How to shuffle the contents of a text file in groups of n lines in Python

Question

Let's say the text file is this:

1
2
3
4
5 
6
...

What I want is to randomely order the contents in groups of N lines, without shuffling the lines in each group, like this:

#In this case, N = 2.
5
6
1
2
7
8
...

My file isn't huge, it will be less than 50 lines for sure.

I've tried to do this using this code:

import random


with open("data.txt", "r") as file:
    lines = []
    groups = []
    for line in file:
        lines.append(line[:-1])
        if len(lines) > 3:
            groups.append(lines)


            lines = []

    random.shuffle(groups)


with open("data.txt", "w") as file:
    file.write("\n".join(groups))

But I get this error:

Traceback (most recent call last):
  File "C:/PYTHON/python/Data.py", line 19, in <module>
    file.write("\n".join(groups))
TypeError: sequence item 0: expected str instance, list found

Is there a simpler way of doing this?

Answer 1

You tried to join a list of lists; flatten them first:

with open("data.txt", "w") as file:
    file.write("\n".join(['\n'.join(g) for g in groups]))

You could use any of the recommended methods of chunking to produce your groups. With a file object, all you need to do is zip() the file with itself:

with open("data.txt", "r") as file:
    groups = list(zip(file, file))

Note that this'll drop the last line if there are an odd number of lines in the file. This includes the newlines now, so join on '' rather than '\\n' .

You could also join the two lines for each group together before shuffling:

with open("data.txt", "r") as file:
    groups = [a + b for a, b in zip(file, file)]

random.shuffle(groups)

with open("data.txt", "w") as file:
    file.write("".join(groups))