fix: Include default data value in function signature

Include default `data` value (an empty dict) in function signature.
We cannot use a normal empty dict because using a mutable
data type as a default argument value in a function is almost
always a bad idea. Indeed, doing this is perhaps the canonical
gotcha in Python programming. See
<https://docs.python-guide.org/writing/gotchas/#mutable-default-arguments>
for a full description of the problem.

With the arrival of type annotations, however, we really do want to
indicate that the default value is an empty dict.  We indicate default
values everywhere else, so not doing it in this case is confusing.

The solution here is to create a superficial implementation of
`frozendict` (which really should be part of the standard library,
alongside `frozenset`). See the rejected PEP416 for why
it is not in the standard library.

Thanks to @smoh for identifying the flaw in the documentation.

stan/model.py

@@ -27,6 +27,12 @@ def default(self, obj):
         return json.JSONEncoder.default(self, obj)
 
 
+# superficial frozendict implementation. Only used for function signatures
+class frozendict(dict):
+    def __setitem__(self, key, value):
+        raise TypeError("'frozendict' object is immutable.")
+
+
 @dataclasses.dataclass(frozen=True)
 class Model:
     """Stores data associated with and proxies calls to a Stan model.
@@ -393,14 +399,15 @@ async def go():
         return asyncio.run(go())
 
 
-def build(program_code: str, data: Optional[Data] = None, random_seed: Optional[int] = None) -> Model:
+def build(program_code: str, data: Data = frozendict(), random_seed: Optional[int] = None) -> Model:
     """Build (compile) a Stan program.
 
     Arguments:
         program_code: Stan program code describing a Stan model.
         data: A Python dictionary or mapping providing the data for the
             model. Variable names are the keys and the values are their
-            associated values. Default is an empty dictionary.
+            associated values. Default is an empty dictionary, suitable
+            for Stan programs with no `data` block.
         random_seed: Random seed, a positive integer for random number
             generation. Used to make sure that results can be reproduced.
 
@@ -413,7 +420,7 @@ def build(program_code: str, data: Optional[Data] = None, random_seed: Optional[
 
     """
     # `data` must be JSON-serializable in order to send to httpstan
-    data = json.loads(DataJSONEncoder().encode(data)) if data else {}
+    data = json.loads(DataJSONEncoder().encode(data))
 
     async def go():
         io = ConsoleIO()