Python编程入门指南：从基础到实践

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Caution

When you are given the option of unchecking any optional components, don't uncheck any!

Some of these components can be useful for you, especially IDLE.

An interesting fact is that about 70% of Python downloads are by Windows users. Of course, this doesn't

give the complete picture since almost all Linux users will have Python installed already on their sys-

tems by default.

Using Python in the Windows command line

If you want to be able to use Python from the Windows command line, then you need to set the

PATH variable appropriately.

For Windows 2000, XP, 2003 , click on Control Panel -> System -> Advanced -> Environment

Variables. Click on the variable named PATH in the 'System Variables' section, then select

Edit and add ;C:\Python23 (without the quotes) to the end of what is already there. Of course,

use the appropriate directory name.

For older versions of Windows, add the following line to the file C:\AUTOEXEC.BAT :

'PATH=%PATH%;C:\Python23' (without the quotes) and restart the system. For Windows

NT, use the AUTOEXEC.NT file.

Summary

For a Linux system, you most probably already have Python installed on your system. Otherwise, you

can install it using the package management software that comes with your distribution. For a Windows

system, installing Python is as easy as downloading the installer and double-clicking on it. From now

on, we will assume that you have Python installed on your system.

Next, we will write our first Python program.

Installing Python

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Chapter 3. First Steps

Introduction

We will now see how to run a traditional 'Hello World' program in Python. This will teach you how to

write, save and run Python programs.

There are two ways of using Python to run your program - using the interactive interpreter prompt or us-

ing a source file. We will now see how to use both the methods.

Using the interpreter prompt

Start the intepreter on the command line by entering python at the shell prompt. Now enter print

'Hello World' followed by the Enter key. You should see the words Hello World as output.

For Windows users, you can run the interpreter in the command line if you have set the PATH variable

appropriately. Alternatively, you can use the IDLE program. IDLE is short for Integrated DeveLopment

Environment. Click on Start -> Programs -> Python 2.3 -> IDLE (Python GUI). Linux users can use

IDLE too.

Note that the <<< signs are the prompt for entering Python statements.

Example 3.1. Using the python interpreter prompt

$ python

Python 2.3.4 (#1, Oct 26 2004, 16:42:40)

[GCC 3.4.2 20041017 (Red Hat 3.4.2-6.fc3)] on linux2

Type "help", "copyright", "credits" or "license" for more information.

>>> print 'hello world'

hello world

>>>

Notice that Python gives you the output of the line immediately! What you just entered is a single Py-

thon statement. We use print to (unsurprisingly) print any value that you supply to it. Here, we are

supplying the text Hello World and this is promptly printed to the screen.

How to quit the Python prompt

To exit the prompt, press Ctrl-d if you are using IDLE or are using a Linux/BSD shell. In case

of the Windows command prompt, press Ctrl-z followed by Enter.

Choosing an Editor

Before we move on to writing Python programs in source files, we need an editor to write the source

files. The choice of an editor is crucial indeed. You have to choose an editor as you would choose a car

you would buy. A good editor will help you write Python programs easily, making your journey more

comfortable and helps you reach your destination (achieve your goal) in a much faster and safer way.

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1 one of the leading Perl6/Parrot hackers and the author of the amazing 'Beginning Perl' book

One of the very basic requirements is syntax highlighting where all the different parts of your Python

program are colorized so that you can see your program and visualize its running.

If you are using Windows, then I suggest that you use IDLE. IDLE does syntax highlighting and a lot

more such as allowing you to run your programs within IDLE among other things. A special note: don't

use Notepad - it is a bad choice because it does not do syntax highlighting and also importantly it does

not support indentation of the text which is very important in our case as we will see later. Good editors

such as IDLE (and also VIM) will automatically help you do this.

If you are using Linux/FreeBSD, then you have a lot of choices for an editor. If you are an experienced

programmer, then you must be already using VIM or Emacs. Needless to say, these are two of the most

powerful editors and you will be benefitted by using them to write your Python programs. I personally

use VIM for most of my programs. If you are a beginner programmer, then you can use Kate which is

one of my favorites. In case you are willing to take the time to learn VIM or Emacs, then I highly re-

commend that you do learn to use either of them as it will be very useful for you in the long run.

If you still want to explore other choices of an editor, see the comprehensive list of Python editors

[http://www.python.org/cgi-bin/moinmoin/PythonEditors] and make your choice. You can also choose

an IDE (Integrated Development Environment) for Python. See the comprehensive list of IDEs that sup-

port Python [http://www.python.org/cgi-bin/moinmoin/IntegratedDevelopmentEnvironments] for more

details. Once you start writing large Python programs, IDEs can be very useful indeed.

I repeat once again, please choose a proper editor - it can make writing Python programs more fun and

easy.

