book

Head First Programming

by David Griffiths, Paul Barry

November 2009

Beginner

440 pages

8h 31m

English

O'Reilly Media, Inc.

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Dedication
A Note Regarding Supplemental Files
Advance Praise for Head First Programming
Praise for other Head First books
Authors of Head First Programming
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 didHere’s what YOU can do to bend your brain into submissionRead MeThe technical review teamAcknowledgmentsSafari® Books Online
1. Starting to Code: Finding your way
Programming lets you do moreBut what are g and guess?So how do you run your code?Create a new program filePrepare and run your codeA program is more than a list of commandsCodeville: Your program is like a network of roadsBranches are code intersectionsif/else branchesThe Python code needs interconnecting pathsPython uses indents to connect pathsLoops let you run the same piece of code over and over againPython’s while loopYour Programming Toolbox
2. Textual Data: Every string has its place
Your new gig at Starbuzz CoffeeHere’s the current Starbuzz codeThe cost is embedded in the HTMLA string is a series of charactersFind characters inside the textBut how do you get at more than one character?Beans‘R’Us is rewarding loyal customersThe price movedSearching is complexPython data is smartStrings and numbers are differentThe program has overloaded the Beans’R’Us ServerTime... if only you had more of itYou’re already using library codeOrder is restoredYour Programming Toolbox
3. Functions: Let’s get organized
Starbuzz is out of beans!What does the new program need to do?Don’t duplicate your code......Reuse your code insteadReuse code with functionsAlways get things in the right orderReturn data with the return commandUse the Force, Web LukeThe function always sends the same messageUse parameters to avoid duplicating functionsSomeone decided to mess with your codeThe rest of the program can’t see the password variableWhen you call a function, the computer creates a fresh list of variablesWhen you leave a function, its variables get thrown awayStarbuzz is fully stocked!Your Programming Toolbox
4. Data in Files and Arrays: Sort it out
Surf’s up in CodevilleFind the highest score in the results fileIterate through the file with the open, for, close patternThe file contains more than numbers...Split each line as you read itThe split() method cuts the stringBut you need more than one top scoreKeeping track of 3 scores makes the code more complexAn ordered list makes code much simplerSorting is easier in memoryKeep the data in memoryYou can’t use a separate variable for each line of dataAn array lets you manage a whole train of dataPython gives you arrays with listsArrays can be extendedSort the array before displaying the resultsSort the scores from highest to lowestAnd the winner is...?You somehow forgot the surfer namesYour Programming Toolbox

