How random is random? This is a weird question to ask, but it is one of paramount importance in cases where information security is concerned. Whenever you’re generating random data, strings, or numbers in Python, it’s a good idea to have at least a rough idea of how that data was generated.
Here, you’ll cover a handful of different options for generating random data in Python, and then build up to a comparison of each in terms of its level of security, versatility, purpose, and speed.
I promise that this tutorial will not be a lesson in mathematics or cryptography, which I wouldn’t be well equipped to lecture on in the first place. You’ll get into just as much math as needed, and no more.
How Random Is Random?
First, a prominent disclaimer is necessary. Most random data generated with Python is not fully random in the scientific sense of the word. Rather, it is pseudorandom: generated with a pseudorandom number generator (PRNG), which is essentially any algorithm for generating seemingly random but still reproducible data.
“True” random numbers can be generated by, you guessed it, a true random number generator (TRNG). One example is to repeatedly pick up a die off the floor, toss it in the air, and let it land how it may.
Assuming that your toss is unbiased, you have truly no idea what number the die will land on. Rolling a die is a crude form of using hardware to generate a number that is not deterministic whatsoever. (Or, you can have the diceomatic do this for you.) TRNGs are out of the scope of this article but worth a mention nonetheless for comparison’s sake.
PRNGs, usually done with software rather than hardware, work slightly differently. Here’s a concise description:
They start with a random number, known as the seed, and then use an algorithm to generate a pseudorandom sequence of bits based on it. (Source)
You’ve likely been told to “read the docs!” at some point. Well, those people are not wrong. Here’s a particularly notable snippet from the random
module’s documentation that you don’t want to miss:
Warning: The pseudorandom generators of this module should not be used for security purposes. (Source)
You’ve probably seen random.seed(999)
, random.seed(1234)
, or the like, in Python. This function call is seeding the underlying random number generator used by Python’s random
module. It is what makes subsequent calls to generate random numbers deterministic: input A always produces output B. This blessing can also be a curse if it is used maliciously.
Perhaps the terms “random” and “deterministic” seem like they cannot exist next to each other. To make that clearer, here’s an extremely trimmed down version of random()
that iteratively creates a “random” number by using x = (x * 3) % 19
. x
is originally defined as a seed value and then morphs into a deterministic sequence of numbers based on that seed:
class NotSoRandom(object):
def seed(self, a=3):
"""Seed the world's most mysterious random number generator."""
self.seedval = a
def random(self):
"""Look, random numbers!"""
self.seedval = (self.seedval * 3) % 19
return self.seedval
_inst = NotSoRandom()
seed = _inst.seed
random = _inst.random
Don’t take this example too literally, as it’s meant mainly to illustrate the concept. If you use the seed value 1234, the subsequent sequence of calls to random()
should always be identical:
>>> seed(1234)
>>> [random() for _ in range(10)]
[16, 10, 11, 14, 4, 12, 17, 13, 1, 3]
>>> seed(1234)
>>> [random() for _ in range(10)]
[16, 10, 11, 14, 4, 12, 17, 13, 1, 3]
You’ll see a more serious illustration of this shortly.
What Is “Cryptographically Secure?”
If you haven’t had enough with the “RNG” acronyms, let’s throw one more into the mix: a CSPRNG, or cryptographically secure PRNG. CSPRNGs are suitable for generating sensitive data such as passwords, authenticators, and tokens. Given a random string, there is realistically no way for Malicious Joe to determine what string came before or after that string in a sequence of random strings.
One other term that you may see is entropy. In a nutshell, this refers to the amount of randomness introduced or desired. For example, one Python module that you’ll cover here defines DEFAULT_ENTROPY = 32
, the number of bytes to return by default. The developers deem this to be “enough” bytes to be a sufficient amount of noise.
Note: Through this tutorial, I assume that a byte refers to 8 bits, as it has since the 1960s, rather than some other unit of data storage. You are free to call this an octet if you so prefer.
A key point about CSPRNGs is that they are still pseudorandom. They are engineered in some way that is internally deterministic, but they add some other variable or have some property that makes them “random enough” to prohibit backing into whatever function enforces determinism.
What You’ll Cover Here
In practical terms, this means that you should use plain PRNGs for statistical modeling, simulation, and to make random data reproducible. They’re also significantly faster than CSPRNGs, as you’ll see later on. Use CSPRNGs for security and cryptographic applications where data sensitivity is imperative.
