Fix argparse default value bug (#2526)

* Add failing test for bug

* Fix bug

pytorch_lightning/trainer/trainer.py

@@ -793,12 +793,32 @@ class Trainer(
         else:
             return int(x)
 
-    @staticmethod
-    def parse_argparser(arg_parser: Union[ArgumentParser, Namespace]) -> Namespace:
+    @classmethod
+    def parse_argparser(cls, arg_parser: Union[ArgumentParser, Namespace]) -> Namespace:
         """Parse CLI arguments, required for custom bool types."""
         args = arg_parser.parse_args() if isinstance(arg_parser, ArgumentParser) else arg_parser
-        args = {k: True if v is None else v for k, v in vars(args).items()}
-        return Namespace(**args)
+
+        types_default = {
+            arg: (arg_types, arg_default) for arg, arg_types, arg_default in cls.get_init_arguments_and_types()
+        }
+
+        modified_args = {}
+        for k, v in vars(args).items():
+            if k in types_default and v is None:
+                # We need to figure out if the None is due to using nargs="?" or if it comes from the default value
+                arg_types, arg_default = types_default[k]
+                if bool in arg_types and isinstance(arg_default, bool):
+                    # Value has been passed as a flag => It is currently None, so we need to set it to True
+                    # We always set to True, regardless of the default value.
+                    # Users must pass False directly, but when passing nothing True is assumed.
+                    # i.e. the only way to disable somthing that defaults to True is to use the long form:
+                    # "--a_default_true_arg False" becomes False, while "--a_default_false_arg" becomes None,
+                    # which then becomes True here.
+
+                    v = True
+
+            modified_args[k] = v
+        return Namespace(**modified_args)
 
     @classmethod
     def from_argparse_args(cls, args: Union[Namespace, ArgumentParser], **kwargs) -> 'Trainer':

tests/trainer/test_trainer_cli.py

@@ -102,7 +102,21 @@ def test_add_argparse_args_redefined_error(cli_args, monkeypatch):
     pytest.param('--tpu_cores=8',
                  {'tpu_cores': 8}),
     pytest.param("--tpu_cores=1,",
-                 {'tpu_cores': '1,'})
+                 {'tpu_cores': '1,'}),
+    pytest.param(
+        "",
+        {
+            # These parameters are marked as Optional[...] in Trainer.__init__, with None as default.
+            # They should not be changed by the argparse interface.
+            "min_steps": None,
+            "max_steps": None,
+            "log_gpu_memory": None,
+            "distributed_backend": None,
+            "weights_save_path": None,
+            "truncated_bptt_steps": None,
+            "resume_from_checkpoint": None,
+            "profiler": None,
+        }),
 ])
 def test_argparse_args_parsing(cli_args, expected):
     """Test multi type argument with bool."""