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Day 6 — Files & Errors

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🟢 Day 6 — Files & Errors (Making Programs Real and Reliable)

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1️⃣ Why Do We Need Files?

Until now, every program had this problem:
When the program ends, everything is forgotten.

Real-world problem

Student records disappear
Expense data resets
Scores are lost
💡 Programs need memory that survives after closing. That memory is called a file.
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2️⃣ Real-World Analogy

Think of:
Notebook → file
Writing → saving data
Reading → loading data
🧠 File = long-term memory Variables = short-term memory
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3️⃣ Writing Data to a File

Basic Example

file = open("data.txt", "w")
file.write("Hello World")
file.close()

How Computer Thinks

Open file in write mode
Write text
Close file (VERY IMPORTANT)
🧠 If you don’t close the file, data may not be saved.
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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
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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.
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5️⃣ Reading Data from a File

file = open("data.txt", "r")
content = file.read()
file.close()

print(content)
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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.
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6️⃣ The with Statement (Best Practice)

with open("data.txt", "r") as file:
content = file.read()
Why better?
Automatically closes file
Safer
Cleaner code
🧠 Always prefer with when possible.
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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.
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8️⃣ Handling Errors Using try / except

try:
x = int(input("Enter number: "))
print(x)
except:
print("Invalid input")

How Computer Thinks

Try code inside try
If error occurs → jump to except
Program continues safely
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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.
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🔁 Key Thinking Pattern of Day 6

Whenever your program:
Talks to user
Reads file
Converts data
Ask:
“What can go wrong here?”
Then handle it.
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⚠️ Common Beginner Mistakes

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