In addition to expanding on the use cases above, in this tutorial, you’ll delve into Python tools for using both PRNGs and CSPRNGs:
 PRNG options include the
random
module from Python’s standard library and its arraybased NumPy counterpart,numpy.random
.  Python’s
os
,secrets
, anduuid
modules contain functions for generating cryptographically secure objects.
You’ll touch on all of the above and wrap up with a highlevel comparison.
PRNGs in Python
The random
Module
Probably the most widely known tool for generating random data in Python is its random
module, which uses the Mersenne Twister PRNG algorithm as its core generator.
Earlier, you touched briefly on random.seed()
, and now is a good time to see how it works. First, let’s build some random data without seeding. The random.random()
function returns a random float in the interval [0.0, 1.0). The result will always be less than the righthand endpoint (1.0). This is also known as a semiopen range:
>>> # Don't call `random.seed()` yet
>>> import random
>>> random.random()
0.35553263284394376
>>> random.random()
0.6101992345575074
If you run this code yourself, I’ll bet my life savings that the numbers returned on your machine will be different. The default when you don’t seed the generator is to use your current system time or a “randomness source” from your OS if one is available.
With random.seed()
, you can make results reproducible, and the chain of calls after random.seed()
will produce the same trail of data:
>>> random.seed(444)
>>> random.random()
0.3088946587429545
>>> random.random()
0.01323751590501987
>>> random.seed(444) # Reseed
>>> random.random()
0.3088946587429545
>>> random.random()
0.01323751590501987
Notice the repetition of “random” numbers. The sequence of random numbers becomes deterministic, or completely determined by the seed value, 444.
Let’s take a look at some more basic functionality of random
. Above, you generated a random float. You can generate a random integer between two endpoints in Python with the random.randint()
function. This spans the full [x, y] interval and may include both endpoints:
>>> random.randint(0, 10)
7
>>> random.randint(500, 50000)
18601
With random.randrange()
, you can exclude the righthand side of the interval, meaning the generated number always lies within [x, y) and will always be smaller than the right endpoint:
>>> random.randrange(1, 10)
5
If you need to generate random floats that lie within a specific [x, y] interval, you can use random.uniform()
, which plucks from the continuous uniform distribution:
>>> random.uniform(20, 30)
27.42639687016509
>>> random.uniform(30, 40)
36.33865802745107
To pick a random element from a nonempty sequence (like a list or a tuple), you can use random.choice()
. There is also random.choices()
for choosing multiple elements from a sequence with replacement (duplicates are possible):
>>> items = ['one', 'two', 'three', 'four', 'five']
>>> random.choice(items)
'four'
>>> random.choices(items, k=2)
['three', 'three']
>>> random.choices(items, k=3)
['three', 'five', 'four']
To mimic sampling without replacement, use random.sample()
:
>>> random.sample(items, 4)
['one', 'five', 'four', 'three']
You can randomize a sequence inplace using random.shuffle()
. This will modify the sequence object and randomize the order of elements:
>>> random.shuffle(items)
>>> items
['four', 'three', 'two', 'one', 'five']
If you’d rather not mutate the original list, you’ll need to make a copy first and then shuffle the copy. You can create copies of Python lists with the copy
module, or just x[:]
or x.copy()
, where x
is the list.
Before moving on to generating random data with NumPy, let’s look at one more slightly involved application: generating a sequence of unique random strings of uniform length.
It can help to think about the design of the function first. You need to choose from a “pool” of characters such as letters, numbers, and/or punctuation, combine these into a single string, and then check that this string has not already been generated. A Python set
works well for this type of membership testing:
import string
def unique_strings(k: int, ntokens: int,
pool: str=string.ascii_letters) > set:
"""Generate a set of unique string tokens.
k: Length of each token
ntokens: Number of tokens
pool: Iterable of characters to choose from
For a highly optimized version:
https://stackoverflow.com/a/48421303/7954504
"""
seen = set()
# An optimization for tightlybound loops:
# Bind these methods outside of a loop
join = ''.join
add = seen.add
while len(seen) < ntokens:
token = join(random.choices(pool, k=k))
add(token)
return seen
''.join()
joins the letters from random.choices()
into a single Python str
of length k
. This token is added to the set, which can’t contain duplicates, and the while
loop executes until the set has the number of elements that you specify.
