10 Chapter 1.Computers and Programs >>print 2 +3 5 >print"2+3=",2+3 2+3=5 Here I have tried out three examples using the Python print statement.The first statement asks Python to display the literal phrase Hello,World.Python responds on the next line by printing the phrase.The second print statement asks Python to print the sum of 2 and 3.The third print combines these two ideas.Python prints the part in quotes"2+3="followed by the result of adding 2 +3,which is 5. This kind of interaction is a great way to try out new things in Python.Snip- pets of interactive sessions are sprinkled throughout this book.When you see the Python prompt>>>in an example,that should tip you off that an inter- active session is being illustrated.It's a good idea to fire up Python and try the examples yourself. Usually we want to move beyond snippets and execute an entire sequence of statements.Python lets us put a sequence of statements together to create a brand-new command called a function.Here is an example of creating a new function called hello: >>def hello(): print "Hello" print "Computers are Fun" >>> The first line tells Python that we are defining a new function called hello.The following lines are indented to show that they are part of the hello function. The blank line (obtained by hitting the <Enter>key twice)lets Python know that the definition is finished,and the interpreter responds with another prompt.No- tice that the definition did not cause anything to happen.We have told Python what should happen when the hello function is used as a command;we haven't actually asked Python to perform it yet. A function is invoked by typing its name.Here's what happens when we use our hello command: >he11o() Hello Computers are Fun >>>
10 Chapter 1. Computers and Programs >» print 2 + 3 5 >» print "2 + 3 =", 2 + 3 2 + 3 = 5 Here I have tried out three examples using the Python print statement. The first statement asks Python to display the literal phrase Hello, World. Python responds on the next line by printing the phrase. The second print statement asks Python to print the sum of 2 and 3. The third print combines these two ideas. Python prints the part in quotes "2 + 3 =" followed by the result of adding 2 + 3, which is 5. This kind of interaction is a great way to try out new things in Python. Snippets of interactive sessions are sprinkled throughout this book. When you see the Python prompt > > > in an example, that should tip you off that an inter- active session is being illustrated. It's a good idea to fire up Python and try the examples yourself. Usually we want to move beyond snippets and execute an entire sequence of statements. Python lets us put a sequence of statements together to create a brand-new command called a function. Here is an example of creating a new function called hello: >» def hello : print "Hello" print "Computers are Fun" >» The first line tells Python that we are defining a new function called hello. The following lines are indented to show that they are part of the hello function. The blank line (obtained by hitting the <Enter> key twice) lets Python know that the definition is finished, and the interpreter responds with another prompt. Notice that the definition did not cause anything to happen. We have told Python what should happen when the hello function is used as a command; we haven't actually asked Python to perform it yet. A function is invoked by typing its name. Here's what happens when we use our hello command: >» helloO Hello Computers are Fun >>>
1.6.The Magic of Python 11 Do you see what this does?The two print statements from the hello function are executed in sequence. You may be wondering about the parentheses in the definition and use of hello.Commands can have changeable parts called parameters that are placed within the parentheses.Let's look at an example of a customized greeting using a parameter.First the definition: >>def greet(person): print "Hello",person print "How are you?" Now we can use our customized greeting. >>greet("John") Hello John How are you? >>greet("Emily") Hello Emily How are you? >>> Can you see what is happening here?When we use greet we can send different names to customize the result.We will discuss parameters in detail later on.For the time being,our functions will not use parameters,so the parentheses will be empty,but you still need to include them when defining and using functions. One problem with entering functions interactively at the Python prompt like this is that the definitions go away when we quit Python.If we want to use them again the next time,we have to type them all over again.Programs are usually created by typing definitions into a separate file called a module or script.This file is saved on a disk so that it can be used over and over again. A module file is just a text file,and you can create one using any program for editing text,like a notepad or word processor program(provided you save your program as a"plain text"file).A special type of program known as a pro- gramming environment simplifies the process.A programming environment is specifically designed to help programmers write programs and includes features such as automatic indenting,color highlighting,and interactive development. The standard Python distribution includes a programming environment called IDLE that you may use for working on the programs in this book. Let's illustrate the use of a module file by writing and running a complete program.Our program will illustrate a mathematical concept known as chaos
