A Caffeinated Crash Course inPython

Python is not….

• Java• C• Perl

The Python Interpreter

• Type “python” at the command prompt• In windows, find the python icon on the

start menu

Dir and Help

help()

dir()

Syntax Errors

• Python Errors show the line number ofthe error

• Check the line above if your error makesno sense

White Space

String Basics

• Not a mutable data type

• String can be delimited with either the “or ‘

More Strings

• Concatenation uses the +

• You can do math with strings!

Output

Indexing• To index into a string, specify the position

inside square brackets

• You can index into a string from the “end” ofthe string.

Slicing• A Substring of a string is a slice

• Your head or tail can be a negativeindex

More Slicing

• You don’t need to specify the beginningand end of the string

• Find the length of a string with len()

Example

Lists

• Lists in python are made of any datatype delimited by commas andsurrounded by brackets.

• Lists are mutable

More on Lists

• You can index into lists

• You can slice lists

Modifying Lists

• You can add lists

• And append to them

List Methods

• sort - sorts the list in place, returns nothing• sorted - does not modify the list, returns new

sorted list• reverse - reverses the list in place, returns

nothing

String Formatting

• The % operator substitutes values into astring

• %s and %d are placeholders for the values(%d makes sure it’s a number)

• “%s has %d letters” %(“colorless”, len(“colorless”))becomes the string “colorless has 9letters”

Converting fromStrings to Lists

• Join a list to make a string

• Split a string to make a list

For and If

• If statements

• For Statements

List Comprehensions

• Applies a function to every element of alist

Dictionaries

• Hash - maps things to things!

Even More Dictionaries

Example: Letter Frequencies

Classes

Importing and the Python path

• Import using the import command• You can import everything from a

module using the syntax “from<module> import *”

Files

Filename = “/home/havasi/input.txt”input = open(Filename, ‘r’)output = open(Filename + ‘.out’, ‘w’)for line in input.readlines():

input.write(‘Cows! \n’)input.close()output.close()

Resources

• Python.org• NLTK Python Tutorial

– http://nltk.org/doc/en/programming.html• IDLE (Windows Development Env.)

– http://www.python.org/idle/