Unix/Python Tutorial

Introduction

You can download all of the files associated with this tutorial (including this description) as a zip archive.

Table of Contents

• Unix Basics
• Python Basics
• Invoking the Interpreter
• Operators
• Strings
• Dir and Help
• Built-in Data Structures
• Lists
• Tuples
• Sets
• Dictionaries
• Writing Scripts
• Indentation
• Writing Functions
• Object Basics
• Defining Classes
• Using Objects
• Tips and Tricks
• Troubleshooting
• More References

Unix Basics

File/Directory Manipulation

• cp copies a file (e.g., cp sourcefile.txt targetfile.txt)

• rm removes (deletes) a file (e.g., rm sourcefile.txt)
• mv moves a file (ie. cut/paste instead of copy/paste)  (e.g., mv old.txt new.txt)
• man displays documentation for a command (e.g., man cp)
  
• pwd prints your current path
• xterm opens a new terminal window
• mozilla 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

Some basic Emacs editing commands (C- means "while holding the Ctrl-key"):

• 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

You can also copy and paste using just the mouse. Using the left button, select a region of text to copy. Click the middle button to paste.

There are two ways you can use Emacs to develop Python code. The most straightforward way is to use it just as a text editor: create and edit Python files in Emacs; then run Python to test the code somewhere else, like in a terminal window. Alternatively, you can run Python inside Emacs: see the options under "Python" in the menubar, or type C-c ! to start a Python interpreter in a split screen. (Use C-x o to switch between the split screens).

If you want to spend some extra set-up time becoming a power user, you can try an IDE like Eclipse (Download the Eclipse Classic package at the bottom). Check out PyDev for Python support in Eclipse.

Python Basics

You may find the Troubleshooting section helpful if you run into problems. It contains a list of the frequent problems previous students have encountered when following this tutorial.

Invoking the Interpreter

Operators

Strings

To see what methods Python provides for a data type, use the dir and help commands:

>>> s = 'abc'

>>> dir(s)
['__add__', '__class__', '__contains__', '__delattr__', '__doc__', '__eq__', '__ge__', '__getattribute__', '__getitem__', '__getnewargs__', '__getslice__', '__gt__', '__hash__', '__init__','__le__', '__len__', '__lt__', '__mod__', '__mul__', '__ne__', '__new__', '__reduce__', '__reduce_ex__','__repr__', '__rmod__', '__rmul__', '__setattr__', '__str__', 'capitalize', 'center', 'count', 'decode', 'encode', 'endswith', 'expandtabs', 'find', 'index', 'isalnum', 'isalpha', 'isdigit', 'islower', 'isspace', 'istitle', 'isupper', 'join', 'ljust', 'lower', 'lstrip', 'replace', 'rfind','rindex', 'rjust', 'rsplit', 'rstrip', 'split', 'splitlines', 'startswith', 'strip', 'swapcase', 'title', 'translate', 'upper', 'zfill']

>>> help(s.find)


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

As with nested lists, you can also create dictionaries of dictionaries.

Exercise: Use dir and help to learn about the functions you can call on 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)        

Test Case:We will check your code by testing that the script correctly outputs

Cost of [('apples', 2.0), ('pears', 3.0), ('limes', 4.0)] is 12.25

Advanced Exercise: Write a quickSort function in Python using list comprehensions. Use the first element as the pivot. Solution: quickSort.py

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

The FruitShop class has some data, the name of the shop and the prices per pound of some fruit, and it provides functions, or methods, on this data. What advantage is there to wrapping this data in a class?

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!")

[jsmith@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

Test Case:Check that, with the following variable definitions:

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]

The following are true:

shopSmart.shopSmart(orders1, shops).getName() == 'shop1'

and

shopSmart.shopSmart(orders2, shops).getName() == '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. reload works similar to import.

More References!

• The place to go for more Python information: www.python.org
  
• A good reference book: Learning Python (more books can be found in the college library)

Tutorial written by John DeNero, Dan Klein, and the UC Berkeley Pac-Man team.