1.6. The Magic of Python 11 Do you see what this does? The two print statements from the hello function are executed in sequence. You may be wondering about the parentheses in the definition and use of hello. Commands can have changeable parts called parameters that are placed within the parentheses. Let's look at an example of a customized greeting using a parameter. First the definition: >» def greet (person) : print "Hello", person print "How are you?" Now we can use our customized greeting. >» greet ("John") Hello John How are you? >» greet ("Emily") Hello Emily How are you? >» Can you see what is happening here? When we use greet we can send different names to customize the result. We will discuss parameters in detail later on. For the time being, our functions will not use parameters, so the parentheses will be empty, but you still need to include them when defining and using functions. One problem with entering functions interactively at the Python prompt like this is that the definitions go away when we quit Python. If we want to use them again the next time, we have to type them all over again. Programs are usually created by typing definitions into a separate file called a module or script. This file is saved on a disk so that it can be used over and over again. A module file is just a text file, and you can create one using any program for editing text, like a notepad or word processor program (provided you save your program as a "plain text" file). A special type of program known as a programming environment simplifies the process. A programming environment is specifically designed to help programmers write programs and includes features such as automatic indenting, color highlighting, and interactive development. The standard Python distribution includes a programming environment called IDLE that you may use for working on the programs in this book. Let's illustrate the use of a module file by writing and running a complete program. Our program will illustrate a mathematical concept known as chaos
12 Chapter 1.Computers and Programs Here is the program as we would type it into IDLE or some other editor and save in a module file: File:chaos.py A simple program illustrating chaotic behavior. def main(): print "This program illustrates a chaotic function" x input("Enter a number between 0 and 1:" for i in range(10): x=3.9*X*(1-x) print x main() This file should be saved with the name chaos.py.The.py extension in- dicates that this is a Python module.You can see that this particular example contains lines to define a new function called main.(Programs are often placed in a function called main.)The last line of the file is the command to invoke this function.Don't worry if you don't understand what main actually does;we will discuss it in the next section.The point here is that once we have a program in a module file,we can run it any time we want. This program can be run in a number of different ways that depend on the actual operating system and programming environment that you are using.If you are using a windowing system,you can run a Python program by clicking (or double-clicking)on the module file's icon.In a command line situation,you might type a command like python chaos.py.If you are using IDLE(or another programming environment)you can run a program by opening it in the editor and then selecting a command like import,run,or execute. One method that should always work is to start the Python interpreter and then import the file.Here is how that looks: >>import chaos This program illustrates a chaotic function Enter a number between 0 and 1:.25 0.73125 0.76644140625 0.698135010439 0.82189581879 0.570894019197
12 Chapter 1. Computers and Programs Here is the program as we would type it into IDLE or some other editor and save in a module file: # File: chaos. py # A simple program illustrating chaotic behavior. def mainO : print "This program illustrates a chaotic function" X = input ("Enter a number between cind 1: ") for i in raiige(lO): X = 3.9 * X * (1 - x) print X mainO This file should be saved with the name chaos. py. The .py extension in- dicates that this is a Python module. You can see that this particular example contains lines to define a new function called main. (Programs are often placed in a function called main.) The last line of the file is the command to invoke this function. Don't worry if you don't understand what main actually does; we will discuss it in the next section. The point here is that once we have a program in a module file, we can run it any time we want. This program can be run in a number of different ways that depend on the actual operating system and programming environment that you are using. If you are using a windowing system, you can run a Python program by clicking (or double-clicking) on the module file's icon. In a command line situation, you might type a command like python chaos . py. If you are using IDLE (or another programming environment) you can run a program by opening it in the editor and then selecting a command like import, run, or execute. One method that should always work is to start the Python interpreter and then import the file. Here is how that looks: >>> import chaos This program illustrates a chaotic function Enter a number between and 1 : .25 0.73125 0.76644140625 0.698135010439 0.82189581879 0.570894019197
