As promised, let’s change a bit our previous code, and make it more effective. If you are reading this tutorial in one-entry mode, let’s check the code

#!/usr/bin/env python
import re
dnafile = "AY162388.seq"
seqlist = open(dnafile, 'r').readlines()
temp = ''.join(seqlist)
sequence = temp.replace('\n', '')
inputfromuser = True
while inputfromuser:
    inmotif = raw_input('Enter motif to search: ')
    if len(inmotif) >= 1:
        motif = re.compile('%s' % inmotif)
        if re.search(motif, sequence):
            print 'Yep, I found it'
        else:
            print 'Sorry, try another one'
    else:
        print 'Done, thanks for using motif_search'
        inputfromuser = False

In order to make it more effective, let’s allow the input of any file, maybe asking for the file as soon as the script is started. To accomplish that we need to “protect” the script and make it error proof (almost at least). Yes, you thought it right: we need to check if the input file exists before opening it.

We can use try ... except statements to do that. Pretty handy. This is called an exception handler, so basically we try the validity of some command/method and depending on the result we continue our program flow or we catch the exception and do something else. Under try we put the expression to evaluate, and under excepts what to do in case of failure. Like this

try:
    opening a file
except:
    as the file does not exist, print something

So, let’s put in our code above. We remove this

dnafile = "AY162388.seq"
seqlist = open(dnafile, 'r').readlines()

and include this lines

fileinput = True
while fileinput == True:
    filename = raw_input('Enter file name:')
    if len(filename) > 0:
        try:
            dnafile = open(filename, 'r')
            fileinput = False
        except:
            print 'File does not exist, please try again'
    else:
        sys.exit()

I know, a lot of new code. But if you take a closer look, there is only three lines we have never seen: try except and the last line with sys.exit(). Here, the script tries to open the file provided as input, if it does find the file normal operation resumes, if does not, the script asks for another input. As in the other while loop, we control it with a boolean variable, and in the case of empty input we end the loop and the script, using a system command exit, in the last line of the new code.

Our script is better now, not bulletproof, but a little bit more efficient. There still a “flaw”, that you can only check one file for each run of the script. At least we not stuck to our usual DNA sequence.

Download the commented code on the repository.