Python Random Function without using random module

The thing is I need to do something that somehow uses the time

You could generate randomness based on a clock drift:

import struct
import time

def lastbit(f):
    return struct.pack('!f', f)[-1] & 1

def getrandbits(k):
    "Return k random bits using a relative drift of two clocks."
    # assume time.sleep() and time.clock() use different clocks
    # though it might work even if they use the same clock
    #XXX it does not produce "good" random bits, see below for details
    result = 0
    for _ in range(k):
        time.sleep(0)
        result <<= 1
        result |= lastbit(time.clock())
    return result

Once you have getrandbits(k), it is straigforward to get a random integer in range [a, b], including both end points. Based on CPython Lib/random.py:

def randint(a, b):
    "Return random integer in range [a, b], including both end points."
    return a + randbelow(b - a + 1)

def randbelow(n):
    "Return a random int in the range [0,n).  Raises ValueError if n<=0."
    # from Lib/random.py
    if n <= 0:
       raise ValueError
    k = n.bit_length()  # don't use (n-1) here because n can be 1
    r = getrandbits(k)          # 0 <= r < 2**k
    while r >= n: # avoid skew
        r = getrandbits(k)
    return r

Example, to generate 20 random numbers from 10 to 110 including:

print(*[randint(10, 110) for _ in range(20)])

Output:

11 76 66 58 107 102 73 81 16 58 43 107 108 98 17 58 18 107 107 77

If getrandbits(k) returns k random bits then randint(a, b) should work as is (no skew due to modulo, etc).

To test the quality of getrandbits(k), dieharder utility could be used:

$ python3 random-from-time.py | dieharder -a -g 200

where random-from-time.py generates infinite (random) binary stream:

#!/usr/bin/env python3

def write_random_binary_stream(write):
    while True:
        write(getrandbits(32).to_bytes(4, 'big'))

if __name__ == "__main__":
    import sys
    write_random_binary_stream(sys.stdout.buffer.write)

where getrandbits(k) is defined above.

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The above assumes that you are not allowed to use os.urandom() or ssl.RAND_bytes(), or some known PRNG algorithm such as Mersenne Twister to implement getrandbits(k).

---

getrandbits(n) implemented using "time.sleep() + time.clock()" fails dieharder tests (too many to be a coincidence).

The idea is still sound: a clock drift may be used as a source of randomness (entropy) but you can't use it directly (the distribution is not uniform and/or some bits are dependent); the bits could be passed as a seed to a PRNG that accepts an arbitrary entropy source instead. See "Mixing" section.

Are you allowed to read random data in some special file? Under Linux, the file `/dev/urandom' provides a convenient way to get random bytes. You could write:

import struct
f = open("/dev/urandom","r")
n = struct.unpack("i",f.read(4))[0]

But this will not work under Windows however.