Resource: Python’s string
module contains a number of useful constants: ascii_lowercase
, ascii_uppercase
, string.punctuation
, ascii_whitespace
, and a handful of others.
Let’s try this function out:
>>> unique_strings(k=4, ntokens=5)
{'AsMk', 'Cvmi', 'GIxv', 'HGsZ', 'eurU'}
>>> unique_strings(5, 4, string.printable)
{"'O*1!", '9Ien%', 'W=m7<', 'mUDz'}
For a finetuned version of this function, this Stack Overflow answer uses generator functions, name binding, and some other advanced tricks to make a faster, cryptographically secure version of unique_strings()
above.
PRNGs for Arrays: numpy.random
One thing you might have noticed is that a majority of the functions from random
return a scalar value (a single int
, float
, or other object). If you wanted to generate a sequence of random numbers, one way to achieve that would be with a Python list comprehension:
>>> [random.random() for _ in range(5)]
[0.021655420657909374,
0.4031628347066195,
0.6609991871223335,
0.5854998250783767,
0.42886606317322706]
But there is another option that is specifically designed for this. You can think of NumPy’s own numpy.random
package as being like the standard library’s random
, but for NumPy arrays. (It also comes loaded with the ability to draw from a lot more statistical distributions.)
Take note that numpy.random
uses its own PRNG that is separate from plain old random
. You won’t produce deterministically random NumPy arrays with a call to Python’s own random.seed()
:
>>> import numpy as np
>>> np.random.seed(444)
>>> np.set_printoptions(precision=2) # Output decimal fmt.
Without further ado, here are a few examples to whet your appetite:
>>> # Return samples from the standard normal distribution
>>> np.random.randn(5)
array([ 0.36, 0.38, 1.38, 1.18, 0.94])
>>> np.random.randn(3, 4)
array([[1.14, 0.54, 0.55, 0.21],
[ 0.21, 1.27, 0.81, 3.3 ],
[0.81, 0.36, 0.88, 0.15]])
>>> # `p` is the probability of choosing each element
>>> np.random.choice([0, 1], p=[0.6, 0.4], size=(5, 4))
array([[0, 0, 1, 0],
[0, 1, 1, 1],
[1, 1, 1, 0],
[0, 0, 0, 1],
[0, 1, 0, 1]])
In the syntax for randn(d0, d1, ..., dn)
, the parameters d0, d1, ..., dn
are optional and indicate the shape of the final object. Here, np.random.randn(3, 4)
creates a 2d array with 3 rows and 4 columns. The data will be i.i.d., meaning that each data point is drawn independent of the others.
Another common operation is to create a sequence of random Boolean values, True
or False
. One way to do this would be with np.random.choice([True, False])
. However, it’s actually about 4x faster to choose from (0, 1)
and then viewcast these integers to their corresponding Boolean values:
>>> # NumPy's `randint` is [inclusive, exclusive), unlike `random.randint()`
>>> np.random.randint(0, 2, size=25, dtype=np.uint8).view(bool)
array([ True, False, True, True, False, True, False, False, False,
False, False, True, True, False, False, False, True, False,
True, False, True, True, True, False, True])
What about generating correlated data? Let’s say you want to simulate two correlated time series. One way of going about this is with NumPy’s multivariate_normal()
function, which takes a covariance matrix into account. In other words, to draw from a single normally distributed random variable, you need to specify its mean and variance (or standard deviation).
To sample from the multivariate normal distribution, you specify the means and covariance matrix, and you end up with multiple, correlated series of data that are each approximately normally distributed.
However, rather than covariance, correlation is a measure that is more familiar and intuitive to most. It’s the covariance normalized by the product of standard deviations, and so you can also define covariance in terms of correlation and standard deviation:
So, could you draw random samples from a multivariate normal distribution by specifying a correlation matrix and standard deviations? Yes, but you’ll need to get the above into matrix form first. Here, S is a vector of the standard deviations, P is their correlation matrix, and C is the resulting (square) covariance matrix:
This can be expressed in NumPy as follows:
def corr2cov(p: np.ndarray, s: np.ndarray) > np.ndarray:
"""Covariance matrix from correlation & standard deviations"""
d = np.diag(s)
return d @ p @ d
Now, you can generate two time series that are correlated but still random:
>>> # Start with a correlation matrix and standard deviations.
