Reading files in Python

When working with data, it is usually provided as a simple text-file, a

.dat-file or something similar. Reading a file so we can use the data in our

program follows few, simple steps. Nevertheless a lot of things one finds

online may seem really complicated, so we start with a really simple example,

that can be modified easily.

Writing the datafile “test.dat”

The file we are going to read should be named “test.dat” and saved in the same

folder as this small program. Doing so you should be able to simply copy and

paste the code snippets.

The file should look like this:

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
x,y

1,1

2,2

3,3

4,4

5,5

6,6

Writing a function read_file

As you usually do not just read one file, we are going to write a function that

takes the name of the file and returns lists with the data.

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
def get_data(name):                     # first stept of writing a function

    with open(name) as file:            # opens the file we want to read

        data = file.read()              # reads the file

    lines = data.split("\n")            # split into lines after each linebreak

    del lines[0]                        # the first line is "x, y"

    del lines[-1]                       # the last line is empty

    x = list()

    y = list()                          # create empty lists

    for line in lines:

        entry = line.split(",")         # split each line at the comma

        x.append(float(entry[0]))

        y.append(float(entry[1]))       # add data to list

    return x, y                         # ending of the function

This is a big bunch of code, but there are only a few important things to

notice: First of all the file is split with “\n” which corresponds to the

linebreak. After that two lines are deleted. Try out what happens if you do

not do that step! After that the lines are split at each “,”. If you have

different things where you want to split that is no problem, for example

1
2
3
entry = data.split("\t")

splits the lines at a tab. The return statement ends the written function and

gives us the two lists that we have created.

Using the written function

Now we can use the written function to read the file that we have created in

the beginning. To do so, simply add some code to your Python-file:

1
2
3
4
5
6
7
x, y = get_data("test.dat") # calls the function with our file

print(x)

print(y)

By giving the function a filename we can now read that file and use a

print-statement to check if everything worked as expected.

Modify this function

If you take a close look at the function, you will notice that you can modify it

for different use. By adding more lists you can work with files with more

entries and as we said before, we can change the characters Python uses to

split a line to many different things.