Deprecate use of None for a demographic model.

doc/misc/deprecated.md

@@ -10,4 +10,10 @@ python -Wd
 
 ```
 
+To treat all warning as errors:
 
+```{code-block} bash
+
+python -We
+
+```

fwdpy11/_evolvets.py

@@ -129,8 +129,9 @@ def evolvets(
         sizes = pop.deme_sizes()[1].tolist()
         msg = "Applying a default demographic model "
         msg += f"where deme sizes are {sizes} "
-        msg += f"and the burn-in length is 10*{sum(sizes)}"
-        warnings.warn(msg, stacklevel=2)
+        msg += f"and the burn-in length is 10*{sum(sizes)}. "
+        msg += "This will raise an error in future releases."
+        warnings.warn(msg, DeprecationWarning, stacklevel=2)
         demographic_model = ForwardDemesGraph.tubes(sizes, 10)
 
     for r in params.sregions:

fwdpy11/_types/model_params.py

@@ -186,9 +186,8 @@ class ModelParams(object):
     recregions = attr.ib(factory=list)
     rates: MutationAndRecombinationRates = attr.ib(converter=_convert_rates)
     gvalue = attr.ib(default=None)
-    demography: typing.Optional[typing.Union[ForwardDemesGraph,
-                                             DemographicModelDetails]] = attr.ib(
-        default=None)
+    demography: typing.Union[ForwardDemesGraph,
+                             DemographicModelDetails] = attr.ib(default=None)
     simlen: int = attr.ib(converter=int, default=0)
     prune_selected: bool = attr.ib(default=True)
     allow_residual_selfing: bool = attr.ib(default=True)
@@ -290,6 +289,9 @@ def validate_gvalue(self, attribute, value):
     @demography.validator
     def validate_demography(self, attribute, value):
         if value is None:
+            warnings.warn("No demographic model specified."
+                          " This will be a hard error in a future release.",
+                          DeprecationWarning)
             return
 
         if isinstance(value, fwdpy11.ForwardDemesGraph):

tests/test_add_mutation.py

@@ -48,6 +48,9 @@ def generate_msprime_ancestry(
             "recregions": [fwdpy11.PoissonInterval(0, 1, 5e-2)],
             "gvalue": fwdpy11.Multiplicative(2.0),
             "rates": (0, 0, None),
+            "demography": fwdpy11.ForwardDemesGraph.tubes([pop.N],
+                                                          burnin=10*pop.N,
+                                                          burnin_is_exact=True),
             "prune_selected": False,
             "simlen": 10 * pop.N,
         }