ENH: default_rng coerces RandomState to Generator

Author: andyfaff

numpy/random/_generator.pyx

@@ -22,6 +22,7 @@ from ._bounded_integers cimport (_rand_bool, _rand_int32, _rand_int64,
          _rand_int16, _rand_int8, _rand_uint64, _rand_uint32, _rand_uint16,
          _rand_uint8, _gen_mask)
 from ._pcg64 import PCG64
+from ._mt19937 import MT19937
 from numpy.random cimport bitgen_t
 from ._common cimport (POISSON_LAM_MAX, CONS_POSITIVE, CONS_NONE,
             CONS_NON_NEGATIVE, CONS_BOUNDED_0_1, CONS_BOUNDED_GT_0_1,
@@ -4990,14 +4991,15 @@ def default_rng(seed=None):
 
     Parameters
     ----------
-    seed : {None, int, array_like[ints], SeedSequence, BitGenerator, Generator}, optional
+    seed : {None, int, array_like[ints], SeedSequence, BitGenerator, Generator, RandomState}, optional
         A seed to initialize the `BitGenerator`. If None, then fresh,
         unpredictable entropy will be pulled from the OS. If an ``int`` or
         ``array_like[ints]`` is passed, then all values must be non-negative and will be
         passed to `SeedSequence` to derive the initial `BitGenerator` state. One may also
         pass in a `SeedSequence` instance.
         Additionally, when passed a `BitGenerator`, it will be wrapped by
         `Generator`. If passed a `Generator`, it will be returned unaltered.
+        When passed a legacy `RandomState` instance it will be coerced to a `Generator`.
 
     Returns
     -------
@@ -5070,6 +5072,14 @@ def default_rng(seed=None):
     elif isinstance(seed, Generator):
         # Pass through a Generator.
         return seed
+    elif isinstance(seed, np.random.RandomState):
+        rs_state = seed.get_state(legacy=False)
+        klass = getattr(np.random, rs_state['bit_generator'])
+        bg = klass()
+        bg.state = rs_state
+        gen = np.random.Generator(bg)
+        return gen
+
     # Otherwise we need to instantiate a new BitGenerator and Generator as
     # normal.
     return Generator(PCG64(seed))

numpy/random/tests/test_direct.py

@@ -559,3 +559,23 @@ def test_passthrough(self):
         rg2 = default_rng(rg)
         assert rg2 is rg
         assert rg2.bit_generator is bg
+
+    def test_coercion_RandomState_Generator(self):
+        # use default_rng to coerce RandomState to Generator
+        rs = RandomState(1234)
+        rg = default_rng(rs)
+        assert isinstance(rg.bit_generator, MT19937)
+
+        assert_allclose(rg.random(), rs.rand())
+
+        # RandomState with a non MT19937 bit generator
+        _original = np.random.get_bit_generator()
+        bg = PCG64(12342298)
+        np.random.set_bit_generator(bg)
+        rs = np.random.mtrand._rand
+        rg = default_rng(rs)
+        assert_allclose(rg.random(), rs.rand())
+
+        # vital to get global state back to original, otherwise
+        # other tests start to fail.
+        np.random.set_bit_generator(_original)