book

Head First Python, 2nd Edition

Name: Head First Python, 2nd Edition
Author: Paul Barry
ISBN: 9781491919538

by Paul Barry

November 2016

Beginner

622 pages

15h 57m

English

O'Reilly Media, Inc.

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Dedication
Advance Praise for Head First Python, Second Edition
Praise for the first editionPraise for other Head First booksPraise for other Head First books
Author of Head First Python, 2nd Edition
How to use this Book: Intro
Who Is This Book For?Who should probably back away from this book?We Know What You’re ThinkingWe know what your brain is thinkingMetacognition: Thinking About ThinkingHere’s What WE Did:Here’s what YOU can do to bend your brain into submissionRead Me, 1 of 2Read Me, 2 of 2The Technical Review TeamAcknowledgments and Thanks
O’Reilly Safari
1. The Basics: Getting Started Quickly
Breaking with TraditionStarting with a meatier exampleJump Right InPython’s IDLE is all you need to get goingUnderstanding IDLE’s WindowsWhat Happens Next...Press F5 to Run Your CodeCode Runs ImmediatelyExecuting Code, One Statement at a TimeLet’s be the Python interpreterFunctions + Modules = The Standard LibraryBatteries IncludedData Structures Come Built-inPython variables are dynamically assignedInvoking Methods Obtains ResultsInvoking built-in module functionalityDeciding When to Run Blocks of CodeWhat Happened to My Curly Braces?A colon introduces an indented suite of codeWhat “else” Can You Have with “if”?Suites Can Contain Embedded SuitesWhat We Already KnowExtending Our Program to Do MoreWhat we need to do:What’s the Best Approach to Solving This Problem?Both approaches work with PythonReturning to the Python ShellExperimenting at the ShellIterating Over a Sequence of ObjectsIterating a Specific Number of TimesApplying the Outcome of Task #1 to Our CodeIndent Suites with Format...Indent RegionArranging to Pause ExecutionImportation ConfusionGenerating Random Integers with PythonAsking the Interpreter for HelpReviewing Our ExperimentsUpdating What We Already KnowA Few Lines of Code Do a LotCoding a Serious Business ApplicationDealing with all that beer...Python Code Is Easy to ReadIs Indentation Driving You Crazy?Getting back to the beer song codeAsking the Interpreter for Help on a FunctionStarting, stopping, and steppingExperimenting with RangesDon’t Forget to Try the Beer Song CodeWrapping up what you already knowWith all the beer gone, what’s next?Chapter 1’s Code
2. List Data: Working with Ordered Data
Numbers, Strings...and ObjectsNumbersStringsObjects“Everything Is an Object”But...what does all this actually mean?Meet the Four Built-in Data StructuresOrdered Collections Are Mutable/ImmutableAn Unordered Data Structure: DictionaryA Data Structure That Avoids Duplicates: SetThe 80/20 data structure rule of thumbA List Is an Ordered Collection of ObjectsHow to spot a list in codeCreating Lists LiterallyPutting Lists to WorkWorking with listsIs one object inside another? Check with “in”Use Your Editor When Working on More Than a Few Lines of CodeDon’t forget: press F5 to run your program“Growing” a List at RuntimeChecking for Membership with “in”Is the object “in” or “not in”?It’s Time to Update Our CodeRemoving Objects from a ListPopping Objects Off a ListExtending a List with ObjectsInserting an Object into a ListWhat About Using Square Brackets?What Happened to “plist”?Lists: What We KnowWhat Looks Like a Copy, But Isn’tHow to Copy a Data StructureSquare Brackets Are EverywhereLists: Updating What We Already KnowLists Extend the Square Bracket NotationLists Understand Start, Stop, and StepYou can put start, stop, and step inside square bracketsList Slices in ActionStarting and Stopping with ListsStepping with ListsSlices are everywherePutting Slices to Work on ListsConverting “Don’t panic!” to “on tap”Putting Slices to Work on Lists, ContinuedWhich Is Better? It Depends...Slicing a List Is NondestructiveSo...which is better?Python’s “for” Loop Understands ListsUnderstanding marvin.py’s codePython’s “for” Loop Understands SlicesMarvin’s Slices in DetailLists: Updating What We KnowWhat’s Wrong with Lists?When Not to Use ListsChapter 2’s Code, 1 of 2Chapter 2’s Code, 2 of 2
3. Structured Data: Working with Structured Data
A Dictionary Stores Key/Value PairsMake Dictionaries Easy to ReadHow to Spot a Dictionary in CodeWhat happened to the insertion order?Insertion Order Is NOT MaintainedDictionaries understand square bracketsValue Lookup with Square BracketsDictionary lookup is fast!Working with Dictionaries at RuntimeRecap: Displaying Found Vowels (Lists)How Can a Dictionary Help Here?Selecting a Frequency Count Data StructureUpdating a Frequency CounterUpdating a Frequency Counter, v2.0Iterating Over a DictionaryIterating Over Keys and ValuesDictionaries: What We Already KnowSpecifying the ordering of a dictionary on outputIterating Over a Dictionary with “items”Just How Dynamic Are Dictionaries?Dictionary keys must be initializedAvoiding KeyErrors at RuntimeChecking for Membership with “in”Ensuring Initialization Before UseSubstituting “not in” for “in”Putting the “setdefault” Method to WorkDictionaries: updating what we already knowAren’t Dictionaries (and Lists) Enough?Sets Don’t Allow DuplicatesSpotting sets in your codeCreating Sets EfficientlyCreating sets from sequencesTaking Advantage of Set Methodsunion Works by Combining SetsWhat happened to the loop code?difference Tells You What’s Not Sharedintersection Reports on CommonalitySets: What You Already KnowUsing a set was the perfect choice here...Making the Case for TuplesHow to spot a tuple in codeTuples Are ImmutableWatch Out for Single-Object TuplesCombining the Built-in Data StructuresStoring a Table of DataA Dictionary Containing a DictionaryA Dictionary of Dictionaries (a.k.a. a Table)Pretty-Printing Complex Data StructuresVisualizing Complex Data StructuresAccessing a Complex Data Structure’s DataData Is As Complex As You Make ItChapter 3’s Code, 1 of 2Chapter 3’s Code, 2 of 2
4. Code Reuse: Functions and Modules
Reusing Code with FunctionsIntroducing FunctionsWhat About Type Information?Naming a Chunk of Code with “def”Invoking Your FunctionEdit your function in an editor, not at the promptUse IDLE’s Editor to Make ChangesWhat’s the Deal with All Those Strings?Understanding the string quote charactersFollow Best Practice As Per the PEPsFunctions Can Accept ArgumentsFunctions Return a ResultReturning One ValueReturning More Than One ValueWhat’s the deal with “set()”?Recalling the Built-in Data StructuresUse Annotations to Improve Your DocsWhy Use Function Annotations?Functions: What We Know AlreadyMaking a Generically Useful FunctionCreating Another Function, 1 of 3Creating Another Function, 2 of 3Creating Another Function, 3 of 3Specifying Default Values for ArgumentsPositional Versus Keyword AssignmentUpdating What We Know About FunctionsFunctions Beget ModulesCreating modules couldn’t be easier, however...How Are Modules Found?Running Python from the Command LineNot Found Modules Produce ImportErrorsImportErrors Occur No Matter the PlatformGetting a Module into Site-packagesUsing “setuptools” to install into site-packagesCreating the Required Setup FilesCreating the Distribution FileCreating a distribution file on WindowsDistribution Files on UNIX-like OSesInstalling Packages with “pip”Step 3 on WindowsStep 3 on UNIX-like OSesModules: What We Know AlreadyGiving your code away (a.k.a. sharing)Demonstrating Call-by-Value SemanticsDemonstrating Call-by-Reference SemanticsCan I Test for PEP 8 Compliance?Getting Ready to Check PEP 8 ComplianceRecalling our codeInstalling pytest and the pep8 plug-inInstall the Testing Developer ToolsHow PEP 8–Compliant Is Our Code?Understanding the Failure MessagesConfirming PEP 8 ComplianceConformance to PEP 8 is a good thingChapter 4’s Code

