Source code: Lib/random.py
This module implements pseudo-random number generators for various distributions.
For integers, there is uniform selection from a range. For sequences, there is uniform selection of a random element, a function to generate a random permutation of a list in-place, and a function for random sampling without replacement.
On the real line, there are functions to compute uniform, normal (Gaussian), lognormal, negative exponential, gamma, and beta distributions. For generating distributions of angles, the von Mises distribution is available.
Almost all module functions depend on the basic function
generates a random float uniformly in the semi-open range [0.0, 1.0). Python
uses the Mersenne Twister as the core generator. It produces 53-bit precision
floats and has a period of 2**19937-1. The underlying implementation in C is
both fast and threadsafe. The Mersenne Twister is one of the most extensively
tested random number generators in existence. However, being completely
deterministic, it is not suitable for all purposes, and is completely unsuitable
for cryptographic purposes.
The functions supplied by this module are actually bound methods of a hidden
instance of the
random.Random class. You can instantiate your own
Random to get generators that don’t share state.
Random can also be subclassed if you want to use a different
basic generator of your own devising: in that case, override the
Optionally, a new generator can supply a
getrandbits() method — this
randrange() to produce selections over an arbitrarily large range.
The pseudo-random generators of this module should not be used for
security purposes. For security or cryptographic uses, see the
Initialize the random number generator.
If a is omitted or
None, the current system time is used. If randomness sources are provided by the operating system, they are used instead of the system time (see the
os.urandom()function for details on availability).
If a is an int, it is used directly.
Changed in version 3.2: Moved to the version 2 scheme which uses all of the bits in a string seed.
Return an object capturing the current internal state of the generator. This object can be passed to
setstate()to restore the state.
Returns a Python integer with k random bits. This method is supplied with the MersenneTwister generator and some other generators may also provide it as an optional part of the API. When available,
randrange()to handle arbitrarily large ranges.
Functions for integers:
randrange(start, stop[, step])
Return a randomly selected element from
range(start, stop, step). This is equivalent to
choice(range(start, stop, step)), but doesn’t actually build a range object.
The positional argument pattern matches that of
range(). Keyword arguments should not be used because the function may use them in unexpected ways.
Changed in version 3.2:
randrange()is more sophisticated about producing equally distributed values. Formerly it used a style like
int(random()*n)which could produce slightly uneven distributions.
Return a random integer N such that
a <= N <= b. Alias for
Functions for sequences:
Return a random element from the non-empty sequence seq. If seq is empty, raises
choices(population, weights=None, *, cum_weights=None, k=1)¶
Return a k sized list of elements chosen from the population with replacement. If the population is empty, raises
If a weights sequence is specified, selections are made according to the relative weights. Alternatively, if a cum_weights sequence is given, the selections are made according to the cumulative weights. For example, the relative weights
[10, 5, 30, 5]are equivalent to the cumulative weights
[10, 15, 45, 50]. Internally, the relative weights are converted to cumulative weights before making selections, so supplying the cumulative weights saves work.
If neither weights nor cum_weights are specified, selections are made with equal probability. If a weights sequence is supplied, it must be the same length as the population sequence. It is a
TypeErrorto specify both weights and cum_weights.
Shuffle the sequence x in place. The optional argument random is a 0-argument function returning a random float in [0.0, 1.0); by default, this is the function
Note that for even rather small
len(x), the total number of permutations of x is larger than the period of most random number generators; this implies that most permutations of a long sequence can never be generated.
Return a k length list of unique elements chosen from the population sequence or set. Used for random sampling without replacement.
Returns a new list containing elements from the population while leaving the original population unchanged. The resulting list is in selection order so that all sub-slices will also be valid random samples. This allows raffle winners (the sample) to be partitioned into grand prize and second place winners (the subslices).
Members of the population need not be hashable or unique. If the population contains repeats, then each occurrence is a possible selection in the sample.
To choose a sample from a range of integers, use an
range()object as an argument. This is especially fast and space efficient for sampling from a large population:
If the sample size is larger than the population size, a
The following functions generate specific real-valued distributions. Function parameters are named after the corresponding variables in the distribution’s equation, as used in common mathematical practice; most of these equations can be found in any statistics text.
Return the next random floating point number in the range [0.0, 1.0).
Return a random floating point number N such that
a <= N <= bfor
a <= band
b <= N <= afor
b < a.
