Beginning Python - From Novice to Professional

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CHAPTER 5 ■ CONDITIONALS, LOOPS, AND SOME OTHER STATEMENTS 89 More Complex Conditions That’s really all there is to know about if statements. Now let’s return to the conditions themselves, because they are the really interesting part of conditional execution. Comparison Operators Perhaps the most basic operators used in conditions are the comparison operators. They are used (surprise, surprise) to compare things. The comparison operators are summed up in Table 5-1. Table 5-1. The Python Comparison Operators Expression Description x == y x equals y. x < y x is less than y. x > y x is greater than y. x >= y x is greater than or equal to y. x
90 CHAPTER 5 ■ CONDITIONALS, LOOPS, AND SOME OTHER STATEMENTS >>> "foo" == "foo" True >>> "foo" == "bar" False Double? Why can’t you just use a single equality sign, like they do in mathematics? I’m sure you’re clever enough to figure this out for yourself, but let’s try it: >>> "foo" = "foo" SyntaxError: can't assign to literal The single equality sign is the assignment operator, which is used to change things, which is not what you want to do when you compare things. is: The Identity Operator The is operator is interesting. It seems to work just like ==, but it doesn’t: >>> x = y = [1, 2, 3] >>> z = [1, 2, 3] >>> x == y True >>> x == z True >>> x is y True >>> x is z False Until the last example, this looks fine, but then you get that strange result, that x is not z even though they are equal. Why? Because is tests for identity, rather than equality. The variables x and y have been bound to the same list, while z is simply bound to another list that happens to contain the same values in the same order. They may be equal, but they aren’t the same object. Does that seem unreasonable? Consider this example: >>> x = [1, 2, 3] >>> y = [2, 4] >>> x is not y True >>> del x[2] >>> y[1] = 1 >>> y.reverse() In this example, I start with two different lists, x and y. As you can see, x is not y (just the inverse of x is y), which you already know. I change the lists around a bit, and though they are now equal, they are still two separate lists: >>> x == y True >>> x is y True
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Beginning Python From Novice to Pro
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For Ranveig
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■CHAPTER 23 Project 4: In the New
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viii ■CONTENTS Strings . . . . .
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x ■CONTENTS ■CHAPTER 5 Conditio
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xii ■CONTENTS ■CHAPTER 8 Except
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xxii ■CONTENTS Preparations . . .
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About the Technical Reviewer ■JER
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Introduction A C program is like a
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CHAPTER 1 ■ ■ ■ Instant Hacki
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140 CHAPTER 7 ■ MORE ABSTRACTION
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CHAPTER 9 ■ ■ ■ Magic Methods
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CHAPTER 9 ■ MAGIC METHODS, PROPER
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256 CHAPTER 11 ■ FILES AND STUFF
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270 CHAPTER 12 ■ GRAPHICAL USER I
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298 CHAPTER 14 ■ NETWORK PROGRAMM
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314 CHAPTER 15 ■ PYTHON AND THE W
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358 CHAPTER 17 ■ EXTENDING PYTHON
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CHAPTER 18 ■ ■ ■ Packaging Yo
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APPENDIX A ■ ■ ■ The Short Ve
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558 APPENDIX B ■ PYTHON REFERENCE
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572 APPENDIX C ■ ONLINE RESOURCES
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Index ■Symbols - operator 558 !=
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■INDEX 583 ■F Factory class twi
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■INDEX 593 playful programming 38
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■INDEX 595 Python C API 365 hand-
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