1.6.The Magic of Python 13 0.955398748364 0.166186721954 0.540417912062 0.9686289303 0.118509010176 >>> Typing the first line import chaos tells the Python interpreter to load the chaos module from the file chaos.py into main memory.Notice that I did not include the.py extension on the import line;Python assumes the module will have a .py extension. As Python imports the module file,each line executes.It's just as if we had typed them one-by-one at the interactive Python prompt.The def in the module causes Python to create the main function.When Python encounters the last line of the module,the main function is invoked,thus running our program. The running program asks the user to enter a number between 0 and 1(in this case,I typed".25")and then prints out a series of 10 numbers. When you first import a module file in this way,Python creates a companion file with a.pyc extension.In this example,Python creates another file on the disk called chaos.pyc.This is an intermediate file used by the Python inter- preter.Technically,Python uses a hybrid compiling/interpreting process.The Python source in the module file is compiled into more primitive instructions called byte code.This byte code (the.pyc)file is then interpreted.Having a pyc file available makes importing a module faster the second time around. However,you may delete the byte code files if you wish to save disk space; Python will automatically recreate them as needed. A module needs to be imported into a session only once.After the mod- ule has been loaded,we can run the program again by asking Python to exe- cute the main command.We do this by using a special dot notation.Typing chaos.main()tells Python to invoke the main function in the chaos module. Continuing with our example,here is how it looks when we rerun the program with.26 as the input: >>>chaos.main() This program illustrates a chaotic function Enter a number between 0 and 1:.26 0.75036 0.73054749456 0.767706625733
1.6. The Magic of Python 13 0.955398748364 0.166186721954 0.540417912062 0.9686289303 0.118509010176 >» Typing the first line import chaos tells the Python interpreter to load the chaos module from the file chaos .py into main memory. Notice that I did not include the .py extension on the import line; Python assumes the module will have a . py extension. As P5^hon imports the module file, each line executes. It's just as if we had typed them one-by-one at the interactive Python prompt. The def in the module causes Python to create the main function. When Python encounters the last line of the module, the main function is invoked, thus running our program. The running program asks the user to enter a number between and 1 (in this case, I typed ".25") and then prints out a series of 10 numbers. when you first import a module file in this way. Python creates a companion file with a . pyc extension. In this example. Python creates another file on the disk called chaos. pyc. This is an intermediate file used by the Python interpreter. Technically, Python uses a hybrid compiling/interpreting process. The Python source in the module file is compiled into more primitive instructions called byte code. This byte code (the .pyc) file is then interpreted. Having a .pyc file available makes importing a module faster the second time around. However, you may delete the byte code files if you wish to save disk space; Python will automatically recreate them as needed. A module needs to be imported into a session only once. After the module has been loaded, we can run the program again by asking Python to exe- cute the main command. We do this by using a special dot notation. Typing chaos. main tells Python to invoke the main function in the chaos module. Continuing with our example, here is how it looks when we rerun the program with .26 as the input: >» chaos . main ( ) This program illustrates a chaotic function Enter a number between and 1 : .26 0.75036 0.73054749456 0.767706625733
14 Chapter 1.Computers and Programs 0.6954993339 0.825942040734 0.560670965721 0.960644232282 0.147446875935 0.490254549376 0.974629602149 >>> 1.7 Inside a Python Program The output from the chaos program may not look very exciting,but it illustrates a very interesting phenomenon known to physicists and mathematicians.Let's take a look at this program line by line and see what it does.Don't worry about understanding every detail right away;we will be returning to all of these ideas in the next chapter. The first two lines of the program start with the character: File:chaos.py A simple program illustrating chaotic behavior. These lines are called comments.They are intended for human readers of the program and are ignored by Python.The Python interpreter always skips any text from the pound sign (#)through the end of a line. The next line of the program begins the definition of a function called main: def main(): Strictly speaking,it would not be necessary to create a main function.Since the lines of a module are executed as they are loaded,we could have written our program without this definition.That is,the module could have looked like this: File:chaos.py A simple program illustrating chaotic behavior. print "This program illustrates a chaotic function" x input("Enter a number between 0 and 1:" for i in range(10): X=3.9*x*(1-x) print x
14 Chapter 1. Computers and Programs 0.6954993339 0.825942040734 0.560670965721 0.960644232282 0.147446875935 0.490254549376 0.974629602149 >» 1.7| Inside a Python Program The output from the chaos program may not look very exciting, but it illustrates a very interesting phenomenon known to physicists and mathematicians. Let's take a look at this program line by line and see what it does. Don't worry about understanding every detail right away; we will be returning to all of these ideas in the next chapter. The first two lines of the program start with the # character: # File: chaos. py # A simple program illustrating chaotic behavior. These lines are called comments. They are intended for human readers of the program and are ignored by Python. The Python interpreter always skips any text from the pound sign (#) through the end of a line. The next line of the program begins the definition of a function called main: def mainO : Strictly speaking, it would not be necessary to create a main function. Since the lines of a module are executed as they are loaded, we could have written our program without this definition. That is, the module could have looked like this: # File: chaos. py # A simple program illustrating chaotic behavior. print "This program illustrates a chaotic function" X = input ("Enter a number between smd 1: ") for i in range (10) : X = 3.9 * X * (1 - x) print X