I’ve been organizing the Boston Python user group for a few years now, and I like it a lot. Except for ordering pizza. But finally I’ve brought some technology to bear on the problem!

First, I’ve taken a poll of people RSVP’ing for tonight’s project night, so I now have an empirical basis for deciding what fraction of the pizzas should be meat, vegetable, cheese, or vegan. Surprise (to me): vegetable wins.

Second, I’ve written what may be the world’s most useful Python program: pizza.py!

"""How many pizzas do we need?"""

import math
import sys

if len(sys.argv) > 1:
    people = int(sys.argv[1])
else:
    people = int(raw_input("How many RSVPs? "))

# The MUC (Meetup Universal Constant)
attending = people * .65

print
print "%d people will show up (guess)" % attending

# Appetite estimation
slices = attending * 2.5

# Basic pizza geometry
pies = slices / 8

print "%.1f pizzas (or so)" % pies

# From answers to the 10/2012 project night:
#   81 answers
#   26 meat 32%
#   37 veg 45%
#   16 cheese 20%
#   2 vegan 3%

vegan = int(.03 * pies) or 1
meat = int(.33 * pies) or 1
veg = int(.45 * pies) or 1
cheese = pies - vegan - meat - veg
if cheese < 1:
    cheese = 1
cheese = int(math.ceil(cheese))

print
print "%2d cheese" % cheese
print "%2d meat" % meat
print "%2d veggie" % veg
print "%2d vegan" % vegan
print
print "%2d total" % (cheese + meat + veg + vegan)

The hard truth here is the Meetup Universal Constant. The MUC has been empirically determined, and says that no matter how much you wheedle people to show up if they say they will, and vice-versa, about one-third of the RSVPs will not attend. This number has proven remarkably stable over the 25 or so events that we’ve measured.

As an example, for tonight’s event, we have 127 RSVPs:

How many RSVPs? 127

82 people will show up (guess)
25.8 pizzas (or so)

 6 cheese
 8 meat
11 veggie
 1 vegan

26 total

Your numbers may vary. Perhaps Boston is a vegetarian hotbed compared to where you are. Maybe your city has more-predictable weather and fewer people abandon their intention to attend. Tweak pizza.py as you see fit!