Fix configuration file unmarshalling of JSON floating-point values (#253)

This PR fixes unmarshalling of `JsonValue` fields if they contain
floating point values; prior to this PR they were incorrectly
unmarshalled as integers, truncating precision.

The reason for this error is that unmarshalling is somewhat forgiving
with coercion allowed between primitives: if the target is a string,
boolean/int/float will be converted to a string. Previous this allowed
`int(float_value)` to succeed; this PR blocks that specifically, and
unit tests have been updated to ensure the values being produced are of
the correct type.

Author: asnare

src/databricks/labs/blueprint/installation.py

@@ -849,11 +849,15 @@ def _unmarshal_primitive(cls, inst: Any, path: list[str], type_ref: type) -> Any
         # No coercion necessary.
         if isinstance(inst, type_ref):
             return inst
+
         # Only attempt coercion between primitive types.
         if type(inst) not in cls._PRIMITIVES:
             msg = f"{'.'.join(path)}: Expected {type_ref.__name__}, got: {inst}"
             raise SerdeError(msg)
-        # Special case for strings to bool
+
+        # Some special cases:
+        #  - str -> bool: only accept "true" or "false" (case-insensitive).
+        #  - float -> int: refuse to truncate
         if type_ref == bool:
             if isinstance(inst, str):
                 if inst.lower() == "true":
@@ -862,6 +866,10 @@ def _unmarshal_primitive(cls, inst: Any, path: list[str], type_ref: type) -> Any
                     return False
             msg = f"{'.'.join(path)}: Expected bool, got: {inst}"
             raise SerdeError(msg)
+        if type_ref == int and isinstance(inst, float):
+            msg = f"{'.'.join(path)}: Expected int, got float: {inst}"
+            raise SerdeError(msg)
+
         # Everything else.
         try:
             converted = type_ref(inst)

tests/unit/test_installation.py

@@ -468,6 +468,7 @@ class SampleClass:
     saved = SampleClass(field={"a": "b", "b": "c"})
     installation.save(saved, filename="backups/SampleClass.json")
     loaded = installation.load(SampleClass, filename="backups/SampleClass.json")
+    assert isinstance(loaded.field["a"], str)
     assert loaded == saved
 
 
@@ -480,6 +481,7 @@ class SampleClass:
     saved = SampleClass(field={"a": 1, "b": 1})
     installation.save(saved, filename="backups/SampleClass.json")
     loaded = installation.load(SampleClass, filename="backups/SampleClass.json")
+    assert isinstance(loaded.field["a"], int)
     assert loaded == saved
 
 
@@ -492,6 +494,7 @@ class SampleClass:
     saved = SampleClass(field={"a": 1.1, "b": 1.2})
     installation.save(saved, filename="backups/SampleClass.json")
     loaded = installation.load(SampleClass, filename="backups/SampleClass.json")
+    assert isinstance(loaded.field["a"], float)
     assert loaded == saved
 
 
@@ -514,10 +517,12 @@ class SampleClass:
 
     installation = MockInstallation()
 
-    json_like: dict[str, JsonValue] = {"a": ["x", "y"], "b": [], "c": 3, "d": True, "e": {"a": "b"}}
+    json_like: dict[str, JsonValue] = {"a": ["x", "y"], "b": [], "c": 3, "d": True, "e": {"a": "b"}, "f": 0.1}
     saved = SampleClass(field=json_like)
     installation.save(saved, filename="backups/SampleClass.json")
     loaded = installation.load(SampleClass, filename="backups/SampleClass.json")
+    assert isinstance(loaded.field["c"], int)
+    assert isinstance(loaded.field["f"], float)
     assert loaded == saved
 
 
@@ -581,6 +586,7 @@ class SampleClass:
         3,
         True,
         {"a": "b"},
+        0.1,
     ]
     saved = SampleClass(field=json_like)
     installation.save(saved, filename="backups/SampleClass.json")