[New-bugs-announce] [issue40346] Redesign random class inheritance

[STINNER Victor]

New submission from STINNER Victor <vstinner at python.org>:

The random.Random class has a strange inheritance which is something uncommon in Python: it inherits from _random.Random which a concrete Mersenne Twister PRNG.

When a class inherit it, like random.SystemRandom, it should carefully override the right method.

I would expect to have something like a base class with abstract methods which raise NotImplementedError, and a concrete implementation which inherits from the base class and the Mersenne Twister C implementation.

A concrete issue is the how subclass handle the newly added randbytes() method:
https://bugs.python.org/issue40286#msg366860

The base class should implement randbytes() using getrandbits(), and the Mersenne Twister would override randbytes() with its fast implementation.

It would avoid the need for such method:

    def __init_subclass__(cls, /, **kwargs):
        """Control how subclasses generate random integers.

        The algorithm a subclass can use depends on the random() and/or
        getrandbits() implementation available to it and determines
        whether it can generate random integers from arbitrarily large
        ranges.
        """

        for c in cls.__mro__:
            if '_randbelow' in c.__dict__:
                # just inherit it
                break
            if 'getrandbits' in c.__dict__:
                cls._randbelow = cls._randbelow_with_getrandbits
                break
            if 'random' in c.__dict__:
                cls._randbelow = cls._randbelow_without_getrandbits
                break

It would also be nice if the base class would support a RNG which only produce bytes, like SystemRandom with os.urandom() and implement getrandbits() from that.


I also had concerns with the implementation of bpo-40282 "Allow random.getrandbits(0)":
https://github.com/python/cpython/pull/19539#pullrequestreview-393728231


It's maybe time to fix the class hierarchy.

----------
components: Library (Lib)
messages: 366892
nosy: vstinner
priority: normal
severity: normal
status: open
title: Redesign random class inheritance
versions: Python 3.9

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<https://bugs.python.org/issue40346>
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