5. Building a Webapp: Getting Real
Python: What You Already KnowLet’s Build SomethingWhat Do We Want Our Webapp to Do?What Happens on the Web Server?What do we need to get going?Let’s Install FlaskInstall Flask from the command-line with pipHow Does Flask Work?Check that Flask is installed and workingRun Flask from your OS command lineRunning Your Flask Webapp for the First TimeHere’s What Happened (Line by Line)Creating a Flask Webapp ObjectDecorating a Function with a URLRunning Your Webapp’s Behavior(s)Exposing Functionalit y to the WebRecall What We’re Trying to BuildHere’s the planBuilding the HTML FormBut...what if you’re new to all this HTML stuff?Create the HTML, then send it to the browserTemplates Relate to Web PagesRendering Templates from FlaskDisplaying the Webapp’s HTML FormPreparing to Run the Template CodeWe’re Ready for a Test RunUnderstanding HTTP Status CodesHandling Posted DataRefining the Edit/Stop/Start/Test CycleAccessing HTML Form Data with FlaskUsing Request Data in Your WebappAutomatic ReloadsProducing the Results As HTMLCalculating the Data We NeedAdding a Finishing TouchRedirect to Avoid Unwanted ErrorsFunctions Can Have Multiple URLsUpdating What We KnowIs that all there is to this chapter?Preparing Your Webapp for the CloudExploiting Dunder Name Dunder MainDeploying to PythonAnywhere (well... almost)Chapter 5’s Code
6. Storing and Manipulating Data: Where to Put Your Data
Doing Something with Your Webapp’s DataPython Supports Open, Process, CloseReading Data from an Existing FileA Better Open, Process, Close: “with”The “with” statement manages contextInvoking the logging functionA Quick ReviewTake your webapp for a spin...Data is logged (behind the scenes)View the Log Through Your WebappWhere to start when things go wrong with your outputExamine the Raw Data with View SourceIt’s Time to Escape (Your Data)Viewing the Entire Log in Your WebappLearning More About the Request ObjectWhat’s all this, then?Logging Specific Web Request AttributesLog a Single Line of Delimited DataOne Final Change to Our Logging CodeFrom Raw Data to Readable OutputDoes This Remind You of Anything?Use a Dict of Dicts...or Something Else?Take a closer look at the logged dataWhat’s Joined Together Can Be Split ApartGetting to a list of lists from a list of stringsWhen Should the Conversion Occur?Processing Data: What We Already KnowTake a closer look at the outputGenerate Readable Output With HTMLEmbed Display Logic in Your TemplateProducing Readable Output with Jinja2The Current State of Our Webapp CodeAsking Questions of Your DataChapter 6’s Code
7. Using a Database: Putting Python’s DB-API to Use
Database-Enabling Your WebappTask 1: Install the MySQL ServerIntroducing Python’s DB-APITask 2: Install a MySQL Database Driver for PythonInstall MySQL-Connector/PythonTask 3: Create Our Webapp’s Database and TablesDecide on a Structure for Your Log DataConfirm Your Table Is Ready for DataTask 4: Create Code to Work with Our Webapp’s Database and TablesStoring Data Is Only Half the BattleThis new function is a big changeHow Best to Reuse Your Database Code?Consider What You’re Trying to ReuseWhat About That Import?Be careful when positioning your import statementsConsider What You’re Trying to DoYou’ve Seen This Pattern BeforeThe Bad News Isn’t Really All That BadChapter 7’s Code
8. A Little Bit of Class: Abstracting Behavior and State
Hooking into the “with” StatementAn Object-Oriented PrimerA class bundles behavior and stateClasses have methods and attributesCreating Objects from ClassesObjects Share Behavior but Not StateDefining what we want CountFromBy to doDoing More with CountFromByIt’s Worth Repeating Ourselves: Objects Share Behavior but Not StateInvoking a Method: Understand the DetailsMethod Invocation: What Actually HappensAdding a Method to a ClassAre You Serious About “self”?The Importance of “self”Coping with ScopingPrefix Your Attribute Names with “self”Initialize (Attribute) Values Before UseDunder “init” Initializes AttributesInitializing Attributes with Dunder “init”Pass any amount of argument data to dunder “init”What have we learned from this Test Drive?Understanding CountFromBy’s RepresentationDefining CountFromBy’s RepresentationProviding Sensible Defaults for CountFromByClasses: What We KnowChapter 8’s Code