5. Hashes and Databases: Putting data in its place
Who won the surfing contest?Associate the name with the scoreUse a hashAssociate a key with a value using a hashIterate hash data with forThe data isn’t sortedPython hashes don’t have a sort() method......but there is a function called sorted()When data gets complexSo what does RSA’s data look like?Return a data structure from a functionHere’s your new board!Meanwhile, down at the studio...The code remains the same; it’s the function that changesTVN’s data is on the money!Your Programming Toolbox
6. Modular Programming: Keeping things straight
Head First Health Club is upgrading some systemsThe program needs to create a transaction fileUse strings to format stringsA late night email ruins your day$50,000... for a donut?!Only the sales from your program were rejectedThe new bank uses a new formatYour coffee bar program still uses the old formatDon’t just update your copySmart programmers write modular codeSo how do you create a module...?...and how do you use it?The transaction file is working great, tooThe health club has a new requirementThe Starbuzz codeThe two discount functions have the same nameFully Qualified Names (FQNs) prevent your programs from getting confusedThe discounts get the customers flooding inYour Programming Toolbox
7. Building a Graphical User Interface: Going all gooey
Head First TVN now produces game showspygame is cross platform0... 2... 1... 9... blast off!tkinter gives you the event loop for freetkinter is packed with optionsThe GUI works, but doesn’t do anythingConnect code to your button eventsThe GUI program’s now ready for a screentestBut TVN is still not happyLabel itYour Programming Toolbox
8. Guis and Data: Data entry widgets
Head-Ex needs a new delivery systemThey’ve already designed the interfaceRead data from the GUIThe Entry and Text widgets let you enter text data into your GUIEntry widget: for entering single lines of textText widget: for longer, multi-line textRead and write data to text fieldsLarge Text fields are harder to handleOne of the Head-Ex deliveries went astrayUsers can enter anything in the fieldsRadio buttons force users to choose a valid depotCreating radio buttons in tkinterThe radio buttons should work togetherThe radio buttons can share a modelThe system tells the other widgets when the model changesSo how do you use models in tkinter?Head-Ex’s business is expandingThere are too many depots on the GUIAn OptionMenu lets you have as many options as neededThe model stays the sameBut wait... you don’t have to list all the values like thatThings are going great at Head-ExYour Programming Toolbox
9. Exceptions and Message Boxes: Get the message?
What’s that smell?Someone changed the file permissionsWhen it couldn’t write to the file, the program threw an exceptionCatch the exceptionWatch for exceptions with try/exceptThere’s an issue with the exception handlerA message box demands attentionCreating message boxes in PythonMessage boxes that say stuffMessage boxes that ask stuffYour Programming Toolbox
10. Graphical Interface Elements: Selecting the right tool
Time to mix it upThe music just kept on playing...Not all events are generated by button clicksControlling the Window ManagerCapturing the protocol event isn’t enoughTwo buttons, or not two buttons? That is the question...The checkbox is an on/off, flip/flop toggleWorking with checkboxes in tkinterUsing the get() methodPump up the volume!Model a slider on a scaleStart with the volumeUse pygame to set the volumeUse tkinter for everything elseThe DJ is over the moon!Your Programming Toolbox
11. Custom Widgets and Classes: With an object in mind
The DJ wants to play more than one trackHow will you create the widgets for each track?Create code for each track as a functionThe new function contains other functionsYour new function needs to create widgets and event handlersThe DJ is confusedGroup widgets togetherCreate a new type of widgetA frame widget contains other widgetsA class is a machine for creating objectsA class has methods that define behaviorBut how does an object call a method?self identifies the widget calling the methodThe SoundPanel class looks a lot like the create_gui() functionclass = methods + dataThe DJ has an entire directory of tracksIt’s party time!The mixer program brought the house down!Congratulations!Your Programming ToolboxLeaving town...It’s been great having you here in Codeville!
A. Leftovers: The Top Ten Things (we didn’t cover)
#1: Doing things “The Python Way”#2: Using Python 2#3: Other programming languages#4: Automated testing techniques#5: Debugging#6: Command-line execution#7: Ooops... we could’ve covered more OOP#8: Algorithms#9: Advanced Scary programming topics#10: Other IDEs, shells, and text editors
Index
About the Authors
Copyright

Content preview from Head First Programming

Chapter 9. Exceptions and Message Boxes: Get the message?

Sometimes things just go wrong. You just need to handle it.

There will always be things beyond your control. Networks will fail. Files will disappear. Smart coders learn how to deal with those kinds of errors and make their programs recover gracefully. The best software keeps the user informed about the bad things that happen and what should be done to recover. By learning how to use exceptions and message boxes, you can take your software to the next level of reliability and quality.

What’s that smell?

Just when it looked like things were going so well, there was a problem in the Head-Ex storeroom.

A trainee was recording the consignment of cheese when there was a problem that prevented the program from recording the delivery. That meant the cheese was never assigned to a delivery truck. So the cheese never left the storeroom and it just sat there for a very long time. And that meant—well, you can just imagine...

To prevent the same thing happening again, you need to find what caused the problem.

Someone changed the file permissions

It turns out the whole thing was caused when someone from Technical Support decided to change the permissions on the deliveries.txt file, making it read-only. When the system tried to write deliveries into ...

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