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

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', that's about 300 lines of text. Right now, I'm trying to get it to print me 10 random lines from that file, then delete those lines.

So if my file has 10 lines:

Carrot
Banana
Strawberry
Canteloupe
Blueberry
Snacks
Apple
Raspberry
Papaya
Watermelon

I need it to randomly pick out from those lines, tell me it's randomly picked blueberry, carrot, watermelon, and banana, and then delete those lines.

The issue is, when Python reads a file, it reads that file and once it gets to the end, it won't go back and delete the lines. My current thinking was that I could write the lines to a list, then reopen the file, match the list to the text file, and if it finds a match, delete the lines.

My current problem is twofold:

It's duplicating the random elements. If it picks a line, I need it to not pick that same line again. However, using random.sample doesn't seem to work, as I need those lines separated out when I later use each line to append to a URL.

I don't feel like my logic (write to array->find matches in text file->delete) is the most ideal logic. Is there a better way to write this?

import webbrowser
import random

"""url= 'http://www.google.com'
webbrowser.open_new_tab(url+myline)""" Eventually, I need a base URL + my 10 random lines opening in each new tab

def ShowMeTheRandoms():
    x=1
    DeleteList= []
    lines=open('TestingDeleteLines.txt').read().splitlines()
for x in range(0,10):
    myline=random.choice(lines)
    print(myline) """debugging, remove later"""
    DeleteList.append(myline)
    x=x+1
    print DeleteList """debugging, remove later"""
ShowMeTheRandoms()

Point is: you dont "delete" from a file, but rewrite the whole file (or another one) with new content. The canonical way is to read the original file line by line, write back the lines you want to keep to a temporary file, then replace the old file with the new one.

with open("/path/to/source.txt") as src, open("/path/to/temp.txt", "w") as dest:
    for line in src:
        if should_we_keep_this_line(line):
            dest.write(line)
os.rename("/path/to/temp.txt", "/path/to/source.txt")

I have a file, 'TestingDeleteLines.txt', that's about 300 lines of text. Right now, I'm trying to get it to print me 10 random lines from that file, then delete those lines.

#!/usr/bin/env python
import random

k = 10
filename = 'TestingDeleteLines.txt'
with open(filename) as file:
    lines = file.read().splitlines()

if len(lines) > k:
    random_lines = random.sample(lines, k)
    print("\n".join(random_lines)) # print random lines

    with open(filename, 'w') as output_file:
        output_file.writelines(line + "\n"
                               for line in lines if line not in random_lines)
elif lines: # file is too small
    print("\n".join(lines)) # print all lines
    with open(filename, 'wb', 0): # empty the file
        pass

It is O(n**2) algorithm that can be improved if necessary (you don't need it for a tiny file such as your input)

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.

What about list.pop - it gives you the item and update the list in one step.

lines = readlines()
deleted = []

indices_to_delete = random.sample(xrange(len(lines)), 10)

# sort to delete biggest index first 
indices_to_delete.sort(reverse=True)

for i in indices_to_delete:
    # lines.pop(i) delete item at index i and return the item
    # do you need it or its index in the original file than
    deleted.append((i, lines.pop(i)))

# write the updated *lines* back to the file or new file ?!
# and you have everything in deleted if you need it again

Lets assume you have a list of lines from your file stored in items

>>> items = ['a', 'b', 'c', 'd', 'e', 'f']
>>> choices = random.sample(items, 2)  # select 2 items
>>> choices  # here are the two
['b', 'c']
>>> for i in choices:
...   items.remove(i)
...
>>> items  # tee daa, no more b or c
['a', 'd', 'e', 'f']

From here you would overwrite your previous text file with the contents of items joining with your preferred line ending \r\n or \n. readlines() does not strip line endings so if you use that method, you do not need to add your own line endings.

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

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