9. The Context Management Protocol: Hooking into Python’s with Statement
What’s the Best Way to Share Our Webapp’s Database Code?Consider What You’re Trying to Do, RevisitedHow best to create a context manager?Managing Context with MethodsDunder “enter” performs setupDunder “exit” does teardown(As you know) dunder “init” initializesYou’ve Already Seen a Context Manager in ActionWhat’s required from youCreate a New Context Manager ClassInitialize the Class with the Database ConfigYour context manager begins to take shapePerform Setup with Dunder “enter”Don’t forget to prefix all attributes with “self”Perform Teardown with Dunder “exit”Your context manager is re ady for testingThere’s not much code here, is there?Reconsidering Your Webapp Code, 1 of 2Reconsidering Your Webapp Code, 2 of 2Recalling the “log_request” FunctionAmending the “log_request” FunctionRecalling the “view_the_log” FunctionIt’s Not Just the Code That ChangesAmending the “view_the_log” FunctionHere’s the SQL query you’ll needIt’s nearly time for one last Test DriveAll That Remains...Answering the Data QuestionsHow many requests have been responded to?What’s the most common list of letters?Which IP addresses are the requests coming from?Which browser is being used the most?Chapter 9’s Code, 1 of 2Chapter 9’s Code, 2 of 2
10. Function Decorators: Wrapping Functions
Your Webapp Is Working Well, But...Only authenticated users gain accessThe Web Is StatelessHTTP is to blame...Your Web Server (Not Your Computer) Runs Your CodeIt’s Time for a Bit of a SessionFlask’s Session Technology Adds StateDictionary Lookup Retrieves StateTime for a Test Drive?Managing Logins with SessionsLet’s Do LoginAmend the webapp’s code to handle loginsLet’s Do Logout and Status CheckingAmend the webapp’s code once moreWhy Not Check for False?Can We Now Restrict Access to URLs?Copy-and-Paste Is Rarely a Good IdeaPut shared code into its own functionCreating a Function Helps, But...You’ve Been Using Decorators All AlongPass a Function to a FunctionFunctions can take a function as an argumentInvoking a Passed FunctionFunctions Can Be Nested Inside FunctionsWhen would you ever use this?Return a Function from a FunctionAccepting a List of ArgumentsUse * to accept an arbitrary list of argumentsProcessing a List of Arguments* works on the way in, tooAccepting a Dictionary of ArgumentsUse ** to accept arbitrary keyword argumentsProcessing a Dictionary of Arguments** works on the way in, tooAccepting Any Number and Type of Function ArgumentsA Recipe for Creating a Function DecoratorRecap: We Need to Restrict Access to Certain URLsCreating a Function DecoratorThat’s almost too easy, isn’t it?Can you see why the nested function is called “wrapper”?The Final Step: Handling ArgumentsWe’re done...or are we?Your Decorator in All Its GloryPutting Your Decorator to WorkThe Beauty of DecoratorsCreating More DecoratorsBack to Restricting Access to /viewlogWhat’s Next?Chapter 10’s Code, 1 of 2Chapter 10’s Code, 2 of 2
11. Exception Handling: What to Do When Things Go Wrong
Databases Aren’t Always AvailableWeb Attacks Are a Real PainInput-Output Is (Sometimes) SlowYour Function Calls Can FailConsidering the Identified Problems1. Your database connection fails2. Your application is subjected to an attack3. Your code takes a long time to execute4. Your function call failsAlways Try to Execute Error-Prone CodeCatching an Error Is Not EnoughThere can be more than one exception raised...try Once, but except Many TimesA Lot of Things Can Go WrongThe Catch-All Exception HandlerHaven’t We Just Lost Something?Learning About Exceptions from “sys”The Catch-All Exception Handler, RevisitedGetting Back to Our Webapp CodeSilently Handling ExceptionsHandling Other Database ErrorsDoes “More Errors” Mean “More excepts”?Avoid Tightly Coupled CodeThe DBcm Module, RevisitedCreating Custom ExceptionsThe empty class isn’t quite empty...What Else Can Go Wrong with “DBcm”?Creating More Custom ExceptionsAre Your Database Credentials Correct?Handling SQLError Is DifferentBe Careful with Code PositioningRaising an SQLErrorA Quick Recap: Adding RobustnessWhat happens if something takes a long time?How to Deal with Wait? It Depends...Chapter 11’s Code, 1 of 3Chapter 11’s Code, 2 of 3Chapter 11’s Code, 3 of 3
