As you might have noticed, BPB generally uses protein sequences. I am a “DNA guy”, and basically in our simple examples either type of sequence (except the example on transcribing) could have been used. I will stick with this molecule for a while, or until I can. We are going to use our good old AY162388.seq file, still assigning the file name inside the script there will be a twist in the end. Also, remember the Regular Expression module? We are going to use it too. I am going to do just as the book: I will paste the code below with comments in the middle and explain it afterwards.

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#!/usr/bin/env python
#finding motifs in DNA sequences
# we use the RegEx module
import re
#still keep the file fixed
dnafile = "AY162388.seq"
#opening the file, reading the sequence and storing in a list
seqlist = open(dnafile, 'r').readlines()
#let's join the the lines in a temporary string
temp = ''.join(seqlist)
#assigning our sequence, with no carriage returns to our
#final variable/object
sequence = temp.replace('\n', '')
#we start to deal with user input
#first we use a boolean variable to check for valid input
inputfromuser = True
#while loop: while there is an motif larger than 0
#the loop continues
while inputfromuser:
    #raw_input received the user input as string
    inmotif = raw_input('Enter motif to search: ')
    #now we check for the size of the input
    if len(inmotif) >= 1:
        #we compile a regex with the input given
        motif = re.compile('%s' % inmotif)
        #looking to see if the entered motif is in the sequence
        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

Now let’s dissect the code, in a biological way. We’ve already seen everything up to the part the list’s lines are joined. First new lines for us is this

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sequence = temp.replace('\\n', '')

Most of the methods in Python have very intuitive names (ok, most languages do), so it is easy to deduce that replace actually replaces something. And why do we need to use this method? First, we joined the lines in one temporary string (yep, strings are immutable), but the lines come with everything, including carriage returns that we need to get rid of. So our “final” string sequence receives the value in temp and we apply the method replace to modify it. replace has two arguments, the first is the string we want to change from, while the second is the string we want to replace with (in fact replace accepts three arguments, the last being the number of times we want to do the change). In our case, we are telling the interpreter to get every substring '\n' and put an empty one in their places. The next line is a simple value assignment:

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inputfromuser = True

and the variable will manage the while that checks input from the user. Basically we will run the loop until a certain type of input is given, that will make the variable value become False. That’s why we have the line

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 while inputfromuser

with no check. Yep, notice that we don’t need to check for the variable’s value, Python assumes that it is True. To understand better, imagine that inputfromuser is a flag that appears when True and disappears when False. Python can only “see” it when it appears, and when it does disappear it needs to check for it. We will elaborate more later. Another different line awaits us

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inmotif = raw_input('Enter motif to search: ')

raw_input is a function that takes a line input by the user and returns a string. In our case, we assign the value returned by the function to a new string called inmotif. Also, raw_input has one prompt argument which is written to the screen with no trailing line, waiting for the user to input something. This takes us to the while loop

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if len(inmotif) >= 1:
    #we compile a regex with the input given 
    motif = re.compile(r'%s' % inmotif)
    #looking to see if the entered motif is in the sequence
    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

After getting the input from the user, we need to check the size of such input: if it is greater or equal to one, we will search it in our sequence, otherwise we will finish the loop. So if we get a valid input (valid in the sense of size, for now) we will compile the regex and search for it. There is a difference in regex compilation. In the DNA transcribing we assigned a string to the regex directly, now we have a string coming from a variable/object

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motif = re.compile(r'%s' % inmotif)

Notice the difference in the argument that is passed to the compile function. In this case the regex to be compiled is coming from the string entered by the user, and we have to pass it by using Python’s string formatting operator, noted as a %. This formatting operator uses two parameters: a string with formatting characters and the data to be formatted, separated by the percent sign. In our case the formatting character will receive a string, hence the %s (s for string), and the data to be formatted that is the input. Previously, we used the regex function to replace characters/substrings in a sequence. This time, we are interested to know if the motif entered by the user is in our sequence. So we use the search method instead

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if re.search(motif, sequence):

Notice that we do the search in at the same time we are testing for its result. The same case as the checking we do at the while loop. If there is a positive result from the regex search a True flag will be raised and the interpreter will execute the code of the initial branch, not testing for the elif and else

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print 'Yep, I found it'
else: print 'Sorry, try another one'

This condition is nested inside another condition, the one that tests for the size of the input entered. So, if our search returns a regex object, we print Yep, I found it, otherwise the user will receive Sorry, try another one. Now back to our upper if, if the user input length is equal to zero (just pressing the Enter key) the interpreter will process the line

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print 'Done, thanks for using motif_search'

and then

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inputfromuser = False

that will tell the while loop that there is no True variable anymore, ending the loop and consequently our script. Let’s review the script and its flow:

1) assign a filename to be opened 
2) read the file 
3) join the lines 
4) remove carriage returns 
5) ask for user input, while is valid 
6) if input length is greater or equal to 1, process it 
 6a) compile regex with input 
 6b) search for it 
   6b1) if it is found print yes 
   6b2) if it is not found print no 
7) if input length is equal to zero, end while loop, end strings

I know it is a lot of information, take your time.