Python: Choose random line from file, then delete that line

Question

I\'m new to Python (in that I learned it through a CodeAcademy course) and could use some help with figuring this out.

I have a file, \'TestingDeleteLines.txt\', tha

Answer 1

To choose a random line from a file, you could use a space efficient single-pass reservoir-sampling algorithm. To delete that line, you could print everything except the chosen line:

#!/usr/bin/env python3
import fileinput

with open(filename) as file:
    k = select_random_it(enumerate(file), default=[-1])[0]

if k >= 0: # file is not empty
    with fileinput.FileInput(filename, inplace=True, backup='.bak') as file:
        for i, line in enumerate(file):
            if i != k: # keep line
                print(line, end='') # stdout is redirected to filename

where select_random_it() implements the reservoir-sampling algorithm:

import random

def select_random_it(iterator, default=None, randrange=random.randrange):
    """Return a random element from iterator.

    Return default if iterator is empty.
    iterator is exhausted.
    O(n)-time, O(1)-space algorithm.
    """
    # from https://stackoverflow.com/a/1456750/4279
    # select 1st item with probability 100% (if input is one item, return it)
    # select 2nd item with probability 50% (or 50% the selection stays the 1st)
    # select 3rd item with probability 33.(3)%
    # select nth item with probability 1/n
    selection = default
    for i, item in enumerate(iterator, start=1):
        if randrange(i) == 0: # random [0..i)
            selection = item
    return selection

---

To print k random lines from a file and delete them:

#!/usr/bin/env python3
import random
import sys

k = 10
filename = 'TestingDeleteLines.txt'
with open(filename) as file:
    random_lines = reservoir_sample(file, k) # get k random lines

if not random_lines: # file is empty
    sys.exit() # do nothing, exit immediately

print("\n".join(map(str.strip, random_lines))) # print random lines
delete_lines(filename, random_lines) # delete them from the file

where reservoir_sample() uses the same algorithm as select_random_it() but allows to choose k items instead of one:

import random

def reservoir_sample(iterable, k,
                     randrange=random.randrange, shuffle=random.shuffle):
    """Select *k* random elements from *iterable*.

    Use O(n) Algorithm R https://en.wikipedia.org/wiki/Reservoir_sampling

    If number of items less then *k* then return all items in random order.
    """
    it = iter(iterable)
    if not (k > 0):
        raise ValueError("sample size must be positive")

    sample = list(islice(it, k)) # fill the reservoir
    shuffle(sample)
    for i, item in enumerate(it, start=k+1):
        j = randrange(i) # random [0..i)
        if j < k:
            sample[j] = item # replace item with gradually decreasing probability
    return sample

and delete_lines() utility function deletes chosen random lines from the file:

import fileinput
import os

def delete_lines(filename, lines):
    """Delete *lines* from *filename*."""
    lines = set(lines) # for amortized O(1) lookup
    with fileinput.FileInput(filename, inplace=True, backup='.bak') as file:
        for line in file:
            if line not in lines:
                print(line, end='')
    os.unlink(filename + '.bak') # remove backup if there is no exception

reservoir_sample(), delete_lines() funciton do not load the whole file into memory and therefore they can work for arbitrary large files.