We already know how to read from files, now we are going to see how to write to them. We will start with a simple example, writing some content to a “fresh” file that does not exist in the system.

Sometime ago, we have seen the print statement in Python, that prints to the system standard output (usually the screen). This output can be redirected using > to a stream/file. But if we are going to creat really professional applications (even to our own use), usyally stream redirection is not really the nicest approach. In some cases the best alternative is to save a file.

Also, some posts ago, we covered the methodology to open a file. We are going to use here the same command, open to (in our case) create the file. Remember that to read the file we used

file = open(dnafile, 'r')

where the 'r' is the mode we are using to open the file. In that case we used the read mode, now we are going to use the write mode. When we open a file with this command in write mode if the file (with the name with pass) does not exist it is created. If it does exist, all the contents of the current file will be erased, so be careful when opening a file from your script. Check for the location, file name, etc before opening the file.

So, basically we have

file = open(output, 'w')

and the file is open and ready to receive data.

Let’s cheat and use the previous script that counts nucleotides and modify it to save a count.txt file wit the results:

#!/usr/bin/env python

#let's keep the file fixed for now
dnafile = "AY162388.seq"
#opening the file, reading the sequence and storing in a list
file = open(dnafile, 'r')
#initialize a string to receive the data
sequence = ''
for line in file:
    sequence += line.strip() #notice the strip, to remove n
#"exploding" the sequence in a list
seqlist = list(sequence)
#initializing integers to store the counts
totalA = 0
totalC = 0
totalG = 0
totalT = 0
#checking each item in the list and updating counts
for base in seqlist:
    if base == 'A':
        totalA += 1
    elif base == 'C':
        totalC += 1
    elif base == 'G':
        totalG += 1
    elif base == 'T':
        totalT += 1
#printing results
resultfile = open('counts.txt', 'w')
resultfile.write(str(totalA) + ' As found \n')
resultfile.write(str(totalC) + ' Cs found \n')
resultfile.write(str(totalG) + ' Gs found \n')
resultfile.write(str(totalT) + ' Ts found \n')

The only difference is at the end of the script. Here instead of print we use write. Notice that write is a method of the opened file.(in our case called resultfile. This method only accepts strings, so we need to convert everything to string before writing in the file. Notice also that we need to add a carriage return/newline at the end of the string to be written. Differently of print, write does not automatically puts a new line at the end of the output.