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Command-Line Examples

Let's look at a simple, but complete program file. The program simulates several dice throws. We've decided that the command-line synopsis should be: 〈-v〉 〈-s samples

The -v option leads to verbose output, where every individual toss of the dice is shown. Without the -v option, only the summary statistics are shown. The -s option tells how many samples to create. If this is omitted, 100 samples are used.

Here is the entire file. This program has a five-part design pattern that we've grouped into three sections.

Example 35.1.

#!/usr/bin/env python

Synopsis: [-v] [-s samples]
-v is for verbose output (show each sample)
-s is the number of samples (default 100)

import dice, getopt, sys

Docstring. The docstring provides the synopsis of the program, plus any other relevant documentation. This should be reasonably complete. Each element of the documentation is separated by blank lines. Several standard document extract utilities expect this kind of formatting.


Imports. The imports line lists the other modules on which this program depends. Each of these modules might have the main-import switch and a separate main program. Our objective is to reuse the imported classes and functions, not the main function.

def dicesim( samples=100, verbose=0 ):
    d= dice.Dice()
    t= 0
    for s in range(samples):
        n= d.roll()
        if verbose: print n
        t += n
    print "%s samples, average is %s" % ( samples, t/float(samples) )

Actual processing in dicesym. This is the actual heart of the program. It is a pure function with no dependencies on a particular operating system. It can be imported by some other program and reused.

def main():
    samples= 100
    verbose= 0
    opts,operands= getopt.getopt( sys.argv[1:], "vs:" )
    for o,v in opts:
        if o == "-v": verbose = 1
        elif o == "-s": samples= int(v)
    dicesim( samples, verbose )

if __name__ == "__main__":

Argument decoding in main. This is the interface between the operating system that initiates this program and the actual work in dicesym. This does not have much reuse potential.


Program vs. import switch. This makes the determination if this is a main program or an import. If it is an import, then __name__ is not "__main__", and no additional processing happens beyond the definitions. If it is the main program, the __name__ is "__main__"; the arguments are parsed by main, which calls dicesym to do the real work.

This is a typical layout for a complete Python main progam. There are two clear objecives. First, keep the main program focused; second, provide as many opportunities for reuse as possible.

  Published under the terms of the Open Publication License Design by Interspire