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Student records disappearExpense data resetsScores are lost
Notebook → fileWriting → saving dataReading → loading data
Open file in write modeWrite textClose file (VERY IMPORTANT)
Automatically closes fileSaferCleaner code
Try code inside tryIf error occurs → jump to exceptProgram continues safely
Talks to userReads fileConverts data
Forgetting to close fileUsing "w" instead of "a"Not handling invalid inputAssuming file always existsPutting too much code inside try
Use .split() to count wordsUse counter variable for linesConvert string to int carefullyUse try / except only where neededAlways close files or use withAssume input can be wrong
Save data permanentlyRead data from filesHandle errors safelyWrite programs that don’t crashThink about failure cases
Why do programs need files?Difference between write and append?Why is with better?Why should errors be handled?
A full mini-projectCombining all conceptsHow to think like a programmerConfidence-building final day
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Day 6 — Files & Errors
🟢 Day 6 — Files & Errors (Making Programs Real and Reliable)
1️⃣ Why Do We Need Files?
Until now, every program had this problem:
When the program ends, everything is forgotten.
Real-world problem
💡 Programs need memory that survives after closing. That memory is called a file.
2️⃣ Real-World Analogy
Think of:
🧠 File = long-term memory Variables = short-term memory
3️⃣ Writing Data to a File
Basic Example
file = open("data.txt", "w")
file.write("Hello World")
file.close()
How Computer Thinks
🧠 If you don’t close the file, data may not be saved.
4️⃣ File Modes (Only What You Need)
Mode
Meaning
"w"
Write (overwrites file)
"a"
Append (adds to file)
"r"
Read
There are no rows in this table
Append Example
file = open("data.txt", "a")
file.write("\nNew Line")
file.close()
💡 Append is safer than write if you don’t want to lose data.
5️⃣ Reading Data from a File
file = open("data.txt", "r")
content = file.read()
file.close()
print(content)
Reading Line by Line
file = open("data.txt", "r")
for line in file:
print(line)
file.close()
🧠 Line-by-line reading is used for large files.
6️⃣ The with Statement (Best Practice)
with open("data.txt", "r") as file:
content = file.read()
Why better?
🧠 Always prefer with when possible.
7️⃣ Errors Are Normal (Crashes Are Not)
Example Crash
x = int(input("Enter number: "))
If user enters "abc" → ❌ program crashes.
💡 Errors WILL happen in real life.
8️⃣ Handling Errors Using try / except
try:
x = int(input("Enter number: "))
print(x)
except:
print("Invalid input")
How Computer Thinks
9️⃣ Handling File Errors
File Not Found Example
try:
with open("missing.txt", "r") as file:
print(file.read())
except:
print("File does not exist")
🧠 Programs should fail gracefully, not crash.
🔁 Key Thinking Pattern of Day 6
Whenever your program:
Ask:
“What can go wrong here?”
Then handle it.
⚠️ Common Beginner Mistakes
🧪 Practice Questions (Very Important)
Think → write steps → then code.
Q1
Write a program to take user input and save it to a file.
Q2
Read a file and print number of lines in it.
Q3
Read a file and count number of words.
Q4
Ask user for filename and display its content.
Handle case when file does not exist.
Q5
Take numbers from user until user types "stop".
Save all numbers to a file.
Q6
Read numbers from file and print their sum.
Q7 (Thinking Question)
Why should we not trust user input or file content?
Answer in words.
🔑 HINTS (Read Together, Only If Stuck)
✅ What You Should Be Able to Do Now
After Day 6, you should:
🧠 Reflection Questions (Answer in Words)
⭐ Golden Rule from Day 6
A real program is not the one that works once, but the one that works even when things go wrong.
If you say “Continue with Day 7”, next we’ll do:
One last step to complete the journey.
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