You are previewing Programming Python, 4th Edition.

Programming Python, 4th Edition

Cover of Programming Python, 4th Edition by Mark Lutz Published by O'Reilly Media, Inc.
  1. Programming Python
  2. A Note Regarding Supplemental Files
  3. Preface
    1. “And Now for Something Completely Different…”
    2. About This Book
      1. This Book’s Ecosystem
      2. What This Book Is Not
    3. About This Fourth Edition
      1. Specific Changes in This Edition
    4. What’s Left, Then?
    5. Python 3.X Impacts on This Book
      1. Specific 3.X Changes
      2. Language Versus Library: Unicode
      3. Python 3.1 Limitations: Email, CGI
    6. Using Book Examples
      1. Where to Look for Examples and Updates
      2. Example Portability
      3. Demo Launchers
      4. Code Reuse Policies
    7. Contacting O’Reilly
    8. Conventions Used in This Book
    9. Acknowledgments
  4. I. The Beginning
    1. 1. A Sneak Preview
      1. “Programming Python: The Short Story”
      2. The Task
      3. Step 1: Representing Records
      4. Step 2: Storing Records Persistently
      5. Step 3: Stepping Up to OOP
      6. Step 4: Adding Console Interaction
      7. Step 5: Adding a GUI
      8. Step 6: Adding a Web Interface
      9. The End of the Demo
  5. II. System Programming
    1. 2. System Tools
      1. “The os.path to Knowledge”
      2. System Scripting Overview
      3. Introducing the sys Module
      4. Introducing the os Module
    2. 3. Script Execution Context
      1. “I’d Like to Have an Argument, Please”
      2. Current Working Directory
      3. Command-Line Arguments
      4. Shell Environment Variables
      5. Standard Streams
    3. 4. File and Directory Tools
      1. “Erase Your Hard Drive in Five Easy Steps!”
      2. File Tools
      3. Directory Tools
    4. 5. Parallel System Tools
      1. “Telling the Monkeys What to Do”
      2. Forking Processes
      3. Threads
      4. Program Exits
      5. Interprocess Communication
      6. The multiprocessing Module
      7. Other Ways to Start Programs
      8. A Portable Program-Launch Framework
      9. Other System Tools Coverage
    5. 6. Complete System Programs
      1. “The Greps of Wrath”
      2. A Quick Game of “Find the Biggest Python File”
      3. Splitting and Joining Files
      4. Generating Redirection Web Pages
      5. A Regression Test Script
      6. Copying Directory Trees
      7. Comparing Directory Trees
      8. Searching Directory Trees
      9. Visitor: Walking Directories “++”
      10. Playing Media Files
      11. Automated Program Launchers (External)
  6. III. GUI Programming
    1. 7. Graphical User Interfaces
      1. “Here’s Looking at You, Kid”
      2. Python GUI Development Options
      3. tkinter Overview
      4. Climbing the GUI Learning Curve
      5. tkinter Coding Alternatives
      6. Adding Buttons and Callbacks
      7. Adding User-Defined Callback Handlers
      8. Adding Multiple Widgets
      9. Customizing Widgets with Classes
      10. Reusable GUI Components with Classes
      11. The End of the Tutorial
      12. Python/tkinter for Tcl/Tk Converts
    2. 8. A tkinter Tour, Part 1
      1. “Widgets and Gadgets and GUIs, Oh My!”
      2. Configuring Widget Appearance
      3. Top-Level Windows
      4. Dialogs
      5. Binding Events
      6. Message and Entry
      7. Checkbutton, Radiobutton, and Scale
      8. Running GUI Code Three Ways
      9. Images
      10. Viewing and Processing Images with PIL
    3. 9. A tkinter Tour, Part 2
      1. “On Today’s Menu: Spam, Spam, and Spam”
      2. Menus
      3. Listboxes and Scrollbars
      4. Text
      5. Canvas
      6. Grids
      7. Time Tools, Threads, and Animation
      8. The End of the Tour
    4. 10. GUI Coding Techniques
      1. “Building a Better Mousetrap”
      2. GuiMixin: Common Tool Mixin Classes
      3. GuiMaker: Automating Menus and Toolbars
      4. ShellGui: GUIs for Command-Line Tools
      5. GuiStreams: Redirecting Streams to Widgets
      6. Reloading Callback Handlers Dynamically
      7. Wrapping Up Top-Level Window Interfaces
      8. GUIs, Threads, and Queues
      9. More Ways to Add GUIs to Non-GUI Code
      10. The PyDemos and PyGadgets Launchers
    5. 11. Complete GUI Programs
      1. “Python, Open Source, and Camaros”
      2. PyEdit: A Text Editor Program/Object
      3. PyPhoto: An Image Viewer and Resizer
      4. PyView: An Image and Notes Slideshow
      5. PyDraw: Painting and Moving Graphics
      6. PyClock: An Analog/Digital Clock Widget
      7. PyToe: A Tic-Tac-Toe Game Widget
      8. Where to Go from Here
  7. IV. Internet Programming
    1. 12. Network Scripting
      1. “Tune In, Log On, and Drop Out”
      2. Python Internet Development Options
      3. Plumbing the Internet
      4. Socket Programming
      5. Handling Multiple Clients
      6. Making Sockets Look Like Files and Streams
      7. A Simple Python File Server
    2. 13. Client-Side Scripting
      1. “Socket to Me!”
      2. FTP: Transferring Files over the Net
      3. Transferring Files with ftplib
