Fix bugs introduced by rewrite (in particular, time-based initialization
got broken). Also added new method .jumpahead(N). This finally gives us
a semi-decent answer to how Python's RNGs can be used safely and efficiently
in multithreaded programs (although it requires the user to use the new
machinery!).
diff --git a/Lib/random.py b/Lib/random.py
index a818f73..1b91886 100644
--- a/Lib/random.py
+++ b/Lib/random.py
@@ -72,8 +72,8 @@
self.gauss_next = None
# Specific to Wichmann-Hill generator. Subclasses wishing to use a
- # different core generator should override seed(), random(), getstate()
- # and setstate().
+ # different core generator should override the seed(), random(),
+ # getstate(), setstate(), and jumpahead() methods.
def __whseed(self, x=0, y=0, z=0):
"""Set the Wichmann-Hill seed from (x, y, z).
@@ -104,6 +104,7 @@
if a is None:
self.__whseed()
+ return
a = hash(a)
a, x = divmod(a, 256)
a, y = divmod(a, 256)
@@ -115,11 +116,10 @@
def getstate(self):
"""Return internal state; can be passed to setstate() later."""
-
return self.VERSION, self._seed, self.gauss_next
def __getstate__(self): # for pickle
- self.getstate()
+ return self.getstate()
def setstate(self, state):
"""Restore internal state from object returned by getstate()."""
@@ -134,6 +134,28 @@
def __setstate__(self, state): # for pickle
self.setstate(state)
+ def jumpahead(self, n):
+ """Act as if n calls to random() were made, but quickly.
+
+ n is an int, greater than or equal to 0.
+
+ Example use: If you have 2 threads and know that each will
+ consume no more than a million random numbers, create two Random
+ objects r1 and r2, then do
+ r2.setstate(r1.getstate())
+ r2.jumpahead(1000000)
+ Then r1 and r2 will use guaranteed-disjoint segments of the full
+ period.
+ """
+
+ if not n >= 0:
+ raise ValueError("n must be >= 0")
+ x, y, z = self._seed
+ x = int(x * pow(171, n, 30269)) % 30269
+ y = int(y * pow(172, n, 30307)) % 30307
+ z = int(z * pow(170, n, 30323)) % 30323
+ self._seed = x, y, z
+
def random(self):
"""Get the next random number in the range [0.0, 1.0)."""
@@ -471,6 +493,17 @@
print 'avg %g, stddev %g, min %g, max %g' % \
(avg, stddev, smallest, largest)
+ s = getstate()
+ N = 1019
+ jumpahead(N)
+ r1 = random()
+ setstate(s)
+ for i in range(N): # now do it the slow way
+ random()
+ r2 = random()
+ if r1 != r2:
+ raise ValueError("jumpahead test failed " + `(N, r1, r2)`)
+
def _test(N=200):
print 'TWOPI =', TWOPI
print 'LOG4 =', LOG4
@@ -515,6 +548,7 @@
weibullvariate = _inst.weibullvariate
getstate = _inst.getstate
setstate = _inst.setstate
+jumpahead = _inst.jumpahead
if __name__ == '__main__':
_test()