fix: check length of args of python model function before accessing it

.changes/unreleased/Fixes-20221011-160715.yaml

@@ -0,0 +1,7 @@
+kind: Fixes
+body: check length of args of python model function before accessing it
+time: 2022-10-11T16:07:15.464093-04:00
+custom:
+  Author: chamini2
+  Issue: "6041"
+  PR: "6042"

core/dbt/parser/models.py

@@ -45,7 +45,7 @@ def __init__(self):
     def visit_FunctionDef(self, node: ast.FunctionDef) -> None:
         if node.name == "model":
             self.num_model_def += 1
-            if not node.args.args[0].arg == "dbt":
+            if node.args.args and not node.args.args[0].arg == "dbt":
                 self.dbt_errors.append("'dbt' not provided for model as the first argument")
             if len(node.args.args) != 2:
                 self.dbt_errors.append(

test/unit/test_parser.py

@@ -623,7 +623,17 @@ def model(dbt):
         self.parser.manifest.files[block.file.file_id] = block.file
         with self.assertRaises(ParsingException):
             self.parser.parse_file(block)
-
+
+    def test_wrong_python_model_def_miss_session(self):
+        py_code = """
+def model():
+    return df
+        """
+        block = self.file_block_for(py_code, 'nested/py_model.py')
+        self.parser.manifest.files[block.file.file_id] = block.file
+        with self.assertRaises(ParsingException):
+            self.parser.parse_file(block)
+
     def test_wrong_python_model_def_wrong_arg(self):
         """ First argument for python model should be dbt
         """