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708 lines
26 KiB
ReStructuredText
:mod:`random` --- Generate pseudo-random numbers
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================================================
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.. module:: random
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:synopsis: Generate pseudo-random numbers with various common distributions.
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**Source code:** :source:`Lib/random.py`
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--------------
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This module implements pseudo-random number generators for various
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distributions.
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For integers, there is uniform selection from a range. For sequences, there is
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uniform selection of a random element, a function to generate a random
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permutation of a list in-place, and a function for random sampling without
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replacement.
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On the real line, there are functions to compute uniform, normal (Gaussian),
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lognormal, negative exponential, gamma, and beta distributions. For generating
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distributions of angles, the von Mises distribution is available.
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Almost all module functions depend on the basic function :func:`.random`, which
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generates a random float uniformly in the half-open range ``0.0 <= X < 1.0``.
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Python uses the Mersenne Twister as the core generator. It produces 53-bit precision
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floats and has a period of 2\*\*19937-1. The underlying implementation in C is
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both fast and threadsafe. The Mersenne Twister is one of the most extensively
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tested random number generators in existence. However, being completely
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deterministic, it is not suitable for all purposes, and is completely unsuitable
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for cryptographic purposes.
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The functions supplied by this module are actually bound methods of a hidden
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instance of the :class:`random.Random` class. You can instantiate your own
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instances of :class:`Random` to get generators that don't share state.
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Class :class:`Random` can also be subclassed if you want to use a different
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basic generator of your own devising: see the documentation on that class for
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more details.
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The :mod:`random` module also provides the :class:`SystemRandom` class which
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uses the system function :func:`os.urandom` to generate random numbers
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from sources provided by the operating system.
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.. warning::
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The pseudo-random generators of this module should not be used for
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security purposes. For security or cryptographic uses, see the
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:mod:`secrets` module.
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.. seealso::
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M. Matsumoto and T. Nishimura, "Mersenne Twister: A 623-dimensionally
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equidistributed uniform pseudorandom number generator", ACM Transactions on
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Modeling and Computer Simulation Vol. 8, No. 1, January pp.3--30 1998.
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`Complementary-Multiply-with-Carry recipe
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<https://code.activestate.com/recipes/576707/>`_ for a compatible alternative
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random number generator with a long period and comparatively simple update
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operations.
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.. note::
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The global random number generator and instances of :class:`Random` are thread-safe.
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However, in the free-threaded build, concurrent calls to the global generator or
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to the same instance of :class:`Random` may encounter contention and poor performance.
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Consider using separate instances of :class:`Random` per thread instead.
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Bookkeeping functions
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---------------------
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.. function:: seed(a=None, version=2)
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Initialize the random number generator.
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If *a* is omitted or ``None``, the current system time is used. If
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randomness sources are provided by the operating system, they are used
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instead of the system time (see the :func:`os.urandom` function for details
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on availability).
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If *a* is an int, it is used directly.
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With version 2 (the default), a :class:`str`, :class:`bytes`, or :class:`bytearray`
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object gets converted to an :class:`int` and all of its bits are used.
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With version 1 (provided for reproducing random sequences from older versions
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of Python), the algorithm for :class:`str` and :class:`bytes` generates a
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narrower range of seeds.
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.. versionchanged:: 3.2
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Moved to the version 2 scheme which uses all of the bits in a string seed.
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.. versionchanged:: 3.11
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The *seed* must be one of the following types:
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``None``, :class:`int`, :class:`float`, :class:`str`,
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:class:`bytes`, or :class:`bytearray`.
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.. function:: getstate()
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Return an object capturing the current internal state of the generator. This
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object can be passed to :func:`setstate` to restore the state.
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.. function:: setstate(state)
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*state* should have been obtained from a previous call to :func:`getstate`, and
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:func:`setstate` restores the internal state of the generator to what it was at
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the time :func:`getstate` was called.
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Functions for bytes
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-------------------
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.. function:: randbytes(n)
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Generate *n* random bytes.
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This method should not be used for generating security tokens.
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Use :func:`secrets.token_bytes` instead.
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.. versionadded:: 3.9
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Functions for integers
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----------------------
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.. function:: randrange(stop)
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randrange(start, stop[, step])
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Return a randomly selected element from ``range(start, stop, step)``.
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This is roughly equivalent to ``choice(range(start, stop, step))`` but
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supports arbitrarily large ranges and is optimized for common cases.
