Open Mind Tree

Introduction

Unix Basics

File/Directory Manipulation

• rm removes (deletes) a file
• mv moves a file (ie. cut/paste instead of copy/paste)
• man displays documentation for a command
• pwd prints your current path
• xterm opens a new terminal window
• firefox opens a web browser
• Press "Ctrl-c" to kill a running process
• Append & to a command to run it in the background
• fg brings a program running in the background to the foreground

The Emacs text editor

• C-x C-s Save the current file
• C-x C-f Open a file, or create a new file it if doesn't exist
• C-k Cut a line, add it to the clipboard
• C-y Paste the contents of the clipboard
• C-_ Undo
• C-g Abort a half-entered command

Python Basics

Invoking the Interpeter

Operators

Strings

Help on built-in function find:



find(...)
    S.find(sub [,start [,end]]) -> int
    
    Return the lowest index in S where substring sub is found,
    such that sub is contained within s[start,end].  Optional
    arguments start and end are interpreted as in slice notation.
    
    Return -1 on failure.

Built-in Data Structures

Lists

>>> help(list.reverse)
Help on built-in function reverse:

reverse(...)
    L.reverse() -- reverse *IN PLACE*

Tuples

Sets

Dictionaries

Writing Scripts

# This is what a comment looks like 
fruits = ['apples','oranges','pears','bananas']
for fruit in fruits:
    print fruit + ' for sale'

fruitPrices = {'apples': 2.00, 'oranges': 1.50, 'pears': 1.75}
for fruit, price in fruitPrices.items():
    if price < 2.00:
        print '%s cost %f a pound' % (fruit, price)
    else:
        print fruit + ' are too expensive!'

nums = [1,2,3,4,5,6]
plusOneNums = [x+1 for x in nums]
oddNums = [x for x in nums if x % 2 == 1]
print oddNums
oddNumsPlusOne = [x+1 for x in nums if x % 2 ==1]
print oddNumsPlusOne

(define nums '(1,2,3,4,5,6))
(map 
   (lambda (x) (+ x 1))  nums)

Beware of Indendation!

if 0 == 1: 
    print 'We are in a world of arithmetic pain' 
print 'Thank you for playing' 

if 0 == 1: 
    print 'We are in a world of arithmetic pain'
    print 'Thank you for playing'

Writing Functions

fruitPrices = {'apples':2.00, 'oranges': 1.50, 'pears': 1.75}

def buyFruit(fruit, numPounds):
    if fruit not in fruitPrices:
        print "Sorry we don't have %s" % (fruit)
    else:
        cost = fruitPrices[fruit] * numPounds
        print "That'll be %f please" % (cost)

# Main Function
if __name__ == '__main__':        
    buyFruit('apples',2.4)
    buyFruit('coconuts',2)        

Object Basics

Defining Classes

class FruitShop:

    def __init__(self, name, fruitPrices):
        """
            name: Name of the fruit shop
            
            fruitPrices: Dictionary with keys as fruit 
            strings and prices for values e.g. 
            {'apples':2.00, 'oranges': 1.50, 'pears': 1.75} 
        """
        self.fruitPrices = fruitPrices
        self.name = name
        print 'Welcome to the %s fruit shop' % (name)
        
    def getCostPerPound(self, fruit):
        """
            fruit: Fruit string
        Returns cost of 'fruit', assuming 'fruit'
        is in our inventory or None otherwise
        """
        if fruit not in self.fruitPrices:
            print "Sorry we don't have %s" % (fruit)
            return None
        return self.fruitPrices[fruit]
        
    def getPriceOfOrder(self, orderList):
        """
            orderList: List of (fruit, numPounds) tuples
            
        Returns cost of orderList. If any of the fruit are  
        """ 
        totalCost = 0.0             
        for fruit, numPounds in orderList:
            costPerPound = self.getCostPerPound(fruit)
            if costPerPound != None:
                totalCost += numPounds * costPerPound
        return totalCost
    
    def getName(self):
        return self.name

1. Encapsulating the data prevents it from being altered or used inappropriately,
2. The abstraction that objects provide make it easier to write general-purpose code.

