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"""
If you execute force_repeatability() then the following things are changed in the runtime:
1. random.random() and its sibling functions, and random.Random.seed() in the random module are seeded with a known seed so that they will return the same sequence on each run.
2. os.urandom() is replaced by a fake urandom that returns a pseudorandom sequence.
3. time.time() is replaced by a fake time that returns an incrementing number. (Original time.time is available as time.realtime.)
Which seed will be used?
If the environment variable REPEATABLE_RANDOMNESS_SEED is set, then it will use that. Else, it will use the current real time. In either case it logs the seed that it used.
Caveats:
1. If some code has acquired a random.Random object before force_repeatability() is executed, then that Random object will produce non-reproducible results. For example, the tempfile module in the Python Standard Library does this.
2. Likewise if some code called time.time() before force_repeatability() was called, then it will have gotten a real time stamp. For example, trial does this. (Then it later subtracts that real timestamp from a faketime timestamp to calculate elapsed time, resulting in a large negative elapsed time.)
3. Fake urandom has an added constraint for performance reasons -- you can't ask it for more than 64 bytes of randomness at a time. (I couldn't figure out how to generate large fake random strings efficiently.)
"""
import os, random, time
if not hasattr(time, "realtime"):
time.realtime = time.time
if not hasattr(os, "realurandom"):
os.realurandom = os.urandom
if not hasattr(random, "realseed"):
random.realseed = random.seed
tdelta = 0
seeded = False
def force_repeatability():
now = 1043659734.0
def faketime():
global tdelta
tdelta += 1
return now + tdelta
time.faketime = faketime
time.time = faketime
from idlib import i2b
def fakeurandom(n):
if n > 64:
raise ("Can't produce more than 64 bytes of pseudorandomness efficiently.")
elif n == 0:
return ''
else:
z = i2b(random.getrandbits(n*8))
x = z + "0" * (n-len(z))
assert len(x) == n
return x
os.fakeurandom = fakeurandom
os.urandom = fakeurandom
global seeded
if not seeded:
SEED = os.environ.get('REPEATABLE_RANDOMNESS_SEED', None)
if SEED is None:
# Generate a seed which is integral and fairly short (to ease cut-and-paste, writing it down, etc.).
t = time.realtime()
subsec = t % 1
t += (subsec * 1000000)
t %= 1000000
SEED = long(t)
import sys
sys.stdout.write("REPEATABLE_RANDOMNESS_SEED: %s\n" % SEED) ; sys.stdout.flush()
sys.stdout.write("In order to reproduce this run of the code, set the environment variable \"REPEATABLE_RANDOMNESS_SEED\" to %s before executing.\n" % SEED) ; sys.stdout.flush()
random.seed(SEED)
def seed_which_refuses(a):
sys.stdout.write("I refuse to reseed to %s. Go away!\n" % (a,)) ; sys.stdout.flush()
return
random.realseed = random.seed
random.seed = seed_which_refuses
seeded = True
import setutil
setutil.RandomSet.DETERMINISTIC = True
def restore_real_clock():
time.time = time.realtime
def restore_real_urandom():
os.urandom = os.realurandom
def restore_real_seed():
random.seed = random.realseed
def restore_non_repeatability():
restore_real_seed()
restore_real_urandom()
restore_real_clock()