17.6. Writing Text Files¶

One of the most commonly performed data processing tasks is to read data from a file, manipulate it in some way, and then write the resulting data out to a new data file to be used for other purposes later. To accomplish this, the open function discussed above can also be used to create a new file prepared for writing. Note in Table 1 above that the only difference between opening a file for writing and opening a file for reading is the use of the 'w' flag instead of the 'r' flag as the second parameter. When we open a file for writing, a new, empty file with that name is created and made ready to accept our data. As before, the function returns a reference to the new file object.

Table 2 above shows one additional file method that we have not used thus far. The write method allows us to add data to a text file. Recall that text files contain sequences of characters. We usually think of these character sequences as being the lines of the file where each line ends with the newline \n character. Be very careful to notice that the write method takes one parameter, a string. When invoked, the characters of the string will be added to the end of the file. This means that it is the programmers job to include the newline characters as part of the string if desired.

As an example, consider the qbdata.txt file once again. Assume that we have been asked to provide a file consisting of only the names of the quarterbacks. In addition, the names should be in the order last name followed by first name with the names separated by a comma. This is a very common type of request, usually due to the fact that someone has a program that requires its data input format to be different from what is available.

To construct this file, we will approach the problem using a similar algorithm as above. After opening the file, we will iterate through the lines, break each line into its parts, choose the parts that we need, and then output them. Eventually, the output will be written to a file.

The program below solves part of the problem. Notice that it reads the data and creates a string consisting of last name followed by a comma followed by the first name. In this example, we simply print the lines as they are created.

infile = open("qbdata.txt", "r")
aline = infile.readline()
while aline:
    items = aline.split()
    dataline = items[1] + ',' + items[0]
    print(dataline)
    aline = infile.readline()

infile.close()

When we run this program, we see the lines of output on the screen. Once we are satisfied that it is creating the appropriate output, the next step is to add the necessary pieces to produce an output file and write the data lines to it. To start, we need to open a new output file by adding another call to the open function, outfile = open("qbnames.txt",'w'), using the 'w' flag. We can choose any file name we like. If the file does not exist, it will be created. However, if the file does exist, it will be reinitialized as empty and you will lose any previous contents.

Once the file has been created, we just need to call the write method passing the string that we wish to add to the file. In this case, the string is already being printed so we will just change the print into a call to the write method. However, there is one additional part of the data line that we need to include. The newline character needs to be concatenated to the end of the line. The entire line now becomes outfile.write(dataline + '\n'). We also need to close the file when we are done.

The complete program is shown below.

infile = open("qbdata.txt", "r")
outfile = open("qbnames.txt", "w")

aline = infile.readline()
while aline:
    items = aline.split()
    dataline = items[1] + ',' + items[0]
    outfile.write(dataline + '\n')
    aline = infile.readline()

infile.close()
outfile.close()

The contents of the qbnames.txt file are as follows.

McCoy,Colt
Freeman,Josh
Vick,Michael
Schaub,Matt
Rivers,Philip
Hasselbeck,Matt
Clausen,Jimmy
Flacco,Joe
Orton,Kyle
Campbell,Jason
Manning,Peyton
Brees,Drew
Ryan,Matt
Cassel,Matt
Sanchez,Mark
Favre,Brett
Garrard,David
Manning,Eli
Palmer,Carson
Smith,Alex
Henne,Chad
Romo,Tony
Cutler,Jay
Kitna,Jon
Brady,Tom
Roethlisberger,Ben
Collins,Kerry
Anderson,Derek
Fitzpatrick,Ryan
McNabb,Donovan
Kolb,Kevin
Rodgers,Aaron
Bradford,Sam
Hill,Shaun