Exception Handling & File Processing

Ⅶ Exception Handling

01. What is Exception Handling?

Programs can encounter unexpected situations at runtime.

int("hello")    # ValueError
10 / 0          # ZeroDivisionError
[1,2,3][10]     # IndexError

When this happens, Python raises an Exception. If unhandled, the program stops.

Exception handling prepares for these error conditions so the program can keep running.

try / except Basic Structure

try:
    x = int(input("Number: "))
    print(10 / x)
except ValueError:
    print("Please enter a number.")
except ZeroDivisionError:
    print("Cannot divide by zero.")

else / finally

try:
    x = int(input("Number: "))
    result = 10 / x
except (ValueError, ZeroDivisionError) as e:
    print(f"Error: {e}")
else:
    print(f"Result: {result}")   # only runs if no exception
finally:
    print("Always runs")         # runs regardless of exception

Clause	When it runs
try	Always attempted
except	When an exception occurs
else	When no exception occurs
finally	Always (exception or not)

---

02. Common Exception Types

Exception	Cause	Example
ValueError	Wrong type conversion	int("hello")
ZeroDivisionError	Division by zero	10 / 0
IndexError	Index out of range	[1,2][5]
KeyError	Missing dictionary key	{"a":1}["b"]
TypeError	Type mismatch	"1" + 1
FileNotFoundError	File doesn't exist	open("x.txt")
AttributeError	Missing attribute/method	"hi".append(1)

raise — Raising Exceptions Manually

def set_age(age):
    if age < 0:
        raise ValueError("Age must be 0 or greater.")
    return age

---

Ⅷ File Processing

01. What is File Processing?

File processing means opening, reading, writing, and closing files in a program.

File Modes

Mode	Meaning
"r"	Read (default; error if file missing)
"w"	Write (creates new; overwrites existing)
"a"	Append (keeps existing; adds to end)
"x"	Create (error if file already exists)

with Statement (Recommended)

with open("hello.txt", "w") as f:
    f.write("Hello, World!")
# file is automatically closed when the with block ends

---

02. Writing to Files

# write() — write a string
with open("memo.txt", "w", encoding="utf-8") as f:
    f.write("Line 1\n")
    f.write("Line 2\n")

# writelines() — write a list of strings
lines = ["apple\n", "banana\n", "strawberry\n"]
with open("fruits.txt", "w", encoding="utf-8") as f:
    f.writelines(lines)

# append mode — add without erasing
with open("memo.txt", "a", encoding="utf-8") as f:
    f.write("Line 3\n")

---

03. Reading Files

# read() — read entire file
with open("memo.txt", "r", encoding="utf-8") as f:
    content = f.read()

# readline() — read one line at a time
with open("memo.txt", "r", encoding="utf-8") as f:
    line = f.readline()   # "Line 1\n"

# readlines() — read all lines as a list
with open("memo.txt", "r", encoding="utf-8") as f:
    lines = f.readlines()

# for loop — best for large files
with open("memo.txt", "r", encoding="utf-8") as f:
    for line in f:
        print(line.strip())

Combining File I/O with Exception Handling

try:
    with open("data.txt", "r", encoding="utf-8") as f:
        content = f.read()
    print(content)
except FileNotFoundError:
    print("File not found.")

---

Key Summary

Concept	Description
try / except	Handle exceptions — prevent program crash
else	Runs only when no exception
finally	Always runs regardless of exception
raise	Manually trigger an exception
open(path, mode)	Open a file
"w" / "a" / "r"	Write / Append / Read mode
with statement	Auto-close — safe even on exception
encoding="utf-8"	Required for non-ASCII (e.g., Korean) files