Compatibility Policy#
numpy.random has a somewhat stricter compatibility policy than the rest of
NumPy. Users of pseudorandomness often have use cases for being able to
reproduce runs in fine detail given the same seed (so-called “stream
compatibility”), and so we try to balance those needs with the flexibility to
enhance our algorithms. NEP 19 describes the evolution of this
policy.
The main kind of compatibility that we enforce is stream-compatibility from run
to run under certain conditions. If you create a Generator with the same
BitGenerator, with the same seed, perform the same sequence of method calls
with the same arguments, on the same build of numpy, in the same
environment, on the same machine, you should get the same stream of numbers.
Note that these conditions are very strict. There are a number of factors
outside of NumPy’s control that limit our ability to guarantee much more than
this. For example, different CPUs implement floating point arithmetic
differently, and this can cause differences in certain edge cases that cascade
to the rest of the stream. Generator.multivariate_normal, for another
example, uses a matrix decomposition from numpy.linalg. Even on the same
platform, a different build of numpy may use a different version of this
matrix decomposition algorithm from the LAPACK that it links to, causing
Generator.multivariate_normal to return completely different (but equally
valid!) results. We strive to prefer algorithms that are more resistant to
these effects, but this is always imperfect.
Note
Most of the Generator methods allow you to draw multiple values from
a distribution as arrays. The requested size of this array is a parameter,
for the purposes of the above policy. Calling rng.random() 5 times is
not guaranteed to give the same numbers as rng.random(5). We reserve
the ability to decide to use different algorithms for different-sized
blocks. In practice, this happens rarely.
Like the rest of NumPy, we generally maintain API source compatibility from version to version. If we must make an API-breaking change, then we will only do so with an appropriate deprecation period and warnings, according to general NumPy policy.
Breaking stream-compatibility in order to introduce new features or
improve performance in Generator or default_rng will be allowed with
caution. Such changes will be considered features, and as such will be no
faster than the standard release cadence of features (i.e. on X.Y releases,
never X.Y.Z). Slowness will not be considered a bug for this purpose.
Correctness bug fixes that break stream-compatibility can happen on bugfix
releases, per usual, but developers should consider if they can wait until the
next feature release. We encourage developers to strongly weight user’s pain
from the break in stream-compatibility against the improvements. One example
of a worthwhile improvement would be to change algorithms for a significant
increase in performance, for example, moving from the Box-Muller transform method of
Gaussian variate generation to the faster Ziggurat algorithm. An example of
a discouraged improvement would be tweaking the Ziggurat tables just a little
bit for a small performance improvement.
Note
In particular, default_rng is allowed to change the default
BitGenerator that it uses (again, with caution and plenty of advance
warning).
In general, BitGenerator classes have stronger guarantees of
version-to-version stream compatibility. This allows them to be a firmer
building block for downstream users that need it. Their limited API surface
makes it easier for them to maintain this compatibility from version to version. See
the docstrings of each BitGenerator class for their individual compatibility
guarantees.
The legacy RandomState and the associated convenience functions have a stricter version-to-version
compatibility guarantee. For reasons outlined in NEP 19, we had made
stronger promises about their version-to-version stability early in NumPy’s
development. There are still some limited use cases for this kind of
compatibility (like generating data for tests), so we maintain as much
compatibility as we can. There will be no more modifications to RandomState,
not even to fix correctness bugs. There are a few gray areas where we can make
minor fixes to keep RandomState working without segfaulting as NumPy’s
internals change, and some docstring fixes. However, the previously-mentioned
caveats about the variability from machine to machine and build to build still
apply to RandomState just as much as it does to Generator.