipedia (and a Python program), determine what's the most common starting letter for the name of one of Snow White's 7 dwarves (or just the most common name). 

(N.B. you can also use python to automatically get the data from the net for you using a module called urllib2 and the method urlopen(url) but it's far easier to start with a text file.)

Sanitising User input

Here's a few nice tricks for sanitising user input - can you figure out what each line does?

Challenge: 

Wrap the above code into a method that takes a text question and a list of acceptable responses and returns the user's answer when they give an appropriate one.

Final Challenge

Create a program that counts all the individual letters in a book and how many times each one occurs, then prints the results (I suggest a copy of Charles Dickens' 'A Tale of Two Cities' which you can get free here). Why? You might be able to use the frequency of certain letters as a sort of 'fingerprint' for the language of that book (i.e. is it in Modern English, Old English, German, French or Latin?), but also this 'frequency analysis' is the most basic method of code cracking. Lets say you receive some encoded text that has been encoded with a simple substitution cypher (i.e. one letter has just been replaced with another). A frequency analysis like this will show (roughly) which letters are which in the encoded message allowing you to start to decode it (for example in English the most common letter is 'e' so the most common character in the encrypted message is probably also 'e'). If you want to test your cracking skills find a nice long string and encode it with the command 'encode':

Then do a frequency analysis on the encoded string and see if you can figure out what character has been replaced with what (ideally you would have someone else encode the message so that you didn't know what the original message was!).