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The positional argument pattern matches the :func:`range` function.
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Keyword arguments should not be used because they can be interpreted
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in unexpected ways. For example ``randrange(start=100)`` is interpreted
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as ``randrange(0, 100, 1)``.
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.. versionchanged:: 3.2
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:meth:`randrange` is more sophisticated about producing equally distributed
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values. Formerly it used a style like ``int(random()*n)`` which could produce
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slightly uneven distributions.
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.. versionchanged:: 3.12
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Automatic conversion of non-integer types is no longer supported.
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Calls such as ``randrange(10.0)`` and ``randrange(Fraction(10, 1))``
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now raise a :exc:`TypeError`.
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.. function:: randint(a, b)
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Return a random integer *N* such that ``a <= N <= b``. Alias for
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``randrange(a, b+1)``.
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.. function:: getrandbits(k)
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Returns a non-negative Python integer with *k* random bits. This method
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is supplied with the Mersenne Twister generator and some other generators
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may also provide it as an optional part of the API. When available,
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:meth:`getrandbits` enables :meth:`randrange` to handle arbitrarily large
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ranges.
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.. versionchanged:: 3.9
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This method now accepts zero for *k*.
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Functions for sequences
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-----------------------
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.. function:: choice(seq)
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Return a random element from the non-empty sequence *seq*. If *seq* is empty,
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raises :exc:`IndexError`.
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.. function:: choices(population, weights=None, *, cum_weights=None, k=1)
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Return a *k* sized list of elements chosen from the *population* with replacement.
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If the *population* is empty, raises :exc:`IndexError`.
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If a *weights* sequence is specified, selections are made according to the
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relative weights. Alternatively, if a *cum_weights* sequence is given, the
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selections are made according to the cumulative weights (perhaps computed
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using :func:`itertools.accumulate`). For example, the relative weights
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``[10, 5, 30, 5]`` are equivalent to the cumulative weights
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``[10, 15, 45, 50]``. Internally, the relative weights are converted to
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cumulative weights before making selections, so supplying the cumulative
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weights saves work.
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If neither *weights* nor *cum_weights* are specified, selections are made
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with equal probability. If a weights sequence is supplied, it must be
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the same length as the *population* sequence. It is a :exc:`TypeError`
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to specify both *weights* and *cum_weights*.
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The *weights* or *cum_weights* can use any numeric type that interoperates
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with the :class:`float` values returned by :func:`random` (that includes
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integers, floats, and fractions but excludes decimals). Weights are assumed
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to be non-negative and finite. A :exc:`ValueError` is raised if all
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weights are zero.
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For a given seed, the :func:`choices` function with equal weighting
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typically produces a different sequence than repeated calls to
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:func:`choice`. The algorithm used by :func:`choices` uses floating
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point arithmetic for internal consistency and speed. The algorithm used
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by :func:`choice` defaults to integer arithmetic with repeated selections
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to avoid small biases from round-off error.
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.. versionadded:: 3.6
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.. versionchanged:: 3.9
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Raises a :exc:`ValueError` if all weights are zero.
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.. function:: shuffle(x)
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Shuffle the sequence *x* in place.
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To shuffle an immutable sequence and return a new shuffled list, use
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``sample(x, k=len(x))`` instead.
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Note that even for small ``len(x)``, the total number of permutations of *x*
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can quickly grow larger than the period of most random number generators.
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This implies that most permutations of a long sequence can never be
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generated. For example, a sequence of length 2080 is the largest that
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can fit within the period of the Mersenne Twister random number generator.
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.. versionchanged:: 3.11
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Removed the optional parameter *random*.
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.. function:: sample(population, k, *, counts=None)
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Return a *k* length list of unique elements chosen from the population
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sequence. Used for random sampling without replacement.
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Returns a new list containing elements from the population while leaving the
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original population unchanged. The resulting list is in selection order so that
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all sub-slices will also be valid random samples. This allows raffle winners
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(the sample) to be partitioned into grand prize and second place winners (the
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subslices).
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Members of the population need not be :term:`hashable` or unique. If the population
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contains repeats, then each occurrence is a possible selection in the sample.
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Repeated elements can be specified one at a time or with the optional
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keyword-only *counts* parameter. For example, ``sample(['red', 'blue'],
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counts=[4, 2], k=5)`` is equivalent to ``sample(['red', 'red', 'red', 'red',
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'blue', 'blue'], k=5)``.