12. 11¾ A Little Bit of Threading: Dealing with Waiting
Waiting: What to Do?How Are You Querying Your Database?Database INSERTs and SELECTs Are DifferentDoing More Than One Thing at OnceConcurrent code: you have optionsDon’t Get Bummed Out: Use ThreadsFirst Things First: Don’t PanicDon’t Get Bummed Out: Flask Can HelpIs Your Webapp Robust Now?Chapter 11¾’s Code, 1 of 2Chapter 11¾’s Code, 2 of 2
13. Advanced Iteration: Looping Like Crazy
Bahamas Buzzers Have Places to GoReading CSV Data As ListsReading CSV Data As DictionariesLet’s Back Up a Little BitStripping, Then Splitting, Your Raw DataBe Careful When Chaining Method CallsTransforming Data into the Format You NeedTransforming into a Dictionary Of ListsThink about the data wrangling that’s needed here...Let’s Do the Basic ConversionsDid You Spot the Pattern in Your Code?Spotting the Pattern with ListsConverting Patterns into ComprehensionsTake a Closer Look at the ComprehensionWhat’s the Big Deal?Comprehensions aren’t just for listsSpecifying a Dictionary ComprehensionExtend Comprehensions with FiltersRecall What You Set Out to DoDeal with Complexity the Python WayExtract a Single Destination’s Flight TimesExtract Flight Times for All DestinationsThat Feeling You Get...The Set Comprehension in ActionHow to Spot a ComprehensionWhat About “Tuple Comprehensions”?Parentheses Around Code == GeneratorGenerators produce data items one at a time...Using a Listcomp to Process URLsUsing a Generator to Process URLsUsing a Generator: What Just Happened?Define What Your Function Needs to DoYield to the Power of Generator FunctionsTracing Your Generator Function, 1 of 2Tracing Your Generator Function, 2 of 2Concluding RemarksOne Final QuestionChapter 12’s CodeIt’s Time to Go…You’re on your way!
A. Installation: Installing Python
Install Python 3 on WindowsDownload, then installCheck Python 3 on WindowsIt’s Python 3 on Windows, sort of...Add to Python 3 on WindowsInstall Python 3 on Mac OS X (macOS)Using a package managerCheck and Configure Python 3 on Mac OS XThe Python 3 folder on Mac OS XYou’re ready to run on Mac OS XInstall Python 3 on Linux
B. Pythonanywhere: Deploying Your Webapp
Step 0: A Little PrepStep 1: Sign Up for PythonAnywhereStep 2: Upload Your Files to the CloudStep 3: Extract and Install Your CodeStep 4: Create a Starter Webapp, 1 of 2Step 4: Create a Starter Webapp, 2 of 2Step 5: Configure Your WebappStep 6: Take Your Cloud-Based Webapp for a Spin!
C. Top Ten Things we Didn’t Cover: There’s Always More to Learn
1. What About Python 2?2. Virtual Programming Environments3. More on Object Orientation4. Formats for Strings and the Like5. Getting Things Sorted6. More from the Standard Librarycollectionsitertoolsfunctools7. Running Your Code ConcurrentlyNew keywords: async and await8. GUIs w ith Tkinter (and Fun w ith Turtles)9. It’s Not Over ’Til It’s Tested10. Debug, Debug, Debug
D. Top Ten Projects not Covered: Even More Tools, Libraries, and Modules
1. Alternatives to >>>2. Alternatives to IDLEWhat does Paul use?3. Jupyter Notebook: The Web-Based IDE4. Doing Data Science5. Web Development TechnologiesBut wait, there’s more6. Working with Web DataScrape that web data!7. More Data SourcesThere’s more to querying data than SQL8. Programming ToolsMore help with testing, too9. Kivy: Our Pick for “Coolest Project Ever”10. Alternative Implementations
E. Getting Involved: The Python Community
BDFL: Benevolent Dictator for LifePSF: The Python Sof t ware FoundationPyCon: The Python ConferenceA Tolerant Community: Respect for DiversitCome for the language, stay for the communityPython PodcastsPython NewslettersThe Zen of PythonWhich Book Should I Read Next?Our Favorite Python Books
Index
About the Author
Copyright