>>> # 0.40 is the correlation between A and B, and the correlation
>>> # of a variable with itself is 1.0.
>>> corr = np.array([[1., 0.40],
... [0.40, 1.]])
>>> # Standard deviations/means of A and B, respectively
>>> stdev = np.array([6., 1.])
>>> mean = np.array([2., 0.5])
>>> cov = corr2cov(corr, stdev)
>>> # `size` is the length of time series for 2d data
>>> # (500 months, days, and so on).
>>> data = np.random.multivariate_normal(mean=mean, cov=cov, size=500)
>>> data[:10]
array([[ 0.58, 1.87],
[7.31, 0.74],
[6.24, 0.33],
[0.77, 1.19],
[ 1.71, 0.7 ],
[3.33, 1.57],
[1.13, 1.23],
[6.58, 1.81],
[0.82, 0.34],
[2.32, 1.1 ]])
>>> data.shape
(500, 2)
You can think of data
as 500 pairs of inversely correlated data points. Here’s a sanity check that you can back into the original inputs, which approximate corr
, stdev
, and mean
from above:
>>> np.corrcoef(data, rowvar=False)
array([[ 1. , 0.39],
[0.39, 1. ]])
>>> data.std(axis=0)
array([5.96, 1.01])
>>> data.mean(axis=0)
array([2.13, 0.49])
Before we move on to CSPRNGs, it might be helpful to summarize some random
functions and their numpy.random
counterparts:
Python random Module 
NumPy Counterpart  Use 

random() 
rand() 
Random float in [0.0, 1.0) 
randint(a, b) 
random_integers() 
Random integer in [a, b] 
randrange(a, b[, step]) 
randint() 
Random integer in [a, b) 
uniform(a, b) 
uniform() 
Random float in [a, b] 
choice(seq) 
choice() 
Random element from seq 
choices(seq, k=1) 
choice() 
Random k elements from seq with replacement 
sample(population, k) 
choice() with replace=False 
Random k elements from seq without replacement 
shuffle(x[, random]) 
shuffle() 
Shuffle the sequence x in place 
normalvariate(mu, sigma) or gauss(mu, sigma) 
normal() 
Sample from a normal distribution with mean mu and standard deviation sigma 
Note: NumPy is specialized for building and manipulating large, multidimensional arrays. If you just need a single value, random
will suffice and will probably be faster as well. For small sequences, random
may even be faster too, because NumPy does come with some overhead.
Now that you’ve covered two fundamental options for PRNGs, let’s move onto a few more secure adaptations.
CSPRNGs in Python
os.urandom()
: About as Random as It Gets
Python’s os.urandom()
function is used by both secrets
and uuid
(both of which you’ll see here in a moment). Without getting into too much detail, os.urandom()
generates operatingsystemdependent random bytes that can safely be called cryptographically secure:

On Unix operating systems, it reads random bytes from the special file
/dev/urandom
, which in turn “allow access to environmental noise collected from device drivers and other sources.” (Thank you, Wikipedia.) This is garbled information that is particular to your hardware and system state at an instance in time but at the same time sufficiently random. 
On Windows, the C++ function
CryptGenRandom()
is used. This function is still technically pseudorandom, but it works by generating a seed value from variables such as the process ID, memory status, and so on.
With os.urandom()
, there is no concept of manually seeding. While still technically pseudorandom, this function better aligns with how we think of randomness. The only argument is the number of bytes to return:
>>> os.urandom(3)
b'\xa2\xe8\x02'
>>> x = os.urandom(6)
>>> x
b'\xce\x11\xe7"!\x84'
>>> type(x), len(x)
(bytes, 6)
Before we go any further, this might be a good time to delve into a minilesson on character encoding. Many people, including myself, have some type of allergic reaction when they see bytes
objects and a long line of \x
characters. However, it’s useful to know how sequences such as x
above eventually get turned into strings or numbers.
os.urandom()
returns a sequence of single bytes:
>>> x
b'\xce\x11\xe7"!\x84'
But how does this eventually get turned into a Python str
or sequence of numbers?