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To choose a sample from a range of integers, use a :func:`range` object as an
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argument. This is especially fast and space efficient for sampling from a large
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population: ``sample(range(10000000), k=60)``.
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If the sample size is larger than the population size, a :exc:`ValueError`
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is raised.
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.. versionchanged:: 3.9
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Added the *counts* parameter.
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.. versionchanged:: 3.11
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The *population* must be a sequence. Automatic conversion of sets
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to lists is no longer supported.
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Discrete distributions
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----------------------
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The following function generates a discrete distribution.
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.. function:: binomialvariate(n=1, p=0.5)
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`Binomial distribution
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<https://mathworld.wolfram.com/BinomialDistribution.html>`_.
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Return the number of successes for *n* independent trials with the
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probability of success in each trial being *p*:
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Mathematically equivalent to::
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sum(random() < p for i in range(n))
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The number of trials *n* should be a non-negative integer.
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The probability of success *p* should be between ``0.0 <= p <= 1.0``.
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The result is an integer in the range ``0 <= X <= n``.
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.. versionadded:: 3.12
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.. _real-valued-distributions:
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Real-valued distributions
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-------------------------
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The following functions generate specific real-valued distributions. Function
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parameters are named after the corresponding variables in the distribution's
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equation, as used in common mathematical practice; most of these equations can
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be found in any statistics text.
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.. function:: random()
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Return the next random floating point number in the range ``0.0 <= X < 1.0``
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.. function:: uniform(a, b)
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Return a random floating point number *N* such that ``a <= N <= b`` for
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``a <= b`` and ``b <= N <= a`` for ``b < a``.
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The end-point value ``b`` may or may not be included in the range
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depending on floating-point rounding in the expression
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``a + (b-a) * random()``.
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.. function:: triangular(low, high, mode)
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Return a random floating point number *N* such that ``low <= N <= high`` and
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with the specified *mode* between those bounds. The *low* and *high* bounds
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default to zero and one. The *mode* argument defaults to the midpoint
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between the bounds, giving a symmetric distribution.
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.. function:: betavariate(alpha, beta)
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Beta distribution. Conditions on the parameters are ``alpha > 0`` and
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``beta > 0``. Returned values range between 0 and 1.
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.. function:: expovariate(lambd = 1.0)
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Exponential distribution. *lambd* is 1.0 divided by the desired
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mean. It should be nonzero. (The parameter would be called
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"lambda", but that is a reserved word in Python.) Returned values
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range from 0 to positive infinity if *lambd* is positive, and from
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negative infinity to 0 if *lambd* is negative.
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.. versionchanged:: 3.12
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Added the default value for ``lambd``.
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.. function:: gammavariate(alpha, beta)
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Gamma distribution. (*Not* the gamma function!) The shape and
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scale parameters, *alpha* and *beta*, must have positive values.
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(Calling conventions vary and some sources define 'beta'
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as the inverse of the scale).
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The probability distribution function is::
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x ** (alpha - 1) * math.exp(-x / beta)
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pdf(x) = --------------------------------------
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math.gamma(alpha) * beta ** alpha
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.. function:: gauss(mu=0.0, sigma=1.0)
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Normal distribution, also called the Gaussian distribution.
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*mu* is the mean,
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and *sigma* is the standard deviation. This is slightly faster than
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the :func:`normalvariate` function defined below.
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Multithreading note: When two threads call this function
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simultaneously, it is possible that they will receive the
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same return value. This can be avoided in three ways.
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1) Have each thread use a different instance of the random
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number generator. 2) Put locks around all calls. 3) Use the
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slower, but thread-safe :func:`normalvariate` function instead.
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.. versionchanged:: 3.11
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*mu* and *sigma* now have default arguments.
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.. function:: lognormvariate(mu, sigma)
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Log normal distribution. If you take the natural logarithm of this
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distribution, you'll get a normal distribution with mean *mu* and standard
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deviation *sigma*. *mu* can have any value, and *sigma* must be greater than
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zero.
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.. function:: normalvariate(mu=0.0, sigma=1.0)
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Normal distribution. *mu* is the mean, and *sigma* is the standard deviation.
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.. versionchanged:: 3.11
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*mu* and *sigma* now have default arguments.
