Python Basics

Required Files

Table of Contents

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

Invoking the Interpreter

Operators

Strings

Exercise: Dir and Help

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

Exercise: Lists

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

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

Tuples

Sets

Dictionaries

Exercise: 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

Exercise: List Comprehensions

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'

Tabs vs Spaces

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)        

Advanced Exercise

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@nova ~]$ 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

More Python Tips and Tricks

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

  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 similarly to import.

More References

• The place to go for more Python information: www.python.org
• A good reference book: Learning Python (From the UCB campus, you can read the whole book online)

Step 1 - Get current page URL

•     http - This is the protocol
•     www.demo.com - This is the hostname.
•     test - This is a directory
•     myscript.php - This is the actual script name
•     name=Bob - This is the first URL parameter (city) and it’s value (Dallas)
•     start=10 - This is the second URL parameter (start) and it’s value (10)

• Get the protocol - http

The url protocol can be read out from the$_SERVER[‘SERVER_PROTOCOL’] variable. 

Code:

1. echo $_SERVER['SERVER_PROTOCOL'];

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However if you check the value you can find that is not simply http or https but a string like this: HTTP/1.1

So we need some string manipulation to get a clean protocol string:

Code:

1. $protocol = strpos(strtolower($_SERVER['SERVER_PROTOCOL']),'https') 

2.                 === FALSE ? 'http' : 'https';

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• Get the hostname - www.demo.com

As next step we need to figure out the hostname. This is in the$_SERVER[‘HTTP_HOST’]. You can read it simply like this:

Code:

1. $host = $_SERVER['HTTP_HOST'];

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• Get the directory and script name - /test/myscript.php

The $_SERVER[‘SCRIPT_NAME’] contains the full path with the name of the actual php script as you can see here:

Code:

1. $script = $_SERVER['SCRIPT_NAME'];

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• Get URL parameters - city=Dallas&start=10

The last part of the current url is in the$_SERVER[‘QUERY_STRING’] and can be accessed similar to the others:

Code:

1. $params = $_SERVER['QUERY_STRING'];

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• Get the actual URI - /test/myscript.php?city=Dallas&start=10

If the protocol and host name is not important for you but only the path, script name and the URL parameters then you can use simply$_SERVER[‘REQUEST_URI’] the as follows:

Code:

1. $uri = $_SERVER['REQUEST_URI'];

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