Python Exception Handling | try-except· else

Python Exception Handling | try-except· else

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Python Exception Handling | try-except· else

이 글의 핵심

Learn Python exceptions: try-except-else-finally, raising errors, custom exception classes, and safe patterns for files, I/O, and retries—with examples.

Introduction

“Handling errors gracefully”

Exception handling is central to building stable, maintainable Python programs.

1. Basic exception handling

try-except

# Basic form
try:
    result = 10 / 0
except ZeroDivisionError:
    print("Cannot divide by zero")
    result = None
# Multiple exception types
try:
    number = int(input("Enter a number: "))
    result = 10 / number
except ValueError:
    print("Please enter a valid number")
except ZeroDivisionError:
    print("Enter a non-zero number")
# Bind the exception object
try:
    file = open('missing.txt', 'r')
except FileNotFoundError as e:
    print(f"Error: {e}")

2. try-except-else-finally

Full structure

try:
    # Code that might fail
    file = open('data.txt', 'r')
    content = file.read()
except FileNotFoundError:
    # Runs when an exception occurs
    print("File not found")
else:
    # Runs only if no exception in try
    print(f"Read OK: {len(content)} characters")
finally:
    # Always runs (cleanup)
    if 'file' in locals():
        file.close()
    print("Done")

3. Raising exceptions (raise)

Basic raise

def divide(a, b):
    if b == 0:
        raise ValueError("b cannot be zero")
    return a / b
try:
    result = divide(10, 0)
except ValueError as e:
    print(f"Error: {e}")

Re-raising

def process_data(data):
    try:
        result = int(data)
    except ValueError:
        print("Conversion failed")
        raise  # propagate to caller
try:
    process_data("abc")
except ValueError:
    print("Handled at outer level")

4. Custom exceptions

User-defined exceptions

class InsufficientBalanceError(Exception):
    """Raised when withdrawal exceeds balance."""
    def __init__(self, balance, amount):
        self.balance = balance
        self.amount = amount
        super().__init__(f"Insufficient balance: {balance} (needed: {amount})")
class BankAccount:
    def __init__(self, owner, balance):
        self.owner = owner
        self.balance = balance
    
    def withdraw(self, amount):
        if amount > self.balance:
            raise InsufficientBalanceError(self.balance, amount)
        self.balance -= amount
        return self.balance
# Usage
account = BankAccount("Alice", 10000)
try:
    account.withdraw(15000)
except InsufficientBalanceError as e:
    print(e)  # Insufficient balance: 10000 (needed: 15000)
    print(f"Current balance: {e.balance}")

5. Common built-in exceptions

Frequently used types

# ValueError: wrong value
try:
    int("abc")
except ValueError:
    print("Conversion failed")
# TypeError: wrong types
try:
    "hello" + 5
except TypeError:
    print("Type mismatch")
# KeyError: missing dict key
try:
    data = {'name': 'Alice'}
    print(data['age'])
except KeyError:
    print("Key missing")
# IndexError: index out of range
try:
    arr = [1, 2, 3]
    print(arr[10])
except IndexError:
    print("Index out of range")
# FileNotFoundError: missing file
try:
    open('missing.txt', 'r')
except FileNotFoundError:
    print("File not found")

6. Practical examples

Safe JSON file reading

import json
def safe_read_json(filename):
    """Read a JSON file safely."""
    try:
        with open(filename, 'r', encoding='utf-8') as f:
            return json.load(f)
    except FileNotFoundError:
        print(f"File not found: {filename}")
        return {}
    except json.JSONDecodeError as e:
        print(f"JSON parse error: {e}")
        return {}
    except Exception as e:
        print(f"Unexpected error: {e}")
        return {}

Retry logic

import time
def retry_operation(func, max_attempts=3):
    """Retry func until success or attempts exhausted."""
    for attempt in range(max_attempts):
        try:
            return func()
        except Exception as e:
            print(f"Attempt {attempt + 1} failed: {e}")
            if attempt < max_attempts - 1:
                time.sleep(1)
            else:
                raise
# Usage
def unstable_operation():
    import random
    if random.random() < 0.7:
        raise ConnectionError("connection failed")
    return "success"
try:
    result = retry_operation(unstable_operation)
    print(result)
except Exception as e:
    print(f"Final failure: {e}")

Practical tips

Exception-handling best practices

# ✅ Catch specific exceptions
try:
    value = int(user_input)
except ValueError:
    print("Please enter a number")
# ❌ Bare except Exception (harder to debug)
try:
    value = int(user_input)
except Exception:
    print("Something went wrong")
# ✅ Use the exception message
try:
    file = open('data.txt', 'r')
except FileNotFoundError as e:
    print(f"File error: {e}")
# ✅ Prefer context managers over manual close in finally
try:
    with open('data.txt', 'r') as file:
        pass
finally:
    pass  # file already closed by with

Summary

Key takeaways

  1. try-except: handle expected failures.
  2. finally: always runs—use for cleanup when not using with.
  3. raise: signal errors; bare raise re-raises the current exception.
  4. Custom exceptions: subclass Exception and store context on self.
  5. Best practice: catch narrow types; avoid swallowing bugs with overly broad handlers.

Next steps

Frequently Asked Questions (FAQ)

Q. When would I use this in practice?

A. Learn Python exceptions: try-except-else-finally, raising errors, custom exception classes, and safe patterns for files,…

Q. What should I read before this?

A. Follow the previous article or related articles links at the bottom of each post to learn in sequence. See the Python series index for the full picture.

Q. Where can I study this more deeply?

A. Check cppreference and the relevant library’s official documentation. The reference links at the end of the article are also worth using.

Other articles related to this topic.

Keywords Covered in This Article (Related Search Terms)

This article covers Python, Exception, try-except, raise, Error Handling, finally.