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.. function:: vonmisesvariate(mu, kappa)
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*mu* is the mean angle, expressed in radians between 0 and 2\*\ *pi*, and *kappa*
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is the concentration parameter, which must be greater than or equal to zero. If
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*kappa* is equal to zero, this distribution reduces to a uniform random angle
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over the range 0 to 2\*\ *pi*.
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.. function:: paretovariate(alpha)
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Pareto distribution. *alpha* is the shape parameter.
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.. function:: weibullvariate(alpha, beta)
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Weibull distribution. *alpha* is the scale parameter and *beta* is the shape
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parameter.
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Alternative Generator
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---------------------
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.. class:: Random([seed])
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Class that implements the default pseudo-random number generator used by the
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:mod:`random` module.
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.. versionchanged:: 3.11
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Formerly the *seed* could be any hashable object. Now it is limited to:
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``None``, :class:`int`, :class:`float`, :class:`str`,
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:class:`bytes`, or :class:`bytearray`.
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Subclasses of :class:`!Random` should override the following methods if they
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wish to make use of a different basic generator:
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.. method:: Random.seed(a=None, version=2)
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Override this method in subclasses to customise the :meth:`~random.seed`
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behaviour of :class:`!Random` instances.
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.. method:: Random.getstate()
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Override this method in subclasses to customise the :meth:`~random.getstate`
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behaviour of :class:`!Random` instances.
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.. method:: Random.setstate(state)
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Override this method in subclasses to customise the :meth:`~random.setstate`
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behaviour of :class:`!Random` instances.
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.. method:: Random.random()
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Override this method in subclasses to customise the :meth:`~random.random`
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behaviour of :class:`!Random` instances.
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Optionally, a custom generator subclass can also supply the following method:
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.. method:: Random.getrandbits(k)
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Override this method in subclasses to customise the
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:meth:`~random.getrandbits` behaviour of :class:`!Random` instances.
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.. class:: SystemRandom([seed])
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Class that uses the :func:`os.urandom` function for generating random numbers
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from sources provided by the operating system. Not available on all systems.
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Does not rely on software state, and sequences are not reproducible. Accordingly,
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the :meth:`seed` method has no effect and is ignored.
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The :meth:`getstate` and :meth:`setstate` methods raise
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:exc:`NotImplementedError` if called.
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Notes on Reproducibility
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------------------------
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Sometimes it is useful to be able to reproduce the sequences given by a
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pseudo-random number generator. By reusing a seed value, the same sequence should be
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reproducible from run to run as long as multiple threads are not running.
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Most of the random module's algorithms and seeding functions are subject to
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change across Python versions, but two aspects are guaranteed not to change:
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* If a new seeding method is added, then a backward compatible seeder will be
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offered.
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* The generator's :meth:`~Random.random` method will continue to produce the same
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sequence when the compatible seeder is given the same seed.
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.. _random-examples:
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Examples
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--------
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Basic examples::
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>>> random() # Random float: 0.0 <= x < 1.0
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0.37444887175646646
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>>> uniform(2.5, 10.0) # Random float: 2.5 <= x <= 10.0
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3.1800146073117523
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>>> expovariate(1 / 5) # Interval between arrivals averaging 5 seconds
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5.148957571865031
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>>> randrange(10) # Integer from 0 to 9 inclusive
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7
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>>> randrange(0, 101, 2) # Even integer from 0 to 100 inclusive
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26
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>>> choice(['win', 'lose', 'draw']) # Single random element from a sequence
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'draw'
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>>> deck = 'ace two three four'.split()
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>>> shuffle(deck) # Shuffle a list
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>>> deck
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['four', 'two', 'ace', 'three']
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>>> sample([10, 20, 30, 40, 50], k=4) # Four samples without replacement
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[40, 10, 50, 30]
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Simulations::
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>>> # Six roulette wheel spins (weighted sampling with replacement)
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>>> choices(['red', 'black', 'green'], [18, 18, 2], k=6)
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['red', 'green', 'black', 'black', 'red', 'black']
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>>> # Deal 20 cards without replacement from a deck
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>>> # of 52 playing cards, and determine the proportion of cards
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>>> # with a ten-value: ten, jack, queen, or king.
