We have seen, briefly, how to define and use a function in Python. Now we are going to jump forward a bit and create a new function and at the same time take a look on command line parameters that can be passed to the script.

If you have used command line applications before, you might have encountered programs that asks for a file name, a calculation parameter, etc to be passed in the command line. Python scripts are no different, they accept such parameters. For this we have the sys module that has system specific parameters and functions. We have used before the sys.exit, imported as an extra module function. Every operating system (even Windows) has arguments in its command line, and programming languages usually call such arguments argv (in the C/C++ you have argv in the parameters of the main function). Lists in Python start at 0 (zero), and for the argument list the first item is the script/program name. Basically if we have this

$> python myscript.py DNA.txt

myscript.py is the argument 0 in the list and DNA.txt is the argument 1. So whenever we create a script that receives arguments in the command line, we have to start (in most cases, be aware) from 1. In Python using system arguments in the CLI will look like

import sys

filename = sys.argv[1]
valueone = sys.argv[2]
...

We will a variation of our previous script that counts the bases, now with command line arguments and a function (with no “error” checking at first)

#!/usr/bin/env python

import sys

def count_nucleotide_types(seq):
    result = []
    totalA = seq.count('A')
    totalC = seq.count('C')
    totalG = seq.count('G')
    totalT = seq.count('T')

    result.append(totalA)
    result.append(totalC)
    result.append(totalG)
    result.append(totalT)

    return result

sequencefile = open(sys.argv[1], 'r').readlines()
sequence = ''.join(sequencefile)
sequence = sequence.replace('n', '')
values = count_nucleotide_types(sequence)
print "Found " + str(result[0] + "As"
print "Found " + str(result[0] + "Cs"
print "Found " + str(result[0] + "Gs"
print "Found " + str(result[0] + "Ts"

Few new things here. We created a function count_nucleotide_types that should receive a string containing the sequence. The “real” first line of the program flow is the one that gets the name of the file from the command line argument, open and read it. We then convert the list to a string, modify it a but and throw it to the function. Get the result back, and done.

With functions we actually don’t save coding time/length (at least here), we make out code more organized, easier to read and somewhat easier to someone else read and understand it. It is not a good coding practice to have long programs/scripts with no functions, no subdivision, no structure. Functions are sometimes good program nuggets that can be reused in the same application or even ported/copied to other applications and reused indefenetely. Soon we will see a function and class that reads a FASTA file in Python that can be used anywhere in any program that needs such feature. Try the code and come back later for more.