diff --git a/pytorch_lightning/utilities/argparse.py b/pytorch_lightning/utilities/argparse.py
index 49cbaf3c6bdcf..46d88184ee190 100644
--- a/pytorch_lightning/utilities/argparse.py
+++ b/pytorch_lightning/utilities/argparse.py
@@ -67,7 +67,7 @@ def parse_argparser(cls, arg_parser: Union[ArgumentParser, Namespace]) -> Namesp
                 # 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:
+                # i.e. the only way to disable something 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.
 
@@ -242,9 +242,6 @@ def add_argparse_args(
         if arg == 'track_grad_norm':
             use_type = float
 
-        if arg_default is inspect._empty:
-            arg_default = None
-
         parser.add_argument(
             f'--{arg}',
             dest=arg,
@@ -291,10 +288,7 @@ def _gpus_allowed_type(x) -> Union[int, str]:
 
 
 def _gpus_arg_default(x) -> Union[int, str]:
-    if ',' in x:
-        return str(x)
-    else:
-        return int(x)
+    return _gpus_allowed_type(x)
 
 
 def _int_or_float_type(x) -> Union[int, float]:
diff --git a/tests/utilities/test_argparse_utils.py b/tests/utilities/test_argparse.py
similarity index 80%
rename from tests/utilities/test_argparse_utils.py
rename to tests/utilities/test_argparse.py
index b2eac514941e6..fdf5ae0cafe65 100644
--- a/tests/utilities/test_argparse_utils.py
+++ b/tests/utilities/test_argparse.py
@@ -1,17 +1,51 @@
 import io
-from argparse import ArgumentParser
+from argparse import ArgumentParser, Namespace
 from typing import List
+from unittest.mock import MagicMock
 
 import pytest
 
 from pytorch_lightning import Trainer
 from pytorch_lightning.utilities.argparse import (
     add_argparse_args,
+    from_argparse_args,
     get_abbrev_qualified_cls_name,
+    parse_argparser,
     parse_args_from_docstring,
+    _gpus_arg_default,
+    _int_or_float_type
 )
 
 
+class ArgparseExample:
+    def __init__(self, a: int = 0, b: str = '', c: bool = False):
+        self.a = a
+        self.b = b
+        self.c = c
+
+
+def test_from_argparse_args():
+    args = Namespace(a=1, b='test', c=True, d='not valid')
+    my_instance = from_argparse_args(ArgparseExample, args)
+    assert my_instance.a == 1
+    assert my_instance.b == 'test'
+    assert my_instance.c
+
+    parser = ArgumentParser()
+    mock_trainer = MagicMock()
+    _ = from_argparse_args(mock_trainer, parser)
+    mock_trainer.parse_argparser.assert_called_once_with(parser)
+
+
+def test_parse_argparser():
+    args = Namespace(a=1, b='test', c=None, d='not valid')
+    new_args = parse_argparser(ArgparseExample, args)
+    assert new_args.a == 1
+    assert new_args.b == 'test'
+    assert new_args.c
+    assert new_args.d == 'not valid'
+
+
 def test_parse_args_from_docstring_normal():
     args_help = parse_args_from_docstring(
         """Constrain image dataset
@@ -168,3 +202,13 @@ def test_add_argparse_args_no_argument_group():
     args = parser.parse_args(fake_argv)
     assert args.main_arg == "abc"
     assert args.my_parameter == 2
+
+
+def test_gpus_arg_default():
+    assert _gpus_arg_default('1,2') == '1,2'
+    assert _gpus_arg_default('1') == 1
+
+
+def test_int_or_float_type():
+    assert isinstance(_int_or_float_type('0.0'), float)
+    assert isinstance(_int_or_float_type('0'), int)