model: Ensure the seed is initialized with current timestamp when it …

…is None

mesa/model.py

@@ -7,6 +7,7 @@
 # Remove this __future__ import once the oldest supported Python is 3.10
 from __future__ import annotations
 
+import datetime
 import random
 
 # mypy
@@ -21,7 +22,16 @@ class Model:
     def __new__(cls, *args: Any, **kwargs: Any) -> Any:
         """Create a new model object and instantiate its RNG automatically."""
         obj = object.__new__(cls)
-        obj._seed = kwargs.get("seed", None)
+        obj._seed = kwargs.get("seed")
+        if obj._seed is None:
+            # See https://docs.python.org/3/library/random.html#random.seed
+            # which says "If a is omitted or None, the current system time is used."
+            # We explicitly specify the seed here so that we know its value.
+            # datetime.now().timestamp() is more cross-platform than using
+            # time.time(), and guarantees that the precision is to the
+            # microsecond.
+            current_timestamp = datetime.datetime.now(datetime.timezone.utc).timestamp()
+            obj._seed = current_timestamp
         obj.random = random.Random(obj._seed)
         return obj