diff --git a/src/collect_others.py b/src/collect_others.py
index 14600be..3af7d8d 100644
--- a/src/collect_others.py
+++ b/src/collect_others.py
@@ -1,3 +1,6 @@
+"""
+This module uses web requests to collect and process other data we are using in our analysis.
+"""
 from dataclasses import dataclass
 
 import requests
diff --git a/src/constants.py b/src/constants.py
index 757e82a..cef8dcb 100644
--- a/src/constants.py
+++ b/src/constants.py
@@ -1,6 +1,10 @@
-# Constants (The instructors said that we can use global constants here:
-# https://piazza.com/class/ksovzjrlsye72f?cid=1664
-# They should not end with "/"
+"""
+This module stores constant variables in our projects.
+
+Instructors said that we can use global constants: https://piazza.com/class/ksovzjrlsye72f?cid=1664
+"""
+
+# Paths, should not end with "/"
 DATA_DIR = '../data'
 TWEETS_DIR = f'{DATA_DIR}/twitter/user-tweets'
 USER_DIR = f'{DATA_DIR}/twitter/user'
diff --git a/src/main.py b/src/main.py
index 4bb6b77..db52016 100644
--- a/src/main.py
+++ b/src/main.py
@@ -1,3 +1,7 @@
+"""
+This module is the main module of our program which runs different functions in different modules
+by steps.
+"""
 from visualization import *
 from collect_twitter import *
 from report import serve_report
diff --git a/src/processing.py b/src/processing.py
index 70eaaea..cd16ad1 100644
--- a/src/processing.py
+++ b/src/processing.py
@@ -162,7 +162,7 @@ def get_english_news_channels() -> list[str]:
 
     Run this after download_all_tweets(api, 'TwitterNews')
 
-    Precondition:
+    Preconditions:
       - <tweets_dir>/user/TwitterNews.json exists.
 
     :return: A list of news channel screen names
@@ -193,7 +193,7 @@ def filter_news_channels() -> None:
     """
     Filter out news channels that don't exist anymore or have been banned by Twitter.
 
-    Precondition:
+    Preconditions:
       - Run this after downloading all tweets from the news channels in Step 2.3 in main.
 
     :return: None
diff --git a/src/report.py b/src/report.py
index 072d279..1d37fbc 100644
--- a/src/report.py
+++ b/src/report.py
@@ -1,3 +1,6 @@
+"""
+This module generates report HTML and serves it in an HTTP server.
+"""
 import json
 import os.path
 import shutil
diff --git a/src/utils.py b/src/utils.py
index 4ee4bec..5b21fc3 100644
--- a/src/utils.py
+++ b/src/utils.py
@@ -3,7 +3,8 @@
 - file I/O
 - statistics functions, removing outliers and averaging values over a period
 - date-related functions
-- classes for configs, reports, statistics, and JSON"""
+- classes for configs, reports, statistics, and JSON
+"""
 
 import dataclasses
 import inspect
@@ -87,6 +88,9 @@ def write(file: str, text: str) -> None:
     """
     Write text to a file
 
+    Preconditions:
+      - file != ''
+
     :param file: File path (will be converted to lowercase)
     :param text: Text
     :return: None
@@ -104,6 +108,9 @@ def read(file: str) -> str:
     """
     Read file content
 
+    Preconditions:
+      - file != ''
+
     :param file: File path (will be converted to lowercase)
     :return: None
     """
@@ -168,6 +175,9 @@ def remove_outliers(points: list[float], z_threshold: float = 3.5) -> list[float
 
     Credit to: https://stackoverflow.com/a/11886564/7346633
 
+    Preconditions:
+      - len(points) > 0
+
     :param points: Input points list
     :param z_threshold: Z threshold for identifying whether or not a point is an outlier
     :return: List with outliers removed
@@ -212,6 +222,9 @@ def get_statistics(points: list[float]) -> Stats:
     """
     Calculate statistics for a set of points
 
+    Preconditions:
+      - len(points) > 0
+
     :param points: Input points
     :return: Statistics
     """
@@ -262,7 +275,7 @@ def parse_date_only(iso: str) -> datetime:
     Parse date faster.
 
     Preconditions:
-      - iso is in the format of "YYYY-MM-DD" (e.g. "2021-10-20")
+      - iso starts with the format of "YYYY-MM-DD" (e.g. "2021-10-20" or "2021-10-20T10:04:14")
       - iso is a valid date (this function does not check for the validity of the input)
 
     :param iso: Input date
@@ -275,6 +288,10 @@ def daterange(start_date: str, end_date: str) -> Generator[tuple[str, datetime],
     """
     Date range for looping, excluding the end date
 
+    Preconditions:
+      - start_date starts with the "YYYY-MM-DD" format
+      - end_date starts with the "YYYY-MM-DD" format
+
     :param start_date: Start date in "YYYY-MM-DD" format
     :param end_date: End date in "YYYY-MM-DD" format
     :return: Generator for looping through the dates one day at a time.
@@ -307,7 +324,7 @@ def filter_days_avg(y: list[float], n: int) -> list[float]:
     """
     Filter y by taking an average over a n-days window. If n = 0, then return y without processing.
 
-    Precondition:
+    Preconditions:
       - n % 2 == 1
       - len(y) > 0
 
@@ -391,6 +408,9 @@ def json_stringify(obj, indent: Union[int, None] = None) -> str:
     Serialize json string with support for dataclasses and datetime and sets and with custom
     configuration.
 
+    Preconditions:
+      - obj != None
+
     :param obj: Objects
     :param indent: Indent size or none
     :return: Json strings
diff --git a/src/visualization.py b/src/visualization.py
index 426f48c..5e394b9 100644
--- a/src/visualization.py
+++ b/src/visualization.py
@@ -87,7 +87,7 @@ class Sample:
         To prevent divide-by-zero, we ignored everyone who didn't post about covid and who didn't
         post at all.
 
-        Precondition:
+        Preconditions:
           - Downloaded tweets data are sorted by date
         """
         debug(f'Calculating sample tweets data for {self.name}...')