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>>> deal = sample(['tens', 'low cards'], counts=[16, 36], k=20)
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>>> deal.count('tens') / 20
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0.15
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>>> # Estimate the probability of getting 5 or more heads from 7 spins
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>>> # of a biased coin that settles on heads 60% of the time.
|
|
>>> sum(binomialvariate(n=7, p=0.6) >= 5 for i in range(10_000)) / 10_000
|
|
0.4169
|
|
|
|
>>> # Probability of the median of 5 samples being in middle two quartiles
|
|
>>> def trial():
|
|
... return 2_500 <= sorted(choices(range(10_000), k=5))[2] < 7_500
|
|
...
|
|
>>> sum(trial() for i in range(10_000)) / 10_000
|
|
0.7958
|
|
|
|
Example of `statistical bootstrapping
|
|
<https://en.wikipedia.org/wiki/Bootstrapping_(statistics)>`_ using resampling
|
|
with replacement to estimate a confidence interval for the mean of a sample::
|
|
|
|
# https://www.thoughtco.com/example-of-bootstrapping-3126155
|
|
from statistics import fmean as mean
|
|
from random import choices
|
|
|
|
data = [41, 50, 29, 37, 81, 30, 73, 63, 20, 35, 68, 22, 60, 31, 95]
|
|
means = sorted(mean(choices(data, k=len(data))) for i in range(100))
|
|
print(f'The sample mean of {mean(data):.1f} has a 90% confidence '
|
|
f'interval from {means[5]:.1f} to {means[94]:.1f}')
|
|
|
|
Example of a `resampling permutation test
|
|
<https://en.wikipedia.org/wiki/Resampling_(statistics)#Permutation_tests>`_
|
|
to determine the statistical significance or `p-value
|
|
<https://en.wikipedia.org/wiki/P-value>`_ of an observed difference
|
|
between the effects of a drug versus a placebo::
|
|
|
|
# Example from "Statistics is Easy" by Dennis Shasha and Manda Wilson
|
|
from statistics import fmean as mean
|
|
from random import shuffle
|
|
|
|
drug = [54, 73, 53, 70, 73, 68, 52, 65, 65]
|
|
placebo = [54, 51, 58, 44, 55, 52, 42, 47, 58, 46]
|
|
observed_diff = mean(drug) - mean(placebo)
|
|
|
|
n = 10_000
|
|
count = 0
|
|
combined = drug + placebo
|
|
for i in range(n):
|
|
shuffle(combined)
|
|
new_diff = mean(combined[:len(drug)]) - mean(combined[len(drug):])
|
|
count += (new_diff >= observed_diff)
|
|
|
|
print(f'{n} label reshufflings produced only {count} instances with a difference')
|
|
print(f'at least as extreme as the observed difference of {observed_diff:.1f}.')
|
|
print(f'The one-sided p-value of {count / n:.4f} leads us to reject the null')
|
|
print(f'hypothesis that there is no difference between the drug and the placebo.')
|
|
|
|
Simulation of arrival times and service deliveries for a multiserver queue::
|
|
|
|
from heapq import heapify, heapreplace
|
|
from random import expovariate, gauss
|
|
from statistics import mean, quantiles
|
|
|
|
average_arrival_interval = 5.6
|
|
average_service_time = 15.0
|
|
stdev_service_time = 3.5
|
|
num_servers = 3
|
|
|
|
waits = []
|
|
arrival_time = 0.0
|
|
servers = [0.0] * num_servers # time when each server becomes available
|
|
heapify(servers)
|
|
for i in range(1_000_000):
|
|
arrival_time += expovariate(1.0 / average_arrival_interval)
|
|
next_server_available = servers[0]
|
|
wait = max(0.0, next_server_available - arrival_time)
|
|
waits.append(wait)
|
|
service_duration = max(0.0, gauss(average_service_time, stdev_service_time))
|
|
service_completed = arrival_time + wait + service_duration
|
|
heapreplace(servers, service_completed)
|
|
|
|
print(f'Mean wait: {mean(waits):.1f} Max wait: {max(waits):.1f}')
|
|
print('Quartiles:', [round(q, 1) for q in quantiles(waits)])
|
|
|
|
.. seealso::
|
|
|
|
`Statistics for Hackers <https://www.youtube.com/watch?v=Iq9DzN6mvYA>`_
|
|
a video tutorial by
|
|
`Jake Vanderplas <https://us.pycon.org/2016/speaker/profile/295/>`_
|
|
on statistical analysis using just a few fundamental concepts
|
|
including simulation, sampling, shuffling, and cross-validation.
