model ADTs with dataclasses and union types

core/dbt/events/events.py

@@ -1,26 +1,42 @@
 
+from dataclasses import dataclass
+from typing import NoReturn, Union
 
-class Singleton:
-   __instance = None
-   @staticmethod 
-   def getInstance():
-      """ Static access method. """
-      if Singleton.__instance == None:
-         Singleton()
-      return Singleton.__instance
-   def __init__(self):
-      """ Virtually private constructor. """
-      if Singleton.__instance != None:
-         raise Exception("This class is a singleton!")
-      else:
-         Singleton.__instance = self
 
+# common trick for getting mypy to do exhaustiveness checks
+# will come up with something like `"assert_never" has incompatible type`
+# if something is missing.
+def assert_never(x: NoReturn) -> NoReturn:
+    raise AssertionError("Unhandled type: {}".format(type(x).__name__))
 
-# All classes that inherit from this one should be defined in this file.
-class BaseEvent():
-    pass
 
+# TODO dummy class
+@dataclass(frozen=True)
+class OK:
+    result: int
 
-class StartParse(BaseEvent):
-    pass
 
+# TODO dummy class
+@dataclass(frozen=True)
+class Failure:
+    msg: str
+
+
+# using a union instead of inheritance means that this set cannot
+# be extended outside this file, and thus mypy can do exhaustiveness
+# checks for us.
+Event = Union[OK, Failure]
+
+
+# function that translates any instance of the above event types
+# into its human-readable message.
+#
+# This could instead be implemented as a method on an ABC for all
+# above classes, but this at least puts all that logic in one place.
+def humanMsg(r: Event) -> str:
+    if isinstance(r, OK):
+        return str(r.result)
+    elif isinstance(r, Failure):
+        return "Failure: " + r.msg
+    else:
+        assert_never(r)