Python - empty range for randrange() (0,0, 0) and ValueError("empty range for randrange() (%d,%d, %d)" % (istart, istop, width))

Question

When I run this program: (python 3.3.1)

import random
import time
from random import randrange
print(' ')
print('I am thinking of a person...')
time.sleep(1)
print('He or she belongs to this group of people:')
people = 'Alice Elise Jack Jill Ricardo David Jane Sacha Thomas'.split()
loop = 0
while loop != 6:
    group = []
    person = randrange(0, len(people))
    personName = people[person]
    int(person)
    group.append(personName)
    del people[person]
    loop = loop + 1

I sometimes get this error message:

Traceback (most recent call last):
  File "C:\Users\user\Python\wsda.py", line 132, in <module>
    person = randrange(0, len(people))
  File "C:\Python33\lib\random.py", line 192, in randrange
    raise ValueError("empty range for randrange() (%d,%d, %d)" % (istart, istop,     width))
ValueError: empty range for randrange() (0,0, 0)

Basically i want it to get 6 random names from the variable 'people' and add it to the variable 'group'...

Also this is part of a larger program based on the guess who game... Could someone please tell me how to fix this? Thx

Solution

The error occurs when the people list is empty (length 0). You may want to test for that:

>>> import random
>>> random.randrange(0, 0)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/Users/mj/Development/Libraries/buildout.python/parts/opt/lib/python2.7/random.py", line 217, in randrange
    raise ValueError, "empty range for randrange() (%d,%d, %d)" % (istart, istop, width)
ValueError: empty range for randrange() (0,0, 0)

If you need to add 6 random choices from the people list it is better to shuffle the people list and then add the first 6 of that list to group:

import random
people = 'Alice Elise Jack Jill Ricardo David Jane Sacha Thomas'.split()
random.shuffle(people)
group.extend(people[:6])
people = people[6:]  # remainder, so the 6 picks have been removed

but chances are of course you end up with an empty list again at some point.

Another approach is to just use random.sample():

people = 'Alice Elise Jack Jill Ricardo David Jane Sacha Thomas'.split()
group.extend(random.sample(people, 6))

This just picks 6 random names from the list, but leaves people unaffected and a future pick of 6 names could repeat names.

Comments

[Lucas]

Thx that worked! BTW what does people[:6] do? Does it take the first 6? The division sign is a bit wierd, hence my confusion.

[Martijn Pieters]

See The Python Slice Notation, it takes the first 6 (indices 0 through to 5).