Add class method DataModel::from_file.

tests/test_model.py

@@ -76,11 +76,8 @@ tree = """+--rw (test:choiA)?
 
 @pytest.fixture
 def data_model():
-    tpath = ["examples/test", "examples/ietf"]
-    with open("examples/test/yang-library-data.json",
-              encoding="utf-8") as ylfile:
-        ylib = ylfile.read()
-    return DataModel(ylib, tpath)
+    return DataModel.from_file("examples/test/yang-library-data.json",
+                               ["examples/test", "examples/ietf"])
 
 @pytest.fixture
 def instance(data_model):

yangson/datamodel.py

@@ -27,6 +27,13 @@ class DataModel:
             raise BadYangLibraryData() from None
         Context.from_yang_library(yl, mod_path)
 
+    @classmethod
+    def from_file(cls, name: str, mod_path: List[str] = ["."]) -> "DataModel":
+        """Return an instance initialised from a file with YANG library data."""
+        with open(name, encoding="utf-8") as infile:
+            yltxt = infile.read()
+        return cls(yltxt, mod_path)
+
     @staticmethod
     def module_set_id():
         """Return numeric id of the current set of modules."""