Week 4 • Python Course

Week 4: Dictionaries, Sets & File I/O

Store key-value pairs, work with unique collections, read and write files — plus remote collaboration on GitHub.

Week 4

📅 Overview: Days 1–4 cover dictionaries, sets, and file I/O. Day 5 is a mini-project combining all concepts. Day 6 reinforces GitHub remote collaboration—pull, push, fetch, merge, and resolving conflicts.

🔗 Prerequisite: Week 1 (variables, conditionals, git basics), Week 2 (loops, git branching), Week 3 (lists, tuples, git push/pull).

🎯 Goal: By the end of this week, you'll confidently use dictionaries for fast lookups, sets for unique data, files for persistent storage, and collaborate effectively on GitHub.

Dictionaries

key value lookup

🔢

Sets

unique union intersect

📂

File I/O

read write open

📝

More File I/O

with modes error

🗂️

Mini Project

phonebook CRUD

🔄

GitHub Collab

fetch merge conflict

Day 1 Dictionaries: Key-Value Stores

Learning Objectives: Create dictionaries, access values via keys, add/update entries, use dictionary methods (keys(), values(), items()), and handle missing keys safely with get().

📖 Dictionary Quick Reference Infographic

"name""Amit"

"age"14

"grade""A"

Creating & Accessing

day1_create.py

# Empty dictionary
empty = {}
# Pre-filled dictionary
student = {"name": "Amit", "age": 14, "grade": "A"}
# Access by key
print(student["name"])        # Amit
# Using get() to avoid KeyError
print(student.get("marks", 0)) # 0 (default)

Adding, Updating, Deleting

day1_modify.py

student["city"] = "Delhi"     # add
student["age"] = 15           # update
del student["grade"]          # delete
print(student) # {'name':'Amit','age':15,'city':'Delhi'}

Looping & Methods

day1_looping.py

for key in student:
    print(key, student[key])
for value in student.values():
    print(value)
for key, value in student.items():
    print(f"{key}: {value}")

🧑‍💻 Exercise: Student Dictionary

Create a dictionary with your name, age, and favourite subject. Print all keys and values using items().

day1_exercise_solution.py

me = {"name": "Riya", "age": 13, "subject": "Math"}
for k, v in me.items():
    print(f"{k}: {v}")

💡 Key takeaway: Dictionaries are unordered* (Python 3.7+ preserves insertion order). Use them for fast lookups by a unique key.

Day 2 Sets: Unique Collections

Learning Objectives: Understand that sets store unique, unordered items. Perform union, intersection, difference. Use set methods like add(), remove(), discard().

🔢 Set Operations Infographic

Union

A | B
All unique items

Intersection

A & B
Common items

Difference

A - B
In A not in B

Set Basics

day2_basics.py

empty_set = set()   # NOT {} (that's dict)
nums = {1, 2, 3, 2, 1}
print(nums)          # {1, 2, 3} duplicates removed
nums.add(4)
nums.discard(2)      # no error if missing

Set Operations

day2_operations.py

a = {1, 2, 3}
b = {2, 3, 4}
print(a | b)  # Union: {1, 2, 3, 4}
print(a & b)  # Intersection: {2, 3}
print(a - b)  # Difference: {1}
print(b - a)  # {4}

🧑‍💻 Exercise: Unique Words

Ask user for a sentence, split into words, create a set to find unique words.

day2_exercise_solution.py

sentence = input("Enter a sentence: ")
words = sentence.split()
unique = set(words)
print(f"Unique words ({len(unique)}): {unique}")

⚠️ Note: Sets are unordered, so you cannot access by index. Convert to list if order matters.

Day 3 File I/O: Read & Write

Learning Objectives: Open files in different modes ('r', 'w', 'a'). Read entire file content, read lines, write strings. Always close files.

📂 File Modes Infographic

'r'
read

'w'
write (overwrites)

'a'
append

'r+'
read+write

Writing to a File

day3_write.py

f = open("data.txt", "w")
f.write("Hello, Python!\n")
f.write("Week 4 is fun.")
f.close()

Reading from a File

day3_read.py

f = open("data.txt", "r")
content = f.read()        # whole string
print(content)
f.seek(0)                 # reset cursor
lines = f.readlines()     # list of strings
f.close()

Appending to a File

day3_append.py

with open("data.txt", "a") as f:
    f.write("\nNew line added!")

🧑‍💻 Exercise: Log Writer

Write a program that asks for user input and appends it to a file called "log.txt" with a timestamp.

day3_exercise_solution.py

from datetime import datetime
entry = input("Log entry: ")
with open("log.txt", "a") as f:
    f.write(f"{datetime.now()}: {entry}\n")
print("Logged!")

💡 Best Practice: Use with statement – it automatically closes the file even if an error occurs.

Day 4 More File I/O & Exceptions

Learning Objectives: Handle file‑related errors with try/except. Work with CSV‑like data. Use with efficiently. Loop over file lines without loading everything into memory.

