SF patch 629637: Add sample(population, k) method to the random module.

Used for random sampling without replacement.

Lib/random.py

@@ -7,6 +7,7 @@
     sequences
     ---------
            pick random element
+           pick random sample
            generate random permutation
 
     distributions on the real line:
@@ -77,7 +78,7 @@ from math import log as _log, exp as _exp, pi as _pi, e as _e
 from math import sqrt as _sqrt, acos as _acos, cos as _cos, sin as _sin
 from math import floor as _floor
 
-__all__ = ["Random","seed","random","uniform","randint","choice",
+__all__ = ["Random","seed","random","uniform","randint","choice","sample",
            "randrange","shuffle","normalvariate","lognormvariate",
            "cunifvariate","expovariate","vonmisesvariate","gammavariate",
            "stdgamma","gauss","betavariate","paretovariate","weibullvariate",
@@ -373,6 +374,43 @@ class Random:
             j = int(random() * (i+1))
             x[i], x[j] = x[j], x[i]
 
+    def sample(self, population, k, random=None, int=int):
+        """Chooses k unique random elements from a population sequence.
+
+        Returns a new list containing elements from the population.  The
+        list itself is in random order so that all sub-slices are also
+        random samples.  The original sequence is left undisturbed.
+
+        If the population has repeated elements, then each occurence is
+        a possible selection in the sample.
+
+        If indices are needed for a large population, use xrange as an
+        argument:  sample(xrange(10000000), 60)
+
+        Optional arg random is a 0-argument function returning a random
+        float in [0.0, 1.0); by default, the standard random.random.
+        """
+
+        n = len(population)
+        if not 0 <= k <= n:
+            raise ValueError, "sample larger than population"
+        if random is None:
+            random = self.random
+        if n < 6 * k:     # if n len list takes less space than a k len dict
+            pool = list(population)
+            for i in xrange(n-1, n-k-1, -1):
+                j = int(random() * (i+1))
+                pool[i], pool[j] = pool[j], pool[i]
+            return pool[-k:]
+        inorder = [None] * k
+        selections = {}
+        for i in xrange(k):
+            j = int(random() * n)
+            while j in selections:
+                j = int(random() * n)
+            selections[j] = inorder[i] = population[j]
+        return inorder     # return selections in the order they were picked
+
 ## -------------------- real-valued distributions  -------------------
 
 ## -------------------- uniform distribution -------------------
@@ -711,7 +749,19 @@ def _test_generator(n, funccall):
     print 'avg %g, stddev %g, min %g, max %g' % \
               (avg, stddev, smallest, largest)
 
-def _test(N=20000):
+def _test_sample(n):
+    # For the entire allowable range of 0 <= k <= n, validate that
+    # the sample is of the correct length and contains only unique items
+    population = xrange(n)
+    for k in xrange(n+1):
+        s = sample(population, k)
+        assert len(dict([(elem,True) for elem in s])) == len(s) == k
+
+def _sample_generator(n, k):
+    # Return a fixed element from the sample.  Validates random ordering.
+    return sample(xrange(n), k)[k//2]
+
+def _test(N=2000):
     print 'TWOPI         =', TWOPI
     print 'LOG4          =', LOG4
     print 'NV_MAGICCONST =', NV_MAGICCONST
@@ -735,6 +785,9 @@ def _test(N=20000):
     _test_generator(N, 'betavariate(3.0, 3.0)')
     _test_generator(N, 'paretovariate(1.0)')
     _test_generator(N, 'weibullvariate(1.0, 1.0)')
+    _test_generator(N, '_sample_generator(50, 5)')  # expected s.d.: 14.4
+    _test_generator(N, '_sample_generator(50, 45)') # expected s.d.: 14.4
+    _test_sample(1000)
 
     # Test jumpahead.
     s = getstate()
@@ -760,6 +813,7 @@ uniform = _inst.uniform
 randint = _inst.randint
 choice = _inst.choice
 randrange = _inst.randrange
+sample = _inst.sample
 shuffle = _inst.shuffle
 normalvariate = _inst.normalvariate
 lognormvariate = _inst.lognormvariate