The end-point value
bmay or may not be included in the range depending on floating-point rounding in the equation
a + (b-a) * random().
triangular(low, high, mode)¶
Return a random floating point number N such that
low <= N <= highand with the specified mode between those bounds. The low and high bounds default to zero and one. The mode argument defaults to the midpoint between the bounds, giving a symmetric distribution.
Beta distribution. Conditions on the parameters are
alpha > 0and
beta > 0. Returned values range between 0 and 1.
Exponential distribution. lambd is 1.0 divided by the desired mean. It should be nonzero. (The parameter would be called “lambda”, but that is a reserved word in Python.) Returned values range from 0 to positive infinity if lambd is positive, and from negative infinity to 0 if lambd is negative.
Gamma distribution. (Not the gamma function!) Conditions on the parameters are
alpha > 0and
beta > 0.
The probability distribution function is:
x ** (alpha - 1) * math.exp(-x / beta) pdf(x) = -------------------------------------- math.gamma(alpha) * beta ** alpha
Gaussian distribution. mu is the mean, and sigma is the standard deviation. This is slightly faster than the
normalvariate()function defined below.
Log normal distribution. If you take the natural logarithm of this distribution, you’ll get a normal distribution with mean mu and standard deviation sigma. mu can have any value, and sigma must be greater than zero.
Normal distribution. mu is the mean, and sigma is the standard deviation.
mu is the mean angle, expressed in radians between 0 and 2*pi, and kappa is the concentration parameter, which must be greater than or equal to zero. If kappa is equal to zero, this distribution reduces to a uniform random angle over the range 0 to 2*pi.
Pareto distribution. alpha is the shape parameter.
Weibull distribution. alpha is the scale parameter and beta is the shape parameter.
Class that uses the
os.urandom()function for generating random numbers from sources provided by the operating system. Not available on all systems. Does not rely on software state, and sequences are not reproducible. Accordingly, the
seed()method has no effect and is ignored. The
M. Matsumoto and T. Nishimura, “Mersenne Twister: A 623-dimensionally equidistributed uniform pseudorandom number generator”, ACM Transactions on Modeling and Computer Simulation Vol. 8, No. 1, January pp.3-30 1998.
Complementary-Multiply-with-Carry recipe for a compatible alternative random number generator with a long period and comparatively simple update operations.
9.6.1. Notes on Reproducibility¶
Sometimes it is useful to be able to reproduce the sequences given by a pseudo random number generator. By re-using a seed value, the same sequence should be reproducible from run to run as long as multiple threads are not running.
Most of the random module’s algorithms and seeding functions are subject to change across Python versions, but two aspects are guaranteed not to change:
- If a new seeding method is added, then a backward compatible seeder will be offered.
- The generator’s
random()method will continue to produce the same sequence when the compatible seeder is given the same seed.
9.6.2. Examples and Recipes¶
>>> random.random() # Random float x, 0.0 <= x < 1.0 0.37444887175646646 >>> random.uniform(1, 10) # Random float x, 1.0 <= x < 10.0 1.1800146073117523 >>> random.randrange(10) # Integer from 0 to 9 7 >>> random.randrange(0, 101, 2) # Even integer from 0 to 100 26 >>> random.choice('abcdefghij') # Single random element 'c' >>> items = [1, 2, 3, 4, 5, 6, 7] >>> random.shuffle(items) >>> items [7, 3, 2, 5, 6, 4, 1] >>> random.sample([1, 2, 3, 4, 5], 3) # Three samples without replacement [4, 1, 5]
A common task is to make a
random.choice() with weighted probabilities.
If the weights are small integer ratios, a simple technique is to build a sample population with repeats:
>>> weighted_choices = [('Red', 3), ('Blue', 2), ('Yellow', 1), ('Green', 4)] >>> population = [val for val, cnt in weighted_choices for i in range(cnt)] >>> population ['Red', 'Red', 'Red', 'Blue', 'Blue', 'Yellow', 'Green', 'Green', 'Green', 'Green'] >>> random.choice(population) 'Green'
>>> choices, weights = zip(*weighted_choices) >>> cumdist = list(itertools.accumulate(weights)) >>> cumdist # [3, 3+2, 3+2+1, 3+2+1+4] [3, 5, 6, 10] >>> x = random.random() * cumdist[-1] >>> choices[bisect.bisect(cumdist, x)] 'Blue'