Randomized tests for Nose 2

nose2-random.py

# -*- coding: utf-8 -*-
"""
nose2-random

This plug-in facilitates capturing and setting the state of the internal
random number generator prior to test execution. Test output is annotated
with a seed value that you can re-use to reproduce randomly-generated test
data across runs.

.. note ::
   Random values produced from external randomness sources (for example, via
   ``os.urandom``) will not be reproducible.

:copyright: (c) 2012 by Ori Livneh
:license: BSD

"""
import os
import random
import base64

from nose2 import session
from nose2.events import Plugin
from nose2.tests._common import FunctionalTestCase


DEFAULT_SEED_SIZE = 32   # Default size, in bytes, of seed value

def get_digest(size=DEFAULT_SEED_SIZE):
    """Encodes `size` random bytes into base64"""
    seed = os.urandom(size)
    return base64.b64encode(seed)

def set_digest(digest):
    """Decodes a base64-encoded array of bytes and uses its value to seed the
    internal random number generator """
    seed = base64.b64decode(digest)
    random.seed(seed)


class Random(Plugin):
    """Capture / set random number generator state"""

    configSection = 'random'
    commandLineSwitch = ('R', 'random', 'Capture/set internal random number '
                         'generator state')

    def __init__(self):
        size = self.config.as_int('seed_size', default=DEFAULT_SEED_SIZE)
        self.digest = get_digest(size)
        self.addArgument(self.setSeedDigest, None, 'seed-digest',
                         'Seed random number generator with this digest')

    def setSeedDigest(self, digest):
        self.digest = digest

    def startTest(self, event):
        set_digest(self.digest)

    def testOutcome(self, event):
        event.metadata['digest'] = self.digest

    def outcomeDetail(self, event):
        event.extraDetail.append('Seed digest: {.digest}'.format(self))