Content preview from Head First Python, 2nd Edition

Chapter 4. Code Reuse: Functions and Modules

Reusing code is key to building a maintainable system.

And when it comes to reusing code in Python, it all starts and ends with the humble function. Take some lines of code, give them a name, and you’ve got a function (which can be reused). Take a collection of functions and package them as a file, and you’ve got a module (which can also be reused). It’s true what they say: it’s good to share, and by the end of this chapter, you’ll be well on your way to sharing and reusing your code, thanks to an understanding of how Python’s functions and modules work.

Reusing Code with Functions

Although a few lines of code can accomplish a lot in Python, sooner or later you’re going to find your program’s codebase is growing...and, when it does, things quickly become harder to manage. What started out as 20 lines of Python code has somehow ballooned to 500 lines or more! When this happens, it’s time to start thinking about what strategies you can use to reduce the complexity of your codebase.

Like many other programming languages, Python supports modularity, in that you can break large chunks of code into smaller, more manageable pieces. You do this by creating functions, which you can think of as named chunks of code. Recall this diagram from Chapter 1, which shows the relationship between functions, modules, and the standard library:

In this chapter, we’re ...

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Head First Python, 2nd Edition

by Paul Barry

Chapter 4. Code Reuse: Functions and Modules

Reusing Code with Functions

Become an O’Reilly member and get unlimited access to this title plus top books and audiobooks from O’Reilly and nearly 200 top publishers, thousands of courses curated by job role, 150+ live events each month,
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