CIS192 Python ProgrammingThe End

Harry Smith

University of Pennsylvania

April 19, 2018

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Outline

1 Good Design in PythonABCs of PythonSingletonObserverMementoCIS 192 in ReviewNext Steps

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The abc module

Stands for the "Abstract Base Class" moduleUseful for giving a template for how some object should behaveRequires some methods to be implemented separately, somecome pre-packaged

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Defining an ABC

Use the ABCMeta class from abc

from abc import ABCMetaclass MyClass(metaclass=ABCMeta):

...

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Specifying abstract methods

Use the ABCMeta class from abc

import abcclass MyClass(metaclass=abc.ABCMeta):

@abc.abstractmethoddef task():raise NotImplementedError()

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Can include partially complete abstract methods

import abcclass MyClass(metaclass=abc.ABCMeta):

@abc.abstractmethoddef partial_task(x):x.sort()

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...or fully implemented methods

import abcclass MyClass(metaclass=abc.ABCMeta):

def complete_task(x):print(x)

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Subclassing (registering)Use the register method gained from the ABCMeta

import abcfrom abc_base import MyClass

@MyClass.registerclass SubClass():

def task():print("Completed task!")

def partial_task(x):super().partial_task(x)print(x[0])

>>> s = SubClass()>>> s.task()"Completed task!">>> s.complete_task("hi")"hi">>> s.partial_task([3, 2])2

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Singleton Pattern

What if I need exactly one instance of something?How can I guarantee that I’m always getting the one thing?Why is this important?

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When to Use Singleton Pattern

File I/OMultithreaded operationsDistributed Computing

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Singleton: Main Idea

Define an inner class for a given objectLet the inner class handle the behaviorLet the outer class check if the inner class has already beencreated

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Observer Pattern

What if I have a many-to-one relation of objects?How can I guarantee that each of the many objects are kept up todate?

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When to Use Observer Pattern

Controlling sub-processes or other instancesSyncing up multiple instances / usersDisplaying server status

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Observer: Main Idea

Let the "one" object inherit from a Notifier classLet the "many" classes inherit from an Observer classOn association, let the notifier store a pointer to observerOn system update, let the notifier send a message to eachobserver

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Memento Pattern

What happens when I might enter an illegal state in my program?How can I procedurally store, observe, and revert to programstates?

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When to Use Memento Pattern

When the legality of some operation cannot be thoroughlychecked before executionWhen race conditions are observable but unavoidableTo programmatically keep track of state

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Memento: Main Idea

Write a "memento" class that is a simple wrapper around someparticular stateExtend the vulnerable/interesting class to have diary and backupmethodsBefore risky operations, or at predetermined periods, write amemento objectWhen a problem is encountered, revert the class using statestored in Memento.

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Python Basics & FundamentalsFunctional ProgrammingObject-Oriented ProgrammingIterators, Generators, Exceptions & IORegular Expressions & Other ModulesHTTP Requests / HTML ParsingData AnalysisMachine LearningNatural Language ProcessingWeb AppsArtificial IntelligenceProbability & Simulations

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If there’s anything I want you to remember...Generators! Use this space-efficient tool in place of iterators.Lambdas & Functional Programming: they look scary but areincredibly useful

I ComparatorsI Elements of good design: strategy design pattern

Make your own classes!Using "with open ... as f:" to safely open files!List/Set/Generator ComprehensionsDictionariesUse pip, installing binaries, and taking advantage of librariesThe power of Python function headers (default kwargs, arbitrarilymany positional args).Testing code is easyUse Decorators to avoid repeating yourselfPython is excellent for scripting. It’s very useful for solving simplemath/probability problems.Python makes coding fun.

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Scratching the Surface

Each special topic has MUCH more depth than what we’vecovered this semester.Many topics we haven’t mentioned

I Parallel & Distributed ComputingI ConcurrencyI Graphical User InterfacesI etc.

Check the website for access to some of these.

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What’s Next?

I hope you find the skills you’ve acquired from CIS 192 useful!Build your own side-projects, big or small!Learn more about Python!

I PyCon conference recordingsI Obey the Testing Goat (TDD in Python + Web Dev)I New Coder: practical tutorials in Python

Join the Python community!/r/Python subredditTrending GitHub Python repositoriesStackOverflow Python questions

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What *Isn’t* Next?

Build a game!I PyGame bookI More rudimentary game advice

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What *Isn’t* Next?

Manage a database!I Advice on database management

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What *Isn’t* Next?

Learn Django!I Django TutorialI Mozilla’s Guides (very nice)

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What *Isn’t* Next?

Deploy Applications!I Fabric (for SSH/sysadmin)I Launch Flask Apps on AWSI Launch Django Apps on AWS

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What *Isn’t* Next?

Build an interpreterWrite your own programming languageBuild a theorem proverAutomate boring computer tasksScrape the webDo beautiful data analysisGenerate other code automaticallyHandle 3D animation and modeling in MayaSend emails automaticallyQuickly create UIBuild maps and GISControl trafficSimulate drug action

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Thank You

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