First, recall one of the fundamental concepts of computing, which is that a byte is made up of 8 bits. You can think of a bit as a single digit that is either 0 or 1. A byte effectively chooses between 0 and 1 eight times, so both 01101100
and 11110000
could represent bytes. Try this, which makes use of Python fstrings introduced in Python 3.6, in your interpreter:
>>> binary = [f'{i:0>8b}' for i in range(256)]
>>> binary[:16]
['00000000',
'00000001',
'00000010',
'00000011',
'00000100',
'00000101',
'00000110',
'00000111',
'00001000',
'00001001',
'00001010',
'00001011',
'00001100',
'00001101',
'00001110',
'00001111']
This is equivalent to [bin(i) for i in range(256)]
, with some special formatting. bin()
converts an integer to its binary representation as a string.
Where does that leave us? Using range(256)
above is not a random choice. (No pun intended.) Given that we are allowed 8 bits, each with 2 choices, there are 2 ** 8 == 256
possible bytes “combinations.”
This means that each byte maps to an integer between 0 and 255. In other words, we would need more than 8 bits to express the integer 256. You can verify this by checking that len(f'{256:0>8b}')
is now 9, not 8.
Okay, now let’s get back to the bytes
data type that you saw above, by constructing a sequence of the bytes that correspond to integers 0 through 255:
>>> bites = bytes(range(256))
If you call list(bites)
, you’ll get back to a Python list that runs from 0 to 255. But if you just print bites
, you get an ugly looking sequence littered with backslashes:
>>> bites
b'\x00\x01\x02\x03\x04\x05\x06\x07\x08\t\n\x0b\x0c\r\x0e\x0f\x10\x11\x12\x13\x14\x15'
'\x16\x17\x18\x19\x1a\x1b\x1c\x1d\x1e\x1f !"#$%&\'()*+,./0123456789:;<=>?@ABCDEFGHIJK'
'LMNOPQRSTUVWXYZ[\\]^_`abcdefghijklmnopqrstuvwxyz{}~\x7f\x80\x81\x82\x83\x84\x85\x86'
'\x87\x88\x89\x8a\x8b\x8c\x8d\x8e\x8f\x90\x91\x92\x93\x94\x95\x96\x97\x98\x99\x9a\x9b'
# ...
These backslashes are escape sequences, and \xhh
represents the character with hex value hh
. Some of the elements of bites
are displayed literally (printable characters such as letters, numbers, and punctuation). Most are expressed with escapes. \x08
represents a keyboard’s backspace, while \x13
is a carriage return (part of a new line, on Windows systems).
If you need a refresher on hexadecimal, Charles Petzold’s Code: The Hidden Language is a great place for that. Hex is a base16 numbering system that, instead of using 0 through 9, uses 0 through 9 and a through f as its basic digits.
Finally, let’s get back to where you started, with the sequence of random bytes x
. Hopefully this makes a little more sense now. Calling .hex()
on a bytes
object gives a str
of hexadecimal numbers, with each corresponding to a decimal number from 0 through 255:
>>> x
b'\xce\x11\xe7"!\x84'
>>> list(x)
[206, 17, 231, 34, 33, 132]
>>> x.hex()
'ce11e7222184'
>>> len(x.hex())
12
One last question: how is b.hex()
12 characters long above, even though x
is only 6 bytes? This is because two hexadecimal digits correspond precisely to a single byte. The str
version of bytes
will always be twice as long as far as our eyes are concerned.
Even if the byte (such as \x01
) does not need a full 8 bits to be represented, b.hex()
will always use two hex digits per byte, so the number 1 will be represented as 01
rather than just 1
. Mathematically, though, both of these are the same size.
Technical Detail: What you’ve mainly dissected here is how a bytes
object becomes a Python str
. One other technicality is how bytes
produced by os.urandom()
get converted to a float
in the interval [0.0, 1.0), as in the cryptographically secure version of random.random()
. If you’re interested in exploring this further, this code snippet demonstrates how int.from_bytes()
makes the initial conversion to an integer, using a base256 numbering system.
With that under your belt, let’s touch on a recently introduced module, secrets
, which makes generating secure tokens much more userfriendly.
Python’s Best Kept secrets
Introduced in Python 3.6 by one of the more colorful PEPs out there, the secrets
module is intended to be the de facto Python module for generating cryptographically secure random bytes and strings.
