diff --git a/pytchat/tool/asyncdl.py b/pytchat/tool/asyncdl.py
index d227118..e87b74e 100644
--- a/pytchat/tool/asyncdl.py
+++ b/pytchat/tool/asyncdl.py
@@ -13,29 +13,29 @@ headers = config.headers
 REPLAY_URL = "https://www.youtube.com/live_chat_replay/" \
              "get_live_chat_replay?continuation="
 
+def _divide(start, end, count):
+    min_interval = 120
+    if (not isinstance(start,int) or 
+        not isinstance(end,int) or 
+        not isinstance(count,int)):
+        raise ValueError("start/end/count must be int")
+    if start>end:
+        raise ValueError("end must be equal to or greater than start.")
+    if count<1:
+        raise ValueError("count must be equal to or greater than 1.")
+    if (end-start)/count < min_interval:
+        count = int((end-start)/min_interval) 
+        if count == 0 : count = 1
+    interval= (end-start)/count 
+    
+    if count == 1:
+        return [start]
+    return sorted(list(set([int(start+interval*j)
+        for j in range(count) ])))
+
 def ready_blocks(video_id, duration, div, callback):
     if div <= 0: raise ValueError
 
-    def _divide(start, end, count):
-        min_interval = 120
-        if (not isinstance(start,int) or 
-            not isinstance(end,int) or 
-            not isinstance(count,int)):
-            raise ValueError("start/end/count must be int")
-        if start>end:
-            raise ValueError("end must be equal to or greater than start.")
-        if count<1:
-            raise ValueError("count must be equal to or greater than 1.")
-        if (end-start)/count < min_interval:
-            count = int((end-start)/min_interval) 
-            if count == 0 : count = 1
-        interval= (end-start)/count 
-        
-        if count == 1:
-            return [start]
-        return sorted(list(set([int(start+interval*j)
-            for j in range(count) ])))
-
     async def _get_blocks( video_id, duration, div, callback):
         async with aiohttp.ClientSession() as session:
             futures = [_create_block(session, video_id, pos, seektime, callback)
@@ -70,7 +70,7 @@ def ready_blocks(video_id, duration, div, callback):
 
 def download_chunk(callback, blocks):
 
-    async def _dl_distribute():
+    async def _allocate_workers():
         workers = [
             DownloadWorker(
                 fetch = _fetch,
@@ -85,16 +85,15 @@ def download_chunk(callback, blocks):
     async def _fetch(continuation,session):
         url = f"{REPLAY_URL}{quote(continuation)}&pbj=1"
         async with session.get(url,headers = config.headers) as resp:
-            text = await resp.text()
-        continuation, actions = parser.parse(json.loads(text))
+            chat_json = await resp.text()
+        continuation, actions = parser.parse(json.loads(chat_json))
         if actions:
             last = parser.get_offset(actions[-1])
             first = parser.get_offset(actions[0])
             if callback:
-                callback(actions,last-first)
-            return actions,continuation,last
+                callback(actions, last - first)
+            return actions, continuation, last
         return continuation, [], None
     
     loop = asyncio.get_event_loop()
-    loop.run_until_complete(
-        _dl_distribute())
+    loop.run_until_complete(_allocate_workers())
diff --git a/pytchat/tool/block.py b/pytchat/tool/block.py
index 60f650a..39fa162 100644
--- a/pytchat/tool/block.py
+++ b/pytchat/tool/block.py
@@ -1,4 +1,30 @@
 class Block:
+    """Block object represents virtual chunk of chatdata.
+
+    Parameter:
+    ---------
+    pos : int
+        index of this block on block list.
+
+    first : int
+        videoOffsetTimeMs of chat_data[0]
+        
+    last : int
+        videoOffsetTimeMs of the last chat_data current read.
+        (chat_data[-1])
+
+        this value increases as fetching chatdata progresses.
+
+    temp_last : int
+        temporary videoOffsetTimeMs of last chat data,
+        equals to first videoOffsetTimeMs of next block.
+        when download worker reaches this offset, the download will stop.
+
+    continuation : str
+        continuation param of last chat data.
+
+    chat_data : List
+    """
     def __init__(self, pos=0, first=0, last=0,
                 continuation='', chat_data=[]):
         self.pos = pos
diff --git a/pytchat/tool/dlworker.py b/pytchat/tool/dlworker.py
index b70f9ef..914949a 100644
--- a/pytchat/tool/dlworker.py
+++ b/pytchat/tool/dlworker.py
@@ -1,23 +1,37 @@
 from . import parser
 
 class DownloadWorker:
+    """
+    DownloadWorker : associates a download session with a block.
+
+    Parameter
+    ----------
+    fetch : func
+        download function of asyncdl
+
+    block : Block
+        chunk of chat_data
+    """
     def __init__(self, fetch, block):
         self.block = block
         self.fetch = fetch
         
-    async def run(self,session):
+    async def run(self, session):
+        """Remove extra chats just after ready_blocks(). """
         temp_last = self.block.temp_last
         self.block.chat_data, continuation = self.cut(
             self.block.chat_data, 
             self.block.continuation, 
             self.block.last,
             temp_last )
+        """download loop """
         while continuation:
             data, cont, fetched_last = await self.fetch(continuation, session)
             data, continuation = self.cut(data, cont, fetched_last, temp_last)
             self.block.chat_data.extend(data)
         
     def cut(self, data, cont, fetched_last, temp_last):
+        """Remove extra chats."""
         if fetched_last < temp_last or temp_last == -1:
             return data, cont
         for i, line in enumerate(data):
diff --git a/pytchat/tool/downloader.py b/pytchat/tool/downloader.py
index 81d3b96..577a950 100644
--- a/pytchat/tool/downloader.py
+++ b/pytchat/tool/downloader.py
@@ -1,8 +1,3 @@
-import asyncio
-import aiohttp
-import json
-import traceback
-from urllib.parse import quote
 from . import asyncdl
 from . import parser
 from . import videoinfo
@@ -10,7 +5,6 @@ from . block import Block
 from . duplcheck import duplicate_head, duplicate_tail, overwrap
 from .. import config
 from .. exceptions import InvalidVideoIdException
-from .. paramgen import arcparam
 
 logger = config.logger(__name__)
 headers=config.headers
@@ -20,8 +14,8 @@ class Downloader:
         self.video_id = video_id
         self.duration = duration
         self.div = div
-        self.blocks = []
         self.callback = callback
+        self.blocks = []
 
     def ready_blocks(self):
         result = asyncdl.ready_blocks(
diff --git a/pytchat/tool/duplcheck.py b/pytchat/tool/duplcheck.py
index ff5dca9..62d5d69 100644
--- a/pytchat/tool/duplcheck.py
+++ b/pytchat/tool/duplcheck.py
@@ -3,16 +3,16 @@ from . import parser
 def check_duplicate(chatdata):
     max_range = len(chatdata)-1 
     tbl_offset = [None] * max_range
-    tbl_id =[None] * max_range
-    tbl_type=[None] * max_range
+    tbl_id = [None] * max_range
+    tbl_type = [None] * max_range
 
-    def create_table(chatdata,max_range):
+    def create_table(chatdata, max_range):
         for i in range(max_range):
             tbl_offset[i] = parser.get_offset(chatdata[i])
             tbl_id[i] = parser.get_id(chatdata[i]) 
             tbl_type[i] = parser.get_type(chatdata[i])
 
-    def is_duplicate(i,j):
+    def is_duplicate(i, j):
         return ( 
             tbl_offset[i] == tbl_offset[j]
                 and