7. How to write a self-executing Python script

So far we have used Python's conversational mode to demonstrate all the features. Now it's time to learn how to write a complete program.

Your program will live in a file called a script. To create your script, use your favorite text editor (emacs, vi, Notepad, whatever), and just type your Python statements into it.

How you make it executable depends on your operating system.

On Windows platforms, be sure to give your script file a name that ends in “.py”. If Python is installed, double-clicking on any script with this ending will use Python to run the script.
Under Linux and MacOS X, the first line of your script must look like this:
```
#!pythonpath
```
The pythonpath tells the operating system where to find Python. This path will usually be “/usr/local/bin/python”, but you can use the “which” shell command to find the path on your computer:
```
$ which python
/usr/local/bin/python
```
Once you have created your script, you must also use this command to make it executable:
```
chmod +x your-script-name
```

Here is a complete script, set up for a typical Linux installation. This script, powersof2, prints a table showing the values of 2ⁿ and 2^-n for n in the range 1, 2, ..., 12.

#!/usr/local/bin/python
print "Table of powers of two"
print
print "{0:>10s} {1:>2s} {2:s}".format("2**n", "n", "2**(-n)")
for n in range(13):
    print "{0:10d} {1:2d} {2:17.15f}".format(2**n, n, 2.0**(-n))

Here we see the invocation of this script under the bash shell, and the output:

$ ./powersof2
Table of powers of two

      2**n  n 2**(-n)
         1  0 1.000000000000000
         2  1 0.500000000000000
         4  2 0.250000000000000
         8  3 0.125000000000000
        16  4 0.062500000000000
        32  5 0.031250000000000
        64  6 0.015625000000000
       128  7 0.007812500000000
       256  8 0.003906250000000
       512  9 0.001953125000000
      1024 10 0.000976562500000
      2048 11 0.000488281250000
      4096 12 0.000244140625000

A Python 2.7 programming tutorial

7. How to write a self-executing Python script