[python] How to make a list of n numbers in Python and randomly select any number?

I have taken a count of something and it came out to N.

Now I would like to have a list, containing 1 to N numbers in it.

Example:

N = 5

then, count_list = [1, 2, 3, 4, 5]

Also, once I have created the list, I would like to randomly select a number from that list and use that number.

After that I would like to select another number from the remaining numbers of the list (N-1) and then use that also.

This goes on it the list is empty.

You don't need to count stuff if you want to pick a random element. Just use random.choice() and pass your iterable:

import random
items = ['foo', 'bar', 'baz']
print random.choice(items)

If you really have to count them, use random.randint(1, count+1).

After that I would like to select another number from the remaining numbers of the list (N-1) and then use that also.

Then you arguably do not really want to create a list of numbers from 1 to N just for the purpose of picking one (why not just ask for a random number in that range directly, instead of explicitly creating it to choose from?), but instead to shuffle such a list. Fortunately, the random module has you covered for this, too: just use random.shuffle.

Of course, if you have a huge list of numbers and you only want to draw a few, then it certainly makes sense to draw each using random.choice and remove it.

But... why do you want to select numbers from a range, that corresponds to the count of some items? Are you going to use the number to select one of the items? Don't do that; that's going out of your way to make things too complicated. If you want to select one of the items, then do so directly - again with random.choice.

Create the list (edited):

count_list = range(1, N+1)

Select random element:

import random
random.choice(count_list)

You can use:

import random
random.choice(range(n))

or:

random.choice(range(1,n+1))

if you want it from 1 to n and not from 0.

Maintain a set and remove a randomly picked-up element (with choice) until the list is empty:

s = set(range(1, 6))
import random

while len(s) > 0:
  s.remove(random.choice(list(s)))
  print(s)

Three runs give three different answers:

>>>
set([1, 3, 4, 5])
set([3, 4, 5])
set([3, 4])
set([4])
set([])
>>>
set([1, 2, 3, 5])
set([2, 3, 5])
set([2, 3])
set([2])
set([])

>>>
set([1, 2, 3, 5])
set([1, 2, 3])
set([1, 2])
set([1])
set([])

As for the first part:

>>> N = 5
>>> count_list = [i+1 for i in xrange(N)]
>>> count_list
[1, 2, 3, 4, 5]
>>>

As for the second, read 9.6. random — Generate pseudo-random numbers.

>>> from random import choice
>>> a = choice(count_list)
>>> a
1
>>> count_list.remove(a)
>>> count_list
[2, 3, 4, 5]

That's the general idea.

By the way, you may also be interested in reading Random selection of elements in a list, with no repeats (Python recipe).

There are a few implementations of fast random selection.

You can try this code

import random
N = 5
count_list = range(1,N+1)
random.shuffle(count_list)

while count_list:
    value = count_list.pop()
    # do whatever you want with 'value'