Using a Source File

Now let's get back to programming. There is a tradition that whenever you learn a new programming

language, the first program that you write and run is the 'Hello World' program - all it does is just say

'Hello World' when you run it. As Simon Cozens 1 puts it, it is the 'traditional incantation to the pro-

gramming gods to help you learn the language better' :) .

Start your choice of editor, enter the following program and save it as helloworld.py

Example 3.2. Using a Source File

#!/usr/bin/python

# Filename : helloworld.py

print 'Hello World'

(Source file: code/helloworld.py)

Run this program by opening a shell (Linux terminal or DOS prompt) and entering the command py-

thon helloworld.py. If you are using IDLE, use the menu Edit -> Run Script or the keyboard short-

cut Ctrl-F5. The output is as shown below.

Output

First Steps

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$ python helloworld.py

Hello World

If you got the output as shown above, congratulations! - you have successfully run your first Python pro-

gram.

In case you got an error, please type the above program exactly as shown and above and run the program

again. Note that Python is case-sensitive i.e. print is not the same as Print - note the lowercase p in

the former and the uppercase P in the latter. Also, ensure there are no spaces or tabs before the first char-

acter in each line - we will see why this is important later.

How It Works

Let us consider the first two lines of the program. These are called comments - anything to the right of

the # symbol is a comment and is mainly useful as notes for the reader of the program.

Python does not use comments except for the special case of the first line here. It is called the shebang

line - whenever the first two characters of the source file are #! followed by the location of a program,

this tells your Linux/Unix system that this program should be run with this interpreter when you execute

the program. This is explained in detail in the next section. Note that you can always run the program on

any platform by specifying the interpreter directly on the command line such as the command python

helloworld.py .

Important

Use comments sensibly in your program to explain some important details of your program -

this is useful for readers of your program so that they can easily understand what the program is

doing. Remember, that person can be yourself after six months!

The comments are followed by a Python statement - this just prints the text 'Hello World'. The

print is actually an operator and 'Hello World' is referred to as a string - don't worry, we will ex-

plore these terminologies in detail later.

Executable Python programs

This applies only to Linux/Unix users but Windows users might be curious as well about the first line of

the program. First, we have to give the program executable permission using the chmod command then

run the source program.

$ chmod a+x helloworld.py

$ ./helloworld.py

Hello World

The chmod command is used here to change the mode of the file by giving execute permission to all

users of the system. Then, we execute the program directly by specifying the location of the source file.

We use the ./ to indicate that the program is located in the current directory.

To make things more fun, you can rename the file to just helloworld and run it as ./helloworld and it

First Steps

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will still work since the system knows that it has to run the program using the interpreter whose location

is specified in the first line in the source file.

You are now able to run the program as long as you know the exact path of the program - but what if

you wanted to be able to run the program from anywhere? You can do this by storing the program in one

of the directories listed in the PATH environment variable. Whenever you run any program, the system

looks for that program in each of the directories listed in the PATH environment variable and then runs

that program. We can make this program available everywhere by simply copying this source file to one

of the directories listed in PATH.

$ echo $PATH

/opt/mono/bin:/usr/local/bin:/usr/bin:/bin:/usr/X11R6/bin:/home/swaroop/bin

$ cp helloworld.py /home/swaroop/bin/helloworld

$ helloworld

Hello World

We can display the PATH variable using the echo command and prefixing the variable name by $ to in-

dicate to the shell that we need the value of this variable. We see that /home/swaroop/bin is one of

the directories in the PATH variable where swaroop is the username I am using in my system. There

will usually be a similar directory for your username on your system. Alternatively, you can add a dir-

ectory of your choice to the PATH variable - this can be done by running

PATH=$PATH:/home/swaroop/mydir where '/home/swaroop/mydir' is the directory I want to

add to the PATH variable.

This method is very useful if you want to write useful scripts that you want to run the program anytime,

anywhere. It is like creating your own commands just like cd or any other commands that you use in the

Linux terminal or DOS prompt.

Caution

W.r.t. Python, a program or a script or software all mean the same thing.

Getting Help

If you need quick information about any function or statement in Python, then you can use the built-in

help functionality. This is very useful especially when using the interpreter prompt. For example, run

help(str) - this displays the help for the str class which is used to store all text (strings) that you

use in your program. Classes will be explained in detail in the chapter on object-oriented programming.

Note

Press q to exit the help.

Similarly, you can obtain information about almost anything in Python. Use help() to learn more

about using help itself!

In case you need to get help for operators like print, then you need to set the PYTHONDOCS environ-

ment variable appropriately. This can be done easily on Linux/Unix using the env command.

$ env PYTHONDOCS=/usr/share/doc/python-docs-2.3.4/html/ python

Python 2.3.4 (#1, Oct 26 2004, 16:42:40)

[GCC 3.4.2 20041017 (Red Hat 3.4.2-6.fc3)] on linux2

First Steps

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Python编程入门指南：从基础到实践

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