Refactor template handler (#1151)

In 0abe669, some unnecessary
refactoring occurred while fixing Issue #1148. This reverts that
refactoring and adds unit tests for the File handler.

tests/test_template_handlers/test_file.py

@@ -0,0 +1,57 @@
+# -*- coding: utf-8 -*-
+
+import pytest
+
+from sceptre.template_handlers.file import File
+from unittest.mock import patch, mock_open
+
+
+class TestFile(object):
+
+    @pytest.mark.parametrize("project_path,path,output_path", [
+        ("my_project_dir",      "my.template.yaml",                               "my_project_dir/templates/my.template.yaml"),        # NOQA
+        ("/src/my_project_dir", "my.template.yaml",                               "/src/my_project_dir/templates/my.template.yaml"),   # NOQA
+        ("my_project_dir",      "/src/my_project_dir/templates/my.template.yaml", "/src/my_project_dir/templates/my.template.yaml"),   # NOQA
+        ("/src/my_project_dir", "/src/my_project_dir/templates/my.template.yaml", "/src/my_project_dir/templates/my.template.yaml"),   # NOQA
+    ])
+    @patch("builtins.open", new_callable=mock_open, read_data="some_data")
+    def test_handler_open(self, mocked_open, project_path, path, output_path):
+        template_handler = File(
+            name="file_handler",
+            arguments={"path": path},
+            stack_group_config={"project_path": project_path}
+        )
+        template_handler.handle()
+        mocked_open.assert_called_with(output_path)
+
+    @pytest.mark.parametrize("project_path,path,output_path", [
+        ("my_project_dir",      "my.template.yaml.j2",                               "my_project_dir/templates/my.template.yaml.j2"),        # NOQA
+        ("/src/my_project_dir", "my.template.yaml.j2",                               "/src/my_project_dir/templates/my.template.yaml.j2"),   # NOQA
+        ("my_project_dir",      "/src/my_project_dir/templates/my.template.yaml.j2", "/src/my_project_dir/templates/my.template.yaml.j2"),   # NOQA
+        ("/src/my_project_dir", "/src/my_project_dir/templates/my.template.yaml.j2", "/src/my_project_dir/templates/my.template.yaml.j2"),   # NOQA
+    ])
+    @patch("sceptre.template_handlers.helper.render_jinja_template")
+    def test_handler_render(self, mocked_render, project_path, path, output_path):
+        template_handler = File(
+            name="file_handler",
+            arguments={"path": path},
+            stack_group_config={"project_path": project_path}
+        )
+        template_handler.handle()
+        mocked_render.assert_called_with(output_path, {"sceptre_user_data": None}, {})
+
+    @pytest.mark.parametrize("project_path,path,output_path", [
+        ("my_project_dir",      "my.template.yaml.py",                               "my_project_dir/templates/my.template.yaml.py"),        # NOQA
+        ("/src/my_project_dir", "my.template.yaml.py",                               "/src/my_project_dir/templates/my.template.yaml.py"),   # NOQA
+        ("my_project_dir",      "/src/my_project_dir/templates/my.template.yaml.py", "/src/my_project_dir/templates/my.template.yaml.py"),   # NOQA
+        ("/src/my_project_dir", "/src/my_project_dir/templates/my.template.yaml.py", "/src/my_project_dir/templates/my.template.yaml.py"),   # NOQA
+    ])
+    @patch("sceptre.template_handlers.helper.call_sceptre_handler")
+    def test_handler_handler(self, mocked_handler, project_path, path, output_path):
+        template_handler = File(
+            name="file_handler",
+            arguments={"path": path},
+            stack_group_config={"project_path": project_path}
+        )
+        template_handler.handle()
+        mocked_handler.assert_called_with(output_path, None)