|
|
|
|
`Economics Simulation
|
|
<https://nbviewer.org/url/norvig.com/ipython/Economics.ipynb>`_
|
|
a simulation of a marketplace by
|
|
`Peter Norvig <https://norvig.com/bio.html>`_ that shows effective
|
|
use of many of the tools and distributions provided by this module
|
|
(gauss, uniform, sample, betavariate, choice, triangular, and randrange).
|
|
|
|
`A Concrete Introduction to Probability (using Python)
|
|
<https://nbviewer.org/url/norvig.com/ipython/Probability.ipynb>`_
|
|
a tutorial by `Peter Norvig <https://norvig.com/bio.html>`_ covering
|
|
the basics of probability theory, how to write simulations, and
|
|
how to perform data analysis using Python.
|
|
|
|
|
|
Recipes
|
|
-------
|
|
|
|
These recipes show how to efficiently make random selections
|
|
from the combinatoric iterators in the :mod:`itertools` module:
|
|
|
|
.. testcode::
|
|
import random
|
|
|
|
def random_product(*args, repeat=1):
|
|
"Random selection from itertools.product(*args, **kwds)"
|
|
pools = [tuple(pool) for pool in args] * repeat
|
|
return tuple(map(random.choice, pools))
|
|
|
|
def random_permutation(iterable, r=None):
|
|
"Random selection from itertools.permutations(iterable, r)"
|
|
pool = tuple(iterable)
|
|
r = len(pool) if r is None else r
|
|
return tuple(random.sample(pool, r))
|
|
|
|
def random_combination(iterable, r):
|
|
"Random selection from itertools.combinations(iterable, r)"
|
|
pool = tuple(iterable)
|
|
n = len(pool)
|
|
indices = sorted(random.sample(range(n), r))
|
|
return tuple(pool[i] for i in indices)
|
|
|
|
def random_combination_with_replacement(iterable, r):
|
|
"Choose r elements with replacement. Order the result to match the iterable."
|
|
# Result will be in set(itertools.combinations_with_replacement(iterable, r)).
|
|
pool = tuple(iterable)
|
|
n = len(pool)
|
|
indices = sorted(random.choices(range(n), k=r))
|
|
return tuple(pool[i] for i in indices)
|
|
|
|
The default :func:`.random` returns multiples of 2⁻⁵³ in the range
|
|
*0.0 ≤ x < 1.0*. All such numbers are evenly spaced and are exactly
|
|
representable as Python floats. However, many other representable
|
|
floats in that interval are not possible selections. For example,
|
|
``0.05954861408025609`` isn't an integer multiple of 2⁻⁵³.
|
|
|
|
The following recipe takes a different approach. All floats in the
|
|
interval are possible selections. The mantissa comes from a uniform
|
|
distribution of integers in the range *2⁵² ≤ mantissa < 2⁵³*. The
|
|
exponent comes from a geometric distribution where exponents smaller
|
|
than *-53* occur half as often as the next larger exponent.
|
|
|
|
::
|
|
|
|
from random import Random
|
|
from math import ldexp
|
|
|
|
class FullRandom(Random):
|
|
|
|
def random(self):
|
|
mantissa = 0x10_0000_0000_0000 | self.getrandbits(52)
|
|
exponent = -53
|
|
x = 0
|
|
while not x:
|
|
x = self.getrandbits(32)
|
|
exponent += x.bit_length() - 32
|
|
return ldexp(mantissa, exponent)
|
|
|
|
All :ref:`real valued distributions <real-valued-distributions>`
|
|
in the class will use the new method::
|
|
|
|
>>> fr = FullRandom()
|
|
>>> fr.random()
|
|
0.05954861408025609
|
|
>>> fr.expovariate(0.25)
|
|
8.87925541791544
|
|
|
|
The recipe is conceptually equivalent to an algorithm that chooses from
|
|
all the multiples of 2⁻¹⁰⁷⁴ in the range *0.0 ≤ x < 1.0*. All such
|
|
numbers are evenly spaced, but most have to be rounded down to the
|
|
nearest representable Python float. (The value 2⁻¹⁰⁷⁴ is the smallest
|
|
positive unnormalized float and is equal to ``math.ulp(0.0)``.)
|
|
|
|
|
|
.. seealso::
|
|
|
|
`Generating Pseudo-random Floating-Point Values
|
|
<https://allendowney.com/research/rand/downey07randfloat.pdf>`_ a
|
|
paper by Allen B. Downey describing ways to generate more
|
|
fine-grained floats than normally generated by :func:`.random`.
|