You can check out the source code for the module, which is short and sweet at about 25 lines of code. secrets
is basically a wrapper around os.urandom()
. It exports just a handful of functions for generating random numbers, bytes, and strings. Most of these examples should be fairly selfexplanatory:
>>> n = 16
>>> # Generate secure tokens
>>> secrets.token_bytes(n)
b'A\x8cz\xe1o\xf9!;\x8b\xf2\x80pJ\x8b\xd4\xd3'
>>> secrets.token_hex(n)
'9cb190491e01230ec4239cae643f286f'
>>> secrets.token_urlsafe(n)
'MJoi7CknFu3YN41m88SEgQ'
>>> # Secure version of `random.choice()`
>>> secrets.choice('rain')
'a'
Now, how about a concrete example? You’ve probably used URL shortener services like tinyurl.com or bit.ly that turn an unwieldy URL into something like https://bit.ly/2IcCp9u. Most shorteners don’t do any complicated hashing from input to output; they just generate a random string, make sure that string has not already been generated previously, and then tie that back to the input URL.
Let’s say that after taking a look at the Root Zone Database, you’ve registered the site short.ly. Here’s a function to get you started with your service:
# shortly.py
from secrets import token_urlsafe
DATABASE = {}
def shorten(url: str, nbytes: int=5) > str:
ext = token_urlsafe(nbytes=nbytes)
if ext in DATABASE:
return shorten(url, nbytes=nbytes)
else:
DATABASE.update({ext: url})
return f'short.ly/{ext}
Is this a fullfledged real illustration? No. I would wager that bit.ly does things in a slightly more advanced way than storing its gold mine in a global Python dictionary that is not persistent between sessions. However, it’s roughly accurate conceptually:
>>> urls = (
... 'https://realpython.com/',
... 'https://docs.python.org/3/howto/regex.html'
... )
>>> for u in urls:
... print(shorten(u))
short.ly/p_Z4fLI
short.ly/fuxSyNY
>>> DATABASE
{'p_Z4fLI': 'https://realpython.com/',
'fuxSyNY': 'https://docs.python.org/3/howto/regex.html'}
Hold On: One thing you may notice is that both of these results are of length 7 when you requested 5 bytes. Wait, I thought that you said the result would be twice as long? Well, not exactly, in this case. There is one more thing going on here: token_urlsafe()
uses base64 encoding, where each character is 6 bits of data. (It’s 0 through 63, and corresponding characters. The characters are AZ, az, 09, and +/.)
If you originally specify a certain number of bytes nbytes
, the resulting length from secrets.token_urlsafe(nbytes)
will be math.ceil(nbytes * 8 / 6)
, which you can prove and investigate further if you’re curious.
The bottom line here is that, while secrets
is really just a wrapper around existing Python functions, it can be your goto when security is your foremost concern.
One Last Candidate: uuid
One last option for generating a random token is the uuid4()
function from Python’s uuid
module. A UUID is a Universally Unique IDentifier, a 128bit sequence (str
of length 32) designed to “guarantee uniqueness across space and time.” uuid4()
is one of the module’s most useful functions, and this function also uses os.urandom()
:
>>> import uuid
>>> uuid.uuid4()
UUID('3e3ef28d3ff049339bbae5ee91ce0e7b')
>>> uuid.uuid4()
UUID('2e115fcb57614fa1828719f4ee2877ac')
The nice thing is that all of uuid
’s functions produce an instance of the UUID
class, which encapsulates the ID and has properties like .int
, .bytes
, and .hex
:
>>> tok = uuid.uuid4()
>>> tok.bytes
b'.\xb7\x80\xfd\xbfIG\xb3\xae\x1d\xe3\x97\xee\xc5\xd5\x81'
>>> len(tok.bytes)
16
>>> len(tok.bytes) * 8 # In bits
128
>>> tok.hex
'2eb780fdbf4947b3ae1de397eec5d581'
>>> tok.int
62097294383572614195530565389543396737
You may also have seen some other variations: uuid1()
, uuid3()
, and uuid5()
. The key difference between these and uuid4()
is that those three functions all take some form of input and therefore don’t meet the definition of “random” to the extent that a Version 4 UUID does:

uuid1()
uses your machine’s host ID and current time by default. Because of the reliance on current time down to nanosecond resolution, this version is where UUID derives the claim “guaranteed uniqueness across time.” 
uuid3()
anduuid5()
both take a namespace identifier and a name. The former uses an MD5 hash and the latter uses SHA1.
uuid4()
, conversely, is entirely pseudorandom (or random). It consists of getting 16 bytes via os.urandom()
, converting this to a bigendian integer, and doing a number of bitwise operations to comply with the formal specification.
