| Chapter 2 |
| 1 | Chapter 2 Variables, expressions and statements 2.1 Values and types A value is one of the basic things a program works with, like a letter or a number. The values we have seen so far are 1, 2, and 'Hello, World!'. These values belong to different types: 2 is an integer, and 'Hello, World!' is a string, so-called because it contains a "string" of letters. You (and the interpreter) can identify strings because they are enclosed in quotation marks. |
| 2 | If you are not sure what type a value has, the interpreter can tell you. >>> type('Hello, World!') <type 'str'> >>> type(17) <type 'int'> Not surprisingly, strings belong to the type str and integers belong to the type int. Less obviously, numbers with a decimal point belong to a type called float, because these numbers are represented in a format called floating-point. >>> type(3.2) <type 'float'> What about values like '17' and '3.2'? |
| 3 | They look like numbers, but they are in quotation marks like strings. >>> type('17') <type 'str'> >>> type('3.2') <type 'str'> They're strings. When you type a large integer, you might be tempted to use commas between groups of three digits, as in 1,000,000. This is not a legal integer in Python, but it is legal: >>> 1,000,000 (1, 0, 0) Well, that's not what we expected at all! Python interprets 1,000,000 as a comma-separated sequence of integers. |
| 4 | This is the first example we have seen of a semantic error: the code runs without producing an error message, but it doesn't do the "right" thing. 2.2 Variables One of the most powerful features of a programming language is the ability to manipulate variables. A variable is a name that refers to a value. An assignment statement creates new variables and gives them values: >>> message = 'And now for something completely different' >>> n = 17 >>> pi = 3.1415926535897932 This example makes three assignments. |
| 5 | The first assigns a string to a new variable named message; the second gives the integer 17 to n; the third assigns the (approximate) value of ? to pi. A common way to represent variables on paper is to write the name with an arrow pointing to the variable's value. This kind of figure is called a state diagram because it shows what state each of the variables is in (think of it as the variable's state of mind). |
| 6 | Figure 2.1 shows the result of the previous example. Figure 2.1: State diagram. The type of a variable is the type of the value it refers to. >>> type(message) <type 'str'> >>> type(n) <type 'int'> >>> type(pi) <type 'float'> 2.3 Variable names and keywords Programmers generally choose names for their variables that are meaningful—they document what the variable is used for. Variable names can be arbitrarily long. |
| 7 | They can contain both letters and numbers, but they have to begin with a letter. It is legal to use uppercase letters, but it is a good idea to begin variable names with a lowercase letter (you'll see why later). The underscore character, _, can appear in a name. It is often used in names with multiple words, such as my_name or airspeed_of_unladen_swallow. If you give a variable an illegal name, you get a syntax error: >>> 76trombones = 'big parade' SyntaxError: invalid syntax >>> more@ = 1000000 SyntaxError: invalid syntax >>> class = 'Advanced Theoretical Zymurgy' SyntaxError: invalid syntax 76trombones is illegal because it does not begin with a letter. |
| 8 | more@ is illegal because it contains an illegal character, @. But what's wrong with class? It turns out that class is one of Python's keywords. The interpreter uses keywords to recognize the structure of the program, and they cannot be used as variable names. Python 2 has 31 keywords: and del from not while as elif global or with assert else if pass yield break except import print class exec in raise continue finally is return def for lambda try In Python 3, exec is no longer a keyword, but nonlocal is. |
| 9 | You might want to keep this list handy. If the interpreter complains about one of your variable names and you don't know why, see if it is on this list. 2.4 Operators and operands Operators are special symbols that represent computations like addition and multiplication. The values the operator is applied to are called operands. The operators +, -, *, / and ** perform addition, subtraction, multiplication, division and exponentiation, as in the following examples: 20+32 hour-1 hour*60+minute minute/60 5**2 (5+9)*(15-7) In some other languages, ^ is used for exponentiation, but in Python it is a bitwise operator called XOR. |
| 10 | I won't cover bitwise operators in this book, but you can read about them at http://wiki.python.org/moin/BitwiseOperators. In Python 2, the division operator might not do what you expect: >>> minute = 59 >>> minute/60 0 The value of minute is 59, and in conventional arithmetic 59 divided by 60 is 0.98333, not 0. The reason for the discrepancy is that Python is performing floor division. When both of the operands are integers, the result is also an integer; floor division chops off the fraction part, so in this example it rounds down to zero. |
| … |
Комментарии