Professional Python

Objects 79 Using type Directly 80 Creating a Class 81 Creating a Subclass 81 The type Chain 82 The Role of type 82 Writing Metaclasses 83 The new Method 83 new Versus init 83 A Trivial Metaclass 84 Metaclass Inheritance 84 Using Metaclasses 87 Python 3 87 Python 2 88 What About Code That Might Run on Either Version? 88 When Is Cross-Compatibility Important? 89 When to Use Metaclasses 89 Declarative Class Declaration 89 An Existing Example 89 How This Works 90 Why This Is a Good Use for Metaclasses 91 Class Verification 91 Non-Inheriting Attributes 93 The Question of Explicit Opt-In 94 Meta-Coding 95 Summary 97 CHAPTER 6: CLASS FACTORIES 99 A Review of type 99 Understanding a Class Factory Function 100 Determining When You Should Write Class Factories 102 Runtime Attributes 102 Understanding Why You Should Do This 103 Attribute Dictionaries 104 Fleshing Out the Credential Class 104 The Form Example 105 Dodging Class Attribute Consistency 106 Class Attributes Versus Instance Attributes 107 The Class Method Limitation 108 Tying This in with Class Factories 109 Answering the Singleton Question 109 Summary 111 CHAPTER 7: ABSTRACT BASE CLASSES 113 Using Abstract Base Classes 113 Declaring a Virtual Subclass 115 Why Declare Virtual Subclasses? 115 Using register as a Decorator 117 __subclasshook__ 117 Declaring a Protocol 119 Other Existing Approaches 119 Using NotImplementedError 120 Using Metaclasses 120 The Value of Abstract Base Classes 122 Abstract Properties 124 Abstract Class or Static Methods 125 Built-in Abstract Base Classes 126 Single-Method ABCs 126 Alternative-Collection ABCs 127 Using Built-In Abstract Base Classes 128 Additional ABCs 128 Summary 128 PART III: DATA CHAPTER 8: STRINGS AND UNICODE 131 Text String Versus Byte String 131 String Data in Python 132 Python 3 Strings 132 Python 2 Strings 134 six 136 Strings with Non-ASCII Characters 136 Observing the Difference 136 Unicode Is a Superset of ASCII 137 Other Encodings 137 Encodings Are Not Cross-Compatible 138 Reading Files 139 Python 3 139 Specifying Encoding 139 Reading Bytes 140 Python 2 140 Reading Other Sources 141 Specifying Python File Encodings 141 Strict Codecs 143 Suppressing Errors 143 Registering Error Handlers 144 Summary 145 CHAPTER 9: REGULAR EXPRESSIONS 147 Why Use Regular Expressions? 147 Regular Expressions in Python 148 Raw Strings 148 Match Objects 149 Finding More Than One Match 149 Basic Regular Expressions 150 Character Classes 150 Ranges 151 Negation 151 Shortcuts 152 Beginning and End of String 153 Any Character 154 Optional Characters 154 Repetition 155 Repetition Ranges 155 Open-Ended Ranges 156 Shorthand 156 Grouping 157 The Zero Group 159 Named Groups 159 Referencing Existing Groups 160 Lookahead 161 Flags 163 Case Insensitivity 163 ASCII and Unicode 163 Dot Matching Newline 163 Multiline Mode 164 Verbose Mode 164 Debug Mode 164 Using Multiple Flags 165 Inline Flags 165 Substitution 165 Compiled Regular Expressions 166 Summary 167 PART IV: EVERYTHING ELSE CHAPTER 10: PYTHON 2 VERSUS PYTHON 3 171 Cross-Compatibility Strategies 171 The __future__ Module 172 2to3 172 Writing Changes 173 Limitations 174 six 174 Changes in Python 3 175 Strings and Unicode 175 The print Function 176 Division 176 Absolute and Relative Imports 177 Removal of "Old-Style" Classes 178 Metaclass Syntax 179 six.with_metaclass 179 six.add_metaclass 180 Exception Syntax 180 Handling Exceptions 181 Exception Chaining 181 Dictionary Methods 182 Function Methods 183 Iterators 183 Standard Library Relocations 184 Merging "Fast" Modules 184 io 184 pickle 184 The URL Modules 185 Renames 185 Other Package Reorganizations 185 Version Detection 186 Summary 186 CHAPTER 11: UNIT TESTING 187 The Testing Continuum 187 The Copied Ecosystem 188 The Isolated Environment 188 Advantages and Disadvantages 189 Speed 189 Interactivity 189 Testing Code 190 Code Layout 190 Testing the Function 191 The assert Statement 192 Unit Testing Frameworks 192 Running Unit Tests 193 Failures 193 Errors 194 Skipped Tests 195 Loading Tests 196 Mocking 197 Mocking a Function Call 197 Asserting Mocked Calls 199 Inspecting Mocks 201 Call Count and Status 201 Multiple Calls 202 Inspecting Calls 203 Other Testing Tools 203 coverage 203 tox 204 Other Test Runners 205 Summary 205 CHAPTER 12: CLI TOOLS 207 optparse 207 A Simple Argument 207 name == ' main__' 208 OptionParser 208 Options 209 Types of Options 209 Adding Options to OptionParser 209 Options with Values 210 Non-String Values 211 Specifying Option Values 212 Positional Arguments 214 Counters 214 List Values 215 Why Use optparse? 216 argparse 216 The Bare Bones 217 Arguments and Options 217 Option Flags 217 Alternate Prefi xes 218 Options with Values 219 Positional Arguments 222 Reading Files 223 Why Use argparse? 224 Summary 224 CHAPTER 13: ASYNCIO 225 The Event Loop 225 A Simple Event Loop 226 Running the Loop 226 Registering Tasks and Running the Loop 227 Delaying Calls 227 Partials 228 Running the Loop until a Task Completes 228 Running a Background Loop 229 Coroutines 230 Nested Coroutines 231 Futures and Tasks 232 Futures 232 Tasks 232 Callbacks 234 No Guarantee of Success 235 Under the Hood 235 Callbacks with Arguments 235 Task Aggregation 236 Gathering Tasks 236 Waiting on Tasks 237 Timeouts 238 Waiting on Any Task 239 Waiting on an Exception 239 Queues 240 Maximum Size 242 Servers 242 Summary 244 CHAPTER 14: STYLE 245 Principles 245 Assume Your Code Will Require Maintenance 245 Be Consistent 246 Think About Ontology, Especially with Data 246 Do Not Repeat Yourself 246 Have Your Comments Explain the Story 247 Occam's Razor 247 Standards 248 Trivial Rules 248 Documentation Strings 248 Blank Lines 249 Imports 249 Variables 250 Comments 250 Line Length 251 Summary 251 INDEX 253