diff --git a/libpurple_to_markdown/__main__.py b/libpurple_to_markdown/__main__.py
index ba8f893..cfd7f91 100644
--- a/libpurple_to_markdown/__main__.py
+++ b/libpurple_to_markdown/__main__.py
@@ -1,7 +1,9 @@
 import argparse
 import dataclasses
 import logging
-from collections.abc import Iterable, Iterator
+import datetime
+from collections.abc import Iterable, Iterator, Mapping
+from typing import Callable
 from pathlib import Path
 
 from . import (
@@ -16,16 +18,16 @@ from .markdown import format_messages
 logger = logging.getLogger(__name__)
 
 
-def group_messages(messages: Iterable[Message], key) -> dict[str, list[Message]]:
+def group_messages(messages: Iterable[Message], key_fn: Callable[[Message], str]) -> dict[str, list[Message]]:
     by_key: dict[str, list[Message]] = {}
     for msg in messages:
-        by_key.setdefault(key(msg), []).append(msg)
+        by_key.setdefault(key_fn(msg), []).append(msg)
         del msg
     return by_key
 
 
 def group_messages_by_chat_id(messages: Iterable[Message]) -> dict[str, list[Message]]:
-    return group_messages(messages, key=lambda msg: msg.chat_id)
+    return group_messages(messages, key_fn=lambda msg: msg.chat_id)
 
 
 def year_and_month_period_key(msg: Message):
@@ -43,7 +45,9 @@ def year_quarter_period_key(msg: Message):
 
 MAX_AVERAGE_MESSAGES_PER_PERIOD = 120
 
-PERIOD_KEYS_BY_NAME = {
+TOO_FEW_MESSAGES_TO_CARE = 2
+
+PERIOD_KEYS_BY_NAME: Mapping[str, Callable[[Message], str]] = {
         'full': (lambda msg: 'full'),
         'year': year_period_key,
         'quarter': year_quarter_period_key,
@@ -51,12 +55,16 @@ PERIOD_KEYS_BY_NAME = {
 }
 
 
-def group_messages_by_period(messages: Iterable[Message], period_key: str | None = None) -> dict[str, list[Message]]:
-    possible_period_keys = PERIOD_KEYS_BY_NAME.values()
+def group_messages_by_period(messages: Iterable[Message], period_key: str | None = None) -> tuple[dict[str, list[Message]], Callable[[Message], str]]:
+    # Determine key function
+    possible_period_keys: Iterable[Callable[[Message], str]] = PERIOD_KEYS_BY_NAME.values()
     if period_key is not None:
         possible_period_keys  = [PERIOD_KEYS_BY_NAME[period_key]]
+    del period_key
+
+    # Group by key
     for period_key_fn in possible_period_keys:
-        grouped = group_messages(messages, key=period_key)
+        grouped = group_messages(messages, key_fn=period_key_fn)
         average_num_messages = sum(len(grouped[k]) for k in grouped) / len(grouped)
         if average_num_messages <= MAX_AVERAGE_MESSAGES_PER_PERIOD:
             break
@@ -81,12 +89,10 @@ def parse_args():
     parser.add_argument('--output', type=Path)
     parser.add_argument('--myself', type=str, default='Myself')
     parser.add_argument('--overwrite', action='store_true', dest='overwrite_files')
-    parser.add_argument('--period', dest='period_key', values=list(PERIOD_KEYS_BY_NAME.keys()))
+    parser.add_argument('--period', dest='period_key', choices=list(PERIOD_KEYS_BY_NAME.keys()))
     parser.add_argument('--skip-this-period', action='store_true', dest='skip_this_period')
     return parser.parse_args()
 
-
-
 def main():
     logging.basicConfig()
     logging.getLogger().setLevel('INFO')
@@ -115,7 +121,7 @@ def main():
 
     for chat_id, messages_in_chat_original in messages_by_chat_id.items():
         messages_in_chat = merge_adjacent_messages(messages_in_chat_original)
-        if len(messages_in_chat) <= 2:
+        if len(messages_in_chat) <= TOO_FEW_MESSAGES_TO_CARE:
             logger.info('  "%s": Skipped due to too few messages', chat_id)
             continue
 
@@ -128,7 +134,7 @@ def main():
             len(messages_in_chat_original) / len(messages_by_period),
         )
 
-        this_period_name = period_key_fn(datetime.datetime.now())
+        this_period_name = period_key_fn(Message(datetime.datetime.now(), '','',''))
 
         for period_key_name, messages in messages_by_period.items():
             file_escaped_chat_id = chat_id.replace(' ', '-')
@@ -146,7 +152,7 @@ def main():
                 continue
 
             # Create folders and file
-            output_file.parent.mkdir(exist_ok=True)
+            output_file.parent.mkdir(exist_ok=True, parents=True)
             with open(output_file, 'w') as f:
                 f.write(
                     format_messages(