      4. Transferring Directories with ftplib
      5. Transferring Directory Trees with ftplib
      6. Processing Internet Email
      7. POP: Fetching Email
      8. SMTP: Sending Email
      9. email: Parsing and Composing Mail Content
      10. A Console-Based Email Client
      11. The mailtools Utility Package
      12. NNTP: Accessing Newsgroups
      13. HTTP: Accessing Websites
      14. The urllib Package Revisited
      15. Other Client-Side Scripting Options
    3. 14. The PyMailGUI Client
      1. “Use the Source, Luke”
      2. Major PyMailGUI Changes
      3. A PyMailGUI Demo
      4. PyMailGUI Implementation
      5. Ideas for Improvement
    4. 15. Server-Side Scripting
      1. “Oh, What a Tangled Web We Weave”
      2. What’s a Server-Side CGI Script?
      3. Running Server-Side Examples
      4. Climbing the CGI Learning Curve
      5. Saving State Information in CGI Scripts
      6. The Hello World Selector
      7. Refactoring Code for Maintainability
      8. More on HTML and URL Escapes
      9. Transferring Files to Clients and Servers
    5. 16. The PyMailCGI Server
      1. “Things to Do When Visiting Chicago”
      2. The PyMailCGI Website
      3. The Root Page
      4. Sending Mail by SMTP
      5. Reading POP Email
      6. Processing Fetched Mail
      7. Utility Modules
      8. Web Scripting Trade-Offs
  8. V. Tools and Techniques
    1. 17. Databases and Persistence
      1. “Give Me an Order of Persistence, but Hold the Pickles”
      2. Persistence Options in Python
      3. DBM Files
      4. Pickled Objects
      5. Shelve Files
      6. The ZODB Object-Oriented Database
      7. SQL Database Interfaces
      8. ORMs: Object Relational Mappers
      9. PyForm: A Persistent Object Viewer (External)
    2. 18. Data Structures
      1. “Roses Are Red, Violets Are Blue; Lists Are Mutable, and So Is Set Foo”
      2. Implementing Stacks
      3. Implementing Sets
      4. Subclassing Built-in Types
      5. Binary Search Trees
      6. Graph Searching
      7. Permuting Sequences
      8. Reversing and Sorting Sequences
      9. PyTree: A Generic Tree Object Viewer
    3. 19. Text and Language
      1. “See Jack Hack. Hack, Jack, Hack”
      2. Strategies for Processing Text in Python
      3. String Method Utilities
      4. Regular Expression Pattern Matching
      5. XML and HTML Parsing
      6. Advanced Language Tools
      7. Custom Language Parsers
      8. PyCalc: A Calculator Program/Object
    4. 20. Python/C Integration
      1. “I Am Lost at C”
      2. Extending Python in C: Overview
      3. A Simple C Extension Module
      4. The SWIG Integration Code Generator
      5. Wrapping C Environment Calls
      6. Wrapping C++ Classes with SWIG
      7. Other Extending Tools
      8. Embedding Python in C: Overview
      9. Basic Embedding Techniques
      10. Registering Callback Handler Objects
      11. Using Python Classes in C
      12. Other Integration Topics
  9. VI. The End
    1. 21. Conclusion: Python and the Development Cycle
      1. “That’s the End of the Book, Now Here’s the Meaning of Life”
      2. “Something’s Wrong with the Way We Program Computers”
      3. The “Gilligan Factor”
      4. Doing the Right Thing
      5. Enter Python
      6. But What About That Bottleneck?
      7. On Sinking the Titanic
      8. “So What’s Python?”: The Sequel
      9. In the Final Analysis…
  10. Index
  11. About the Author
  12. Colophon
  13. Copyright
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Pickled Objects

Probably the biggest limitation of DBM keyed files is in what they can store: data stored under a key must be a simple string. If you want to store Python objects in a DBM file, you can sometimes manually convert them to and from strings on writes and reads (e.g., with str and eval calls), but this takes you only so far. For arbitrarily complex Python objects such as class instances and nested data structures, you need something more. Class instance objects, for example, cannot usually be later re-created from their standard string representations. Moreover, custom to-string conversions and from-string parsers are error prone and not general.

The Python pickle module, a standard part of the Python system, provides the conversion step needed. It’s a sort of super general data formatting and de-formatting tool—pickle converts nearly arbitrary Python in-memory objects to and from a single linear string format, suitable for storing in flat files, shipping across network sockets between trusted sources, and so on. This conversion from object to string is often called serialization—arbitrary data structures in memory are mapped to a serial string form.

The string representation used for objects is also sometimes referred to as a byte stream, due to its linear format. It retains all the content and references structure of the original in-memory object. When the object is later re-created from its byte string, it will be a new in-memory object identical in structure and value ...

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