diff --git a/uroboros/command.py b/uroboros/command.py
index b32f61c..2188031 100644
--- a/uroboros/command.py
+++ b/uroboros/command.py
@@ -226,7 +226,7 @@ class Command(metaclass=abc.ABCMeta):
         Get all `Option` instance of this `Command`.
         :return: List of Option instance
         """
-        return self.options
+        return [opt() if type(opt) == type else opt for opt in self.options]
 
     def print_help(self):
         """

diff --git a/tests/test_command.py b/tests/test_command.py
index a7eb5f0..05a703b 100644
--- a/tests/test_command.py
+++ b/tests/test_command.py
@@ -13,6 +13,18 @@ from .base import RootCommand, SecondCommand, ThirdCommand
 def commands():
     return RootCommand(), SecondCommand(), ThirdCommand()
 
+class Opt1(Option):
+    def build_option(self, parser):
+        return parser
+
+class Opt2(Option):
+    def build_option(self, parser):
+        return parser
+
+class Opt3(Option):
+    def build_option(self, parser):
+        return parser
+
 
 def get_sub_commands(cmd_set):
     if len(cmd_set) == 0:
@@ -338,3 +350,34 @@ class TestCommand(object):
             cmd_parser = cmd.create_default_parser()
             args = cmd_parser.parse_args(argv)
             assert args.test == 'test'
+
+    @pytest.mark.parametrize(
+        'option_objs', [
+            [Opt1(), Opt2(), Opt3()],
+        ]
+    )
+    def test_get_options(self, option_objs):
+        additional_opt = Opt1()
+        class SourceCommand(RootCommand):
+            options = option_objs
+        class Cmd(SourceCommand):
+            def get_options(self):
+                opts =super(Cmd, self).get_options()
+                opts.append(additional_opt)
+                return opts
+        root = SourceCommand()
+        source_opts = root.get_options()
+        cmd = Cmd()
+        actual_options = cmd.get_options()
+        expected_options = option_objs + [additional_opt]
+        assert len(actual_options) == len(expected_options) 
+        # All options are instantiated
+        types = map(type, actual_options)
+        bools = map(lambda x: x != type, types)
+        assert all(bools)
+        # All class is correct
+        actual_classes = map(lambda x: type(x), actual_options)
+        expected_classes = map(lambda x: x if type(x) == type else type(x), expected_options)
+        assert list(actual_classes) == list(expected_classes)
+        # Inheritance source class is not modified
+        assert RootCommand().get_options() == []

Operations on `options` in the inheritance destination class also affect the inheritance source.
Sample code is as follows.

```python
class Opt(uroboros.Option):
    def build_option(self, parser):
        parser.add_argument("--test", type=str)
        return parser

class RootCommand(uroboros.Command):
    name = 'root'
    def run(self, args):
        self.print_help()
        return 0

class BaseCommand(uroboros.Command):
    options = []
    def run(self, args):
        print(args.test)
        return 0

class FirstCommand(BaseCommand):
    name = 'first'
    def get_options(self):
        options = super(FirstCommand, self).get_options()
        options.append(Opt())
        return options


class SecondCommand(BaseCommand):
    name = 'second'

root = RootCommand()
root.add_command(
    FirstCommand(),
    SecondCommand(),
)
root.execute()
```

## Expected

```sh
$ pipenv run python sample.py -h
usage: root [-h] {first,second} ...

optional arguments:
  -h, --help      show this help message and exit

Sub commands:
  {first,second}
    first
    second
＄pipenv run python sample.py first -h
usage: root first [-h] [--test TEST]

optional arguments:
  -h, --help   show this help message and exit
  --test TEST
＄pipenv run python sample.py second -h
usage: root second [-h]

optional arguments:
  -h, --help  show this help message and exit
```

## Actual

```
$ pipenv run python sample.py -h
Traceback (most recent call last):
  File "/home/pudding/.local/share/virtualenvs/swat-analyzer-EWaBsfe_/lib/python3.7/site-packages/uroboros/command.py", line 57, in execute
    self._check_initialized()
  File "/home/pudding/.local/share/virtualenvs/swat-analyzer-EWaBsfe_/lib/python3.7/site-packages/uroboros/command.py", line 322, in _check_initialized
    raise errors.CommandNotRegisteredError(self.name)
uroboros.errors.CommandNotRegisteredError: Command 'root' has not been registered yet.

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "test.py", line 36, in <module>
    root.execute()
  File "/home/pudding/.local/share/virtualenvs/swat-analyzer-EWaBsfe_/lib/python3.7/site-packages/uroboros/command.py", line 59, in execute
    self.initialize()
  File "/home/pudding/.local/share/virtualenvs/swat-analyzer-EWaBsfe_/lib/python3.7/site-packages/uroboros/command.py", line 110, in initialize
    self.initialize_sub_parsers(self._parser)
  File "/home/pudding/.local/share/virtualenvs/swat-analyzer-EWaBsfe_/lib/python3.7/site-packages/uroboros/command.py", line 124, in initialize_sub_parsers
    parents=[o.get_parser() for o in cmd.get_options()],
  File "/home/pudding/.local/share/virtualenvs/swat-analyzer-EWaBsfe_/lib/python3.7/site-packages/uroboros/command.py", line 124, in <listcomp>
    parents=[o.get_parser() for o in cmd.get_options()],
  File "/home/s-kokuryo/.local/share/virtualenvs/swat-analyzer-EWaBsfe_/lib/python3.7/site-packages/uroboros/option.py", line 15, in get_parser
    return self.build_option(self.parser)
  File "test.py", line 5, in build_option
    parser.add_argument("--test", type=str)
  File "/home/pudding/.anyenv/envs/pyenv/versions/3.7.4/lib/python3.7/argparse.py", line 1367, in add_argument
    return self._add_action(action)
  File "/home/pudding/.anyenv/envs/pyenv/versions/3.7.4/lib/python3.7/argparse.py", line 1730, in _add_action
    self._optionals._add_action(action)
  File "/home/pudding/.anyenv/envs/pyenv/versions/3.7.4/lib/python3.7/argparse.py", line 1571, in _add_action
    action = super(_ArgumentGroup, self)._add_action(action)
  File "/home/pudding/.anyenv/envs/pyenv/versions/3.7.4/lib/python3.7/argparse.py", line 1381, in _add_action
    self._check_conflict(action)
  File "/home/pudding/.anyenv/envs/pyenv/versions/3.7.4/lib/python3.7/argparse.py", line 1520, in _check_conflict
    conflict_handler(action, confl_optionals)
  File "/home/pudding/.anyenv/envs/pyenv/versions/3.7.4/lib/python3.7/argparse.py", line 1529, in _handle_conflict_error
    raise ArgumentError(action, message % conflict_string)
argparse.ArgumentError: argument --test: conflicting option string: --test
```

