Restore call convention compatibility in get_model (microsoft#304)

A bug surfaced where first time evaluation of a model fails due to the
Model constructor throwing if the model does not exist.

Looking deeper, we see that most calls to get_model expect a possible
None response and check at the call site. Unfortunately we get the same
WebserviceException class for a model not being found as we do a REST
error or similar.

This change is a stopgap mitigation to restore compatibility with the
existing callers, and compromises by allowing the model version
dependent behavior to continue passing on exceptions.

In a future follow up we should settle on a convention and allow version
checks to propagate failure while still giving the possibility for
handling a service exception in the caller.

diabetes_regression/util/model_helper.py

@@ -8,8 +8,8 @@
 
 def get_current_workspace() -> Workspace:
     """
-    Retrieves and returns the latest model from the workspace
-    by its name and tag. Will not work when ran locally.
+    Retrieves and returns the current workspace.
+    Will not work when ran locally.
 
     Parameters:
     None
@@ -30,8 +30,8 @@ def get_model(
     aml_workspace: Workspace = None
 ) -> AMLModel:
     """
-    Retrieves and returns the latest model from the workspace
-    by its name and (optional) tag.
+    Retrieves and returns a model from the workspace by its name
+    and (optional) tag.
 
     Parameters:
     aml_workspace (Workspace): aml.core Workspace that the model lives.
@@ -40,25 +40,40 @@ def get_model(
     (optional) tag (str): the tag value & name the model was registered under.
 
     Return:
-    A single aml model from the workspace that matches the name and tag.
+    A single aml model from the workspace that matches the name and tag, or
+    None.
     """
     if aml_workspace is None:
         print("No workspace defined - using current experiment workspace.")
         aml_workspace = get_current_workspace()
 
-    if tag_name is not None and tag_value is not None:
+    tags = None
+    if tag_name is not None or tag_value is not None:
+        # Both a name and value must be specified to use tags.
+        if tag_name is None or tag_value is None:
+            raise ValueError(
+                "model_tag_name and model_tag_value should both be supplied"
+                + "or excluded"  # NOQA: E501
+            )
+        tags = [[tag_name, tag_value]]
+
+    model = None
+    if model_version is not None:
+        # TODO(tcare): Finding a specific version currently expects exceptions
+        # to propagate in the case we can't find the model. This call may
+        # result in a WebserviceException that may or may not be due to the
+        # model not existing.
         model = AMLModel(
             aml_workspace,
             name=model_name,
             version=model_version,
-            tags=[[tag_name, tag_value]])
-    elif (tag_name is None and tag_value is not None) or (
-        tag_value is None and tag_name is not None
-    ):
-        raise ValueError(
-            "model_tag_name and model_tag_value should both be supplied"
-            + "or excluded"  # NOQA: E501
-        )
+            tags=tags)
     else:
-        model = AMLModel(aml_workspace, name=model_name, version=model_version)  # NOQA: E501
+        models = AMLModel.list(
+            aml_workspace, name=model_name, tags=tags, latest=True)
+        if len(models) == 1:
+            model = models[0]
+        elif len(models) > 1:
+            raise Exception("Expected only one model")
+
     return model