Python Desktop Server Weblog 27.12.2004

Futures are just Promises - I was reminded of this fact by the Alice Tour. So I just added futures to my lazy evaluation module. Futures are just promises that are not calculated when the result is needed, but that start calculating the value right along - in parallel to your main thread. So every future represents a thread that calculates some result. When you force the result of a future that isn't already done, your main thread will block until the future is done and then continue with either the value or the exception of the calculated future. It's a nice and clean way to do multithreading in a small scale:

from LazyEvaluation import *

# define the fibonacci function
def fib(n):
if n in (0,1): return 1
else: return fib(n-1) + fib(n-2)

# make fib a future generating function
ffib = future(fib)

# create some future promises
fib5 = ffib(5)
fib10 = ffib(10)
fib20 = ffib(20)
fib30 = ffib(30) # that one takes quite long

# calculate some fibonacci number in the foreground while
# other calculations proceed in the background
# (due to the GIL in Python this won't make use of multiple
# processors and so can be seen as a purely academic
# exercise!)
fib15 = fib(15)

# now print the values - this will force the values
# and will block until the value is available
print fib5
print fib10
print fib15
print fib20
print fib30

Python Desktop Server Weblog 27.12.2004

Futures as a different kind of Promise