## Why this happened?

Python's list object is mutable. `uroboros.Option`'s `get_options()` function return the pointer of `options` attribute itself. So the operation on it in the inheritance class also affect its source class.

## How to fix

Change the implementation of `uroboros.Command.get_options` like [permission mechanism of django-restframework's view class](https://github.com/encode/django-rest-framework/blob/3.10.3/rest_framework/views.py#L266).
`uroboros.Command.options` accepts both of `uroboros.Option` class and its instance for backward compatibility. Check the type of the attribute of `uroboros.Command.options` and instantiate the class if need, and generate new list and return.
tests/test_command.py::TestCommand::test_get_options[option_objs0]
tests/test_command.py::TestCommand::test_build_option[command0] tests/test_command.py::TestCommand::test_build_option[command1] tests/test_command.py::TestCommand::test_build_option[command2] tests/test_command.py::TestCommand::test_validate[command0] tests/test_command.py::TestCommand::test_validate[command1] tests/test_command.py::TestCommand::test_validate[command2] tests/test_command.py::TestCommand::test_increment_nest[command_set0] tests/test_command.py::TestCommand::test_increment_nest[command_set1] tests/test_command.py::TestCommand::test_increment_nest[command_set2] tests/test_command.py::TestCommand::test_register_parent[command_set0] tests/test_command.py::TestCommand::test_register_parent[command_set1] tests/test_command.py::TestCommand::test_register_parent[command_set2] tests/test_command.py::TestCommand::test_add_command[command_set0] tests/test_command.py::TestCommand::test_add_command[command_set1] tests/test_command.py::TestCommand::test_add_command[command_set2] tests/test_command.py::TestCommand::test_multiple_add_command[root_cmd0-add_commands0] tests/test_command.py::TestCommand::test_multiple_add_command[root_cmd1-add_commands1] tests/test_command.py::TestCommand::test_add_others tests/test_command.py::TestCommand::test_get_all_sub_commands[command_set0] tests/test_command.py::TestCommand::test_get_all_sub_commands[command_set1] tests/test_command.py::TestCommand::test_get_all_sub_commands[command_set2] tests/test_command.py::TestCommand::test_get_all_sub_commands[command_set3] tests/test_command.py::TestCommand::test_get_sub_commands[command_set0-argv0] tests/test_command.py::TestCommand::test_get_sub_commands[command_set1-argv1] tests/test_command.py::TestCommand::test_get_sub_commands[command_set2-argv2] tests/test_command.py::TestCommand::test_get_sub_commands[command_set3-argv3] tests/test_command.py::TestCommand::test_get_sub_commands[command_set4-argv4] tests/test_command.py::TestCommand::test_add_duplicate_command[command_set0] tests/test_command.py::TestCommand::test_add_duplicate_command[command_set1] tests/test_command.py::TestCommand::test_execute[command_set0-root tests/test_command.py::TestCommand::test_execute[command_set1-root tests/test_command.py::TestCommand::test_execute[command_set2-root-False] tests/test_command.py::TestCommand::test_execute[command_set3-root tests/test_command.py::TestCommand::test_execute[command_set4-root tests/test_command.py::TestCommand::test_execute[command_set5-root-False] tests/test_command.py::TestCommand::test_violates_validation_argv[command_set0-root tests/test_command.py::TestCommand::test_violates_validation_argv[command_set1-root tests/test_command.py::TestCommand::test_violates_validation_argv[command_set2-root tests/test_command.py::TestCommand::test_violates_validation_argv[command_set3-root tests/test_command.py::TestCommand::test_violates_validation_argv[command_set4-root tests/test_command.py::TestCommand::test_violates_validation_argv[command_set5-root tests/test_command.py::TestCommand::test_before_validate tests/test_command.py::TestCommand::test_pre_hook[commands0] tests/test_command.py::TestCommand::test_pre_hook[commands1] tests/test_command.py::TestCommand::test_pre_hook[commands2] tests/test_command.py::TestCommand::test_after_validate tests/test_command.py::TestCommand::test_pre_hook_validated[commands0] tests/test_command.py::TestCommand::test_pre_hook_validated[commands1] tests/test_command.py::TestCommand::test_pre_hook_validated[commands2] tests/test_command.py::TestCommand::test_create_default_parser