Improve startup time with lazy loading some args (#1221)

* Improve startup time with lazy loading some args

* add some tests

* Add changelog entry

* Update CHANGELOG.md

Co-authored-by: Jakub Roztocil <jakub@roztocil.co>

tests/test_cli_utils.py

@@ -0,0 +1,86 @@
+import pytest
+from argparse import ArgumentParser
+from unittest.mock import Mock
+from httpie.cli.utils import LazyChoices
+
+
+def test_lazy_choices():
+    mock = Mock()
+    getter = mock.getter
+    getter.return_value = ['a', 'b', 'c']
+
+    parser = ArgumentParser()
+    parser.register('action', 'lazy_choices', LazyChoices)
+    parser.add_argument(
+        '--option',
+        help="the regular option",
+        default='a',
+        metavar='SYMBOL',
+        choices=['a', 'b'],
+    )
+    parser.add_argument(
+        '--lazy-option',
+        help="the lazy option",
+        default='a',
+        metavar='SYMBOL',
+        action='lazy_choices',
+        getter=getter,
+        cache=False  # for test purposes
+    )
+
+    # Parser initalization doesn't call it.
+    getter.assert_not_called()
+
+    # If we don't use --lazy-option, we don't retrieve it.
+    parser.parse_args([])
+    getter.assert_not_called()
+
+    parser.parse_args(['--option', 'b'])
+    getter.assert_not_called()
+
+    # If we pass a value, it will retrieve to verify.
+    parser.parse_args(['--lazy-option', 'c'])
+    getter.assert_called()
+    getter.reset_mock()
+
+    with pytest.raises(SystemExit):
+        parser.parse_args(['--lazy-option', 'z'])
+    getter.assert_called()
+    getter.reset_mock()
+
+
+def test_lazy_choices_help():
+    mock = Mock()
+    getter = mock.getter
+    getter.return_value = ['a', 'b', 'c']
+
+    help_formatter = mock.help_formatter
+    help_formatter.return_value = '<my help>'
+
+    parser = ArgumentParser()
+    parser.register('action', 'lazy_choices', LazyChoices)
+    parser.add_argument(
+        '--lazy-option',
+        default='a',
+        metavar='SYMBOL',
+        action='lazy_choices',
+        getter=getter,
+        help_formatter=help_formatter,
+        cache=False  # for test purposes
+    )
+
+    # Parser initalization doesn't call it.
+    getter.assert_not_called()
+
+    # If we don't use `--help`, we don't use it.
+    parser.parse_args([])
+    getter.assert_not_called()
+    help_formatter.assert_not_called()
+
+    parser.parse_args(['--lazy-option', 'b'])
+    help_formatter.assert_not_called()
+
+    # If we use --help, then we call it with styles
+    with pytest.raises(SystemExit):
+        parser.parse_args(['--help'])
+    help_formatter.assert_called_once_with(['a', 'b', 'c'])