SOLVED How to iterate over every n-th line from a file?

[sweet friends]

I have a file called data.txt containing lines of data:

This is line one
This is line two 
This is line three 
This is line four 
This is line five 
This is line six 
This is line seven

…
I have the following code:

with open('data.txt', 'r') as f:
    for x, line in enumerate(f):
        if x == 3:
            print(line)
In this case it only prints

“This is line four”.
I do understand why but how do I take it from here and have it print the lines 4, 7, 10, 13, …?

[zaasmi]

Demo:

data.txt:

line1
line2
line3
line4
line5
line6
line7
line8
line9
line10
line11
line12
line13

Code:

from itertools import islice

with open('data.txt') as f:
    for line in islice(f, 3, None, 3):
        print line,  # Python3: print(line, end='')

Produces:

line4
line7
line10
line13

Reff

[zaasmi]

Demo:

data.txt:

line1
line2
line3
line4
line5
line6
line7
line8
line9
line10
line11
line12
line13

Code:

from itertools import islice

with open('data.txt') as f:
    for line in islice(f, 3, None, 3):
        print line,  # Python3: print(line, end='')

Produces:

line4
line7
line10
line13

Reff