Hopefully, by now you have a good idea of the distinction between different “types” of random data and how to create them. However, one other issue that might come to mind is that of collisions.
In this case, a collision would simply refer to generating two matching UUIDs. What is the chance of that? Well, it is technically not zero, but perhaps it is close enough: there are 2 ** 128
or 340 undecillion possible uuid4
values. So, I’ll leave it up to you to judge whether this is enough of a guarantee to sleep well.
One common use of uuid
is in Django, which has a UUIDField
that is often used as a primary key in a model’s underlying relational database.
Why Not Just “Default to” SystemRandom
?
In addition to the secure modules discussed here such as secrets
, Python’s random
module actually has a littleused class called SystemRandom
that uses os.urandom()
. (SystemRandom
, in turn, is also used by secrets
. It’s all a bit of a web that traces back to urandom()
.)
At this point, you might be asking yourself why you wouldn’t just “default to” this version? Why not “always be safe” rather than defaulting to the deterministic random
functions that aren’t cryptographically secure ?
I’ve already mentioned one reason: sometimes you want your data to be deterministic and reproducible for others to follow along with.
But the second reason is that CSPRNGs, at least in Python, tend to be meaningfully slower than PRNGs. Let’s test that with a script, timed.py
, that compares the PRNG and CSPRNG versions of randint()
using Python’s timeit.repeat()
:
# timed.py
import random
import timeit
# The "default" random is actually an instance of `random.Random()`.
# The CSPRNG version uses `SystemRandom()` and `os.urandom()` in turn.
_sysrand = random.SystemRandom()
def prng() > None:
random.randint(0, 95)
def csprng() > None:
_sysrand.randint(0, 95)
setup = 'import random; from __main__ import prng, csprng'
if __name__ == '__main__':
print('Best of 3 trials with 1,000,000 loops per trial:')
for f in ('prng()', 'csprng()'):
best = min(timeit.repeat(f, setup=setup))
print('\t{:8s} {:0.2f} seconds total time.'.format(f, best))
Now to execute this from the shell:
$ python3 ./timed.py
Best of 3 trials with 1,000,000 loops per trial:
prng() 1.07 seconds total time.
csprng() 6.20 seconds total time.
A 5x timing difference is certainly a valid consideration in addition to cryptographic security when choosing between the two.
Odds and Ends: Hashing
One concept that hasn’t received much attention in this tutorial is that of hashing, which can be done with Python’s hashlib
module.
A hash is designed to be a oneway mapping from an input value to a fixedsize string that is virtually impossible to reverse engineer. As such, while the result of a hash function may “look like” random data, it doesn’t really qualify under the definition here.
Recap
You’ve covered a lot of ground in this tutorial. To recap, here is a highlevel comparison of the options available to you for engineering randomness in Python:
Package/Module  Description  Cryptographically Secure 

random 
Fasty & easy random data using Mersenne Twister  No 
numpy.random 
Like random but for (possibly multidimensional) arrays 
No 
os 
Contains urandom() , the base of other functions covered here 
Yes 
secrets 
Designed to be Python’s de facto module for generating secure random numbers, bytes, and strings  Yes 
uuid 
Home to a handful of functions for building 128bit identifiers  Yes, uuid4() 
Feel free to leave some totally random comments below, and thanks for reading.
Additional Links
 Random.org offers “true random numbers to anyone on the Internet” derived from atmospheric noise.
 The Recipes section from the
random
module has some additional tricks.  The seminal paper on the Mersienne Twister appeared in 1997, if you’re into that kind of thing.
 The Itertools Recipes define functions for choosing randomly from a combinatoric set, such as from combinations or permutations.
 ScikitLearn includes various random sample generators that can be used to build artificial datasets of controlled size and complexity.
 Eli Bendersky digs into
random.randint()
in his article Slow and Fast Methods for Generating Random Integers in Python.  Peter Norvig’s a Concrete Introduction to Probability using Python is a comprehensive resource as well.
 The Pandas library includes a context manager that can be used to set a temporary random state.
 From Stack Overflow: