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Safari | Python Developer's Handbook -> Programs





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Programs

Until now, all the examples were written directly in the interpreter environment. However, most Python programs are executed as external scripts, being loaded from files.

You can write your own Python scripts by using any text editor of your choice. Remember to always save your files using the .py extension.

As with any other UNIX scripting language, Python scripts (for UNIX) need a special handling.

First, you need to put a "shebang" in the first line of the script. This line declares the location of Python's interpreter in your system. For example

					
#!/usr/local/bin/python

				

Note that this example works only if Python was installed under the given mounting point. Most Linux systems have Python installed under /usr by default, so the preceding example will not work. Today, the following line of code seems to be more common, and does not depend on where Python is installed:

					
#!/usr/bin/env python

				

If you are running your scripts on an MS Windows environment, you can keep this line of code for portability purposes because the literal # is only used to identify comment lines that are ignored by the interpreter, so it will cause no harm to your programs.

Tip

The "shebang" line is only meaningful when you work on a UNIX system.



If you don't know where Python is located on your UNIX system, use the following command:

					
$ whereis python

				

Also, remember to set the permissions on your script to 755 in order to let every user be able to execute it.

					
$ chmod +x scriptname.py

				

or

					
$ chmod 755 scriptname.py

				

As you cannot directly execute Python scripts in the MS Windows systems through the command line, you have two options: Either double-click the file using Windows Explorer or call the interpreter, passing the filename as an argument. For example,

					
c:\>python scriptname.py

				

Another way to call the interpreter on Windows systems is by typing start scriptname.py at the shell prompt. This command will find and execute the program associated with the extension .py.

If you want to open the interpreter after executing a program, use the -i argument when calling the script. The interpreter will run your script, and after it executes all the commands, it will open its command-line interface for you. Here's how to call the script with a command-line option:

					
c:\python -i scriptname.py

				

Otherwise, after the script finishes its execution, it will automatically close the interpreter.

After spending some time creating Python programs, you might find some .pyc files in the same directory in which you are saving your .py scripts. See Chapter 17, "Development Tools," to know more about this other file extension.

Indentation

Python delimits code blocks by using indentation. There is no concept of { }s or Begin/Ends as in other languages. When you indent a block of code, you define the way the statements are grouped. It also reduces errors due to bad indentation. For instance, the following C or Perl code looks like a single if statement, but the second statement is always executed:

						
if (expression)
    statement1;
    statement2;

					

Python doesn't suffer from this problem because indentation defines block structure.

Another great aspect of this implementation is that you can reduce the size of your code while using indentation instead of conventional block delimiters.

Tip

Keep in mind that tabs are internally converted to spaces (1 tab = 8 spaces), and blank lines are ignored when part of scripts.



I suggest you write one statement per line, using a newline (ENTER) to terminate each line. If you decide to have more than one statement in the same line, you need to separate them by using semicolons, as shown in the following:

						
>>> print "When AH "; print "were young"

					

Remember that you must put a backslash \ at the end of lines that need to be broken into two lines:

						
>>> t = "Nobody expects " + \
 "the Spanish inquisition"
				
					

Lexical Analysis

It is unnecessary to declare the type of a variable in Python programs. The same variable name might have different types at different occasions because it is re-initialized every time a value gets assigned to it, as illustrated in the following:

						
>>> x = "Albatross!!"
>>> print x
Albatross!!
>>> x = 123
>>> print x
123

					

You can assign any object type to a variable (for example, functions, classes, and modules). The following example shows how you can create a variable that references the round() function object:

						

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