Using Objects

import shop

shopName = 'the Berkeley Bowl'
fruitPrices = {'apples': 1.00, 'oranges': 1.50, 'pears': 1.75}
berkeleyShop = shop.FruitShop(shopName, fruitPrices)
applePrice = berkeleyShop.getCostPerPound('apples')
print applePrice
print('Apples cost $%.2f at %s.' % (applePrice, shopName))

otherName = 'the Stanford Mall'
otherFruitPrices = {'kiwis':6.00, 'apples': 4.50, 'peaches': 8.75}
otherFruitShop = shop.FruitShop(otherName, otherFruitPrices)
otherPrice = otherFruitShop.getCostPerPound('apples')
print otherPrice
print('Apples cost $%.2f at %s.' % (otherPrice, otherName))
print("My, that's expensive!")

[cs188-ta@midway ~]$ python shopTest.py
Welcome to the Berkeley Bowl fruit shop
1.0
Apples cost $1.00 at the Berkeley Bowl.
Welcome to the Stanford Mall fruit shop
4.5
Apples cost $4.50 at the Stanford Mall.
My, that's expensive!

Static vs Instance Variables

class Person:
    population = 0
    def __init__(self, myAge):
        self.age = myAge
        Person.population += 1
    def get_population(self):
        return Person.population
    def get_age(self):
        return self.age

orders1 = [('apples',1.0), ('oranges',3.0)]
orders2 = [('apples',3.0)]    
dir1 = {'apples': 2.0, 'oranges':1.0}
shop1 =  shop.FruitShop('shop1',dir1)
dir2 = {'apples': 1.0, 'oranges': 5.0}
shop2 = shop.FruitShop('shop2',dir2)
shops = [shop1, shop2]

More Python Tips and Tricks

• Use range to generate a sequence of integers, useful for generating traditional indexed for loops:

  for index in range(3):
      print lst[index]
  

• After importing a file, if you edit a source file, the changes will not be immediately propagated in the interpreter. For this, use the reload command: 

>>> reload(shop)


Troubleshooting

• Problem:
  ImportError: No module named py
  
  Solution:
  When using import, do not include the ".py" from the filename.
  For example, you should say: import shop
  NOT: import shop.py
• Problem:
  NameError: name 'MY VARIABLE' is not defined
  Even after importing you may see this.
  
  Solution:
  To access a member of a module, you have to type MODULE NAME.MEMBER NAME, where MODULE NAME is the name of the .py file, and MEMBER NAME is the name of the variable (or function) you are trying to access.
• Problem:
  TypeError: 'dict' object is not callable
  
  Solution:
  Dictionary looks up are done using square brackets: [ and ]. NOT parenthesis: ( and ).
• Problem:
  ValueError: too many values to unpack
  
  Solution:
  Make sure the number of variables you are assigning in a for loop matches the number of elements in each item of the list. Similarly for working with tuples.
  For example, if pair is a tuple of two elements (e.g. pair =('apple', 2.0)) then the following code would cause the "too many values to unpack error":
  (a,b,c) = pair
  Here is a problematic scenario involving a for loop:

  pairList = [('apples', 2.00), ('oranges', 1.50), ('pears', 1.75)]
  for fruit, price, color in pairList:
      print '%s fruit costs %f and is the color %s' % (fruit, price, color)
  

• Problem:
  AttributeError: 'list' object has no attribute 'length' (or something similar)
  
  Solution:
  Finding length of lists is done using len(NAME OF LIST).
• Problem:
  Changes to a file are not taking effect.
  
  Solution:
  1. Make sure you are saving all your files after any changes.
  2. If you are editing a file in a window different from the one you are using to execute python, make sure you reload(YOUR_MODULE) to guarantee your changes are being reflected. reloadworks similar to import.