Reading Line by Line (Memory Efficient)

day4_readlines.py

with open("bigfile.txt", "r") as f:
    for line in f:
        print(line.strip())

Writing a CSV (Comma-Separated)

day4_csv.py

students = [("Amit",14,"A"), ("Sara",15,"B")]
with open("students.csv", "w") as f:
    for name, age, grade in students:
        f.write(f"{name},{age},{grade}\n")

Handling File Errors

day4_exceptions.py

try:
    with open("missing.txt", "r") as f:
        print(f.read())
except FileNotFoundError:
    print("File not found! Creating a new one.")
    with open("missing.txt", "w") as f:
        f.write("Start")

🧑‍💻 Exercise: Grade Reader

Read students.csv and print a dictionary mapping name → grade.

day4_exercise_solution.py

grades = {}
with open("students.csv", "r") as f:
    for line in f:
        name, age, grade = line.strip().split(',')
        grades[name] = grade
print(grades)

⚠️ Remember: Always close files. Prefer with to avoid resource leaks.

Day 5 Mini-Project: Phonebook Manager

Learning Objectives: Build a CRUD phonebook using dictionaries and file storage. Save contacts to a text file, load them on startup. Use sets to find duplicate names.

Project Structure

Load existing contacts from phonebook.txt (name,phone per line).
Provide menu: 1. Add, 2. Search, 3. Delete, 4. View all, 5. Save & Exit.
Use a dictionary: name → phone number.
Use set operations to detect duplicate names before adding.

phonebook.py

import os
contacts = {}
filename = "phonebook.txt"
# Load existing
if os.path.exists(filename):
    with open(filename, "r") as f:
        for line in f:
            name, phone = line.strip().split(',')
            contacts[name] = phone

while True:
    print("\n--- Phonebook ---")
    print("1.Add 2.Search 3.Delete 4.View 5.Save & Exit")
    choice = input("Choice: ")
    if choice == '1':
        name = input("Name: ")
        if name in contacts:
            print("Contact exists. Use a different name.")
            continue
        phone = input("Phone: ")
        contacts[name] = phone
        print("Added.")
    elif choice == '2':
        name = input("Name to search: ")
        print(contacts.get(name, "Not found"))
    elif choice == '3':
        name = input("Name to delete: ")
        if contacts.pop(name, None):
            print("Deleted.")
        else:
            print("Not found.")
    elif choice == '4':
        for n, p in contacts.items():
            print(f"{n}: {p}")
    elif choice == '5':
        with open(filename, "w") as f:
            for n, p in contacts.items():
                f.write(f"{n},{p}\n")
        print("Saved. Goodbye!")
        break

🚀 Extension: Add input validation (phone numbers only digits), or a “list duplicates” feature using a set.

Day 6 GitHub Collaboration: Fetch, Merge & Conflicts

Learning Objectives: Understand remote tracking branches. Use git fetch and git pull. Merge branches and resolve simple conflicts. Simulate a collaborative workflow.

🔄 Remote Collaboration Visual

💻 Local
feature branch

⬆️ push

☁️ GitHub

⬇️ fetch

💻 origin/main

fetch = download updates without merging | pull = fetch + merge

Important Commands

Command	Purpose
`git fetch origin`	Download remote changes, don't integrate
`git merge origin/main`	Merge fetched changes into current branch
`git pull origin main`	Fetch + merge in one step
`git push origin feature`	Upload branch to remote

Resolving a Simple Conflict

Conflict occurs when two branches change the same line. Git marks the file like:

conflict_example.txt

<<<<<<< HEAD
print("Hello from main")
=======
print("Hello from feature")
>>>>>>> feature

Edit file to keep the correct version, remove the markers, then git add and git commit.

Collaboration Flow (Student Practice)

terminal simulation

# Partner A pushes
git push origin main
# Partner B gets updates
git fetch origin
git merge origin/main
# If conflict → resolve → add → commit
git push origin main

💡 Pro Tip: Always git pull before starting new work. Use feature branches and pull requests (GitHub UI) for team projects.

Week 4 Practice Problems

🔰 Beginner Level

1. Word Frequency (Dictionary)

text = "apple banana apple orange banana apple"
words = text.split()
freq = {}
for w in words:
    freq[w] = freq.get(w, 0) + 1
print(freq)

2. Common Elements in Two Lists (Set)

list1 = [1,2,3,4]
list2 = [3,4,5,6]
common = set(list1) & set(list2)
print(common)  # {3,4}

⚡ Intermediate Level

3. File Copy (with error handling)

try:
    with open("source.txt","r") as src, open("dest.txt","w") as dest:
        dest.write(src.read())
    print("Copied.")
except FileNotFoundError:
    print("Source file missing.")

Unsolved Exercises

🔰 Beginner

1. Student Grades Dictionary

Create dict from 3 student names & marks. Print average marks.

Write your solution here.

2. Unique Characters (Set)

Input a word, print the set of unique letters.

Write your solution here.

⚡ Intermediate

3. JSON-like file reader

Read a file where each line is "key:value", store in dict.

Write your solution here.