Return unrecognized random state unmodified (#2622)

* return unrecognized random state unmodified

* lint

* document type and fix docstring

* edit doc

* doc

cirq/value/random_state.py

@@ -12,20 +12,49 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
-from typing import cast, Optional, Union
+from typing import cast, Any
 
 import numpy as np
 
-RANDOM_STATE_LIKE = Optional[Union[np.random.RandomState, int]]
+from cirq._doc import document
+
+RANDOM_STATE_LIKE = Any
+document(
+    RANDOM_STATE_LIKE,
+    """A pseudorandom number generator or object that can be converted to one.
+
+    If None, turns into the module `np.random`.
+
+    If an integer, turns into a `np.random.RandomState` seeded with that
+    integer.
+
+    If none of the above, it is used unmodified. In this case, it is assumed
+    that the object implements whatever methods are required for the use case
+    at hand. For example, it might be an existing instance of
+    `np.random.RandomState` or a custom pseudorandom number generator
+    implementation.
+    """)
 
 
 def parse_random_state(random_state: RANDOM_STATE_LIKE
                       ) -> np.random.RandomState:
+    """Interpret an object as a pseudorandom number generator.
+
+    If `random_state` is None, returns the module `np.random`.
+    If `random_state` is an integer, returns
+    `np.random.RandomState(random_state)`.
+    Otherwise, returns `random_state` unmodified.
+
+    Args:
+        random_state: The object to be used as or converted to a pseudorandom
+            number generator.
+
+    Returns:
+        The pseudorandom number generator object.
+    """
     if random_state is None:
         return cast(np.random.RandomState, np.random)
-    elif (isinstance(random_state, np.random.RandomState) or
-          random_state == np.random):
-        return cast(np.random.RandomState, random_state)
     elif isinstance(random_state, int):
         return np.random.RandomState(random_state)
-    raise TypeError(f'Argument must be of type cirq.value.RANDOM_STATE_LIKE.')
+    else:
+        return cast(np.random.RandomState, random_state)

cirq/value/random_state_test.py

@@ -13,7 +13,6 @@
 # limitations under the License.
 
 import numpy as np
-import pytest
 
 import cirq
 
@@ -43,6 +42,3 @@ def rand(prng):
     vals = [prng.rand() for prng in prngs]
     eq = cirq.testing.EqualsTester()
     eq.add_equality_group(*vals)
-
-    with pytest.raises(TypeError):
-        cirq.value.parse_random_state('random_state')