add required for positional arguments

CHANGELOG.md

@@ -43,7 +43,7 @@ The format is based on [Keep a Changelog](http://keepachangelog.com/en/1.0.0/).
 - Marked `swa_lrs` argument in `StochasticWeightAveraging` callback as required ([#12556](https://github.com/PyTorchLightning/pytorch-lightning/pull/12556))
 
 
--
+- Make positional arguments required for classes passed into the `add_argparse_args` function. ([#1250](https://github.com/PyTorchLightning/pytorch-lightning/pull/12504))
 
 
 -

pytorch_lightning/utilities/argparse.py

@@ -266,7 +266,13 @@ def add_argparse_args(
             use_type = _precision_allowed_type
 
         parser.add_argument(
-            f"--{arg}", dest=arg, default=arg_default, type=use_type, help=args_help.get(arg), **arg_kwargs
+            f"--{arg}",
+            dest=arg,
+            default=arg_default,
+            type=use_type,
+            help=args_help.get(arg),
+            required=(arg_default == inspect._empty),
+            **arg_kwargs,
         )
 
     if use_argument_group:

tests/utilities/test_argparse.py

@@ -19,7 +19,7 @@
 
 
 class ArgparseExample:
-    def __init__(self, a: int = 0, b: str = "", c: bool = False):
+    def __init__(self, a: int, b: str = "", c: bool = False):
         self.a = a
         self.b = b
         self.c = c
@@ -147,6 +147,16 @@ def __init__(self, invalid_class: SomeClass):
         pass
 
 
+class AddArgparseArgsExampleClassNoDefault:
+    """
+    Args:
+        my_parameter: A thing.
+    """
+
+    def __init__(self, my_parameter: int):
+        pass
+
+
 def extract_help_text(parser):
     help_str_buffer = io.StringIO()
     parser.print_help(file=help_str_buffer)
@@ -160,6 +170,7 @@ def extract_help_text(parser):
         [AddArgparseArgsExampleClass, "AddArgparseArgsExampleClass"],
         [AddArgparseArgsExampleClassViaInit, "AddArgparseArgsExampleClassViaInit"],
         [AddArgparseArgsExampleClassNoDoc, "AddArgparseArgsExampleClassNoDoc"],
+        [AddArgparseArgsExampleClassNoDefault, "AddArgparseArgsExampleClassNoDefault"],
     ],
 )
 def test_add_argparse_args(cls, name):
@@ -185,6 +196,15 @@ def test_add_argparse_args(cls, name):
     assert args.main_arg == "abc"
     assert args.my_parameter == 2
 
+    fake_argv = ["--main_arg=abc"]
+    if cls is AddArgparseArgsExampleClassNoDefault:
+        with pytest.raises(SystemExit):
+            parser.parse_args(fake_argv)
+    else:
+        args = parser.parse_args(fake_argv)
+        assert args.main_arg == "abc"
+        assert args.my_parameter == 0
+
 
 def test_negative_add_argparse_args():
     with pytest.raises(RuntimeError, match="Please only pass an ArgumentParser instance."):