Guide to Programming with Python - PowerPoint Presentation

Slide1

Guide to Programming with Python

Chapter

Nine

Working with/Creating ModulesSlide2

2

Fm: http://xkcd.com/353/Slide3

Creating Modules

Create, use, and even share your own modules

Reuse code

Could reuse the

Card, Hand, and

Deck

classes for different card games

Manage large projectsProfessional projects can be hundreds of thousands of lines longWould be nearly impossible to maintain in one file

3Slide4

Using

Modules

Module

imported using filename, just like built-in

modules Import programmer-created module the same way you import a built-in module

import random

random.randrange(10)

from random import *

randrange(10)

(import * operation may not work very well on Windows platforms, where the

filesystem does not always have accurate information about the case of a filename!)

4

What else have you tried?Slide5

Writing

Modules

Write module as a collection of related programming components, like functions and classes,

in a single file

File is just Python file with extension .

py

(

games module is file

games.py

)

class Player(object):

""" A player for a game. """ def __

init__(self, name, score = 0): self.name

= name self.score = score

def __

str__(sefl

): return "name=" + name + " score=" + str(score)def ask_yes_no(question): """Ask a yes or no question.""" response = None while response not in ("y", "n"): response = raw_input(question).lower() return response

5Slide6

if __name ==

"

__main__

"

class

Player(object

):

""" A player for a game. """ def __init__(self

, name, score = 0):

...

if __name__ == "__main__": test =

Player("Tom

", 100) print tet

raw_input("\n\nPress the enter key to exit.")Why do I need to do this? To make the file usable as a script as well as an importable module, because the code that parses the command line only runs if the module is executed as the “main” file (__name__ == "__main__"

is true )

6Slide7

Using Imported Functions and Classes

num =

games.ask_number(question

= "How many?",

low = 2, high = 5

)

player

= games.Player(name, score)

Use imported programmer-created modules the same way as you use imported built-in modules

7Slide8

What’s are Defined in a Module?

The built-in function dir() is used to find out which names a module defines. It returns a sorted list of strings:

import sys

dir(sys)Slide9

Module Not Found?The Module Search Path

When a module named spam is imported, the interpreter searches for a file named

spam.py

in the current directory, and then in the list of directories specified by the environment variable PYTHONPATH.

Modules are searched in the list of directories given by the variable sys.path import sys

sys.path.append('/home/yye/lib/python

')Slide10

Packages

Packages are a way of structuring Python’s module namespace by using “dotted module names”. For example, the module name A.B designates a

submodule

named B in a package named A.

sound/ Top-level package __init__.py

Initialize the sound package

formats/

Subpackage for file format conversions

__

init__.py

wavread.py wavwrite.py ... effects/ Subpackage

for sound effects __init__.py

echo.pyimport sound.effects.echo #or

from sound.effects import echoSlide11

The Blackjack Game -

Pseudocode

Deal each player and dealer initial two cards

For each player

While the player asks for a hit and the player is not busted

Deal the player an additional card

If there are no players still playing

Show the dealer’s two cards

Otherwise

While the dealer must hit and the dealer is not busted

Deal the dealer an additional card

If the dealer is busted

For each player who is still playing The player winsOtherwise For each player who is still playing If the player’s total is greater than the dealer’s total

The player wins Otherwise, if the player’s total is less than the dealer’s total The player loses Otherwise The player pushes

11Slide12

The Blackjack Game – Class Hierarchy

Figure 9.8: Blackjack classes

Inheritance hierarchy of classes for the Blackjack game

12Slide13

The Blackjack Game - Classes

Table 9.1: Blackjack classes

Guide to Programming with Python

13Slide14

Summary

You can write, import, and even share your own modules

You

write a module as a collection of related programming components, like functions and classes, in a single Python file

Programmer-created modules can be imported the same way that built-in modules are, with an

import

statement

You test to see if

__name__

is equal to

"__main__" to make a module identify itself as such

14Slide15

Key OOP Concepts

Inheritance

Inheriting vs. overriding (Implementing a new one or adding)

Polymorphism means that the same message can be sent to objects of different classes

related by inheritance and achieve different and appropriate results. For example, when Human or Comput object calls flip_or_take

() methods, they do things differently. Polymorphism enables objects of different types to be substituted (e.g., the human-

vs

-compute Flip game to human-vs-human Flip game). EncapsulationThe inclusion within a program object of all the resources needed for the object to function: the methods and the data to hide the details of the inner workings from the client code.

Encapsulation is a principle of abstraction -- a concept or idea not associated with any specific instance (OOP attempts to abstract both data and code)