Python Try Except：例外を正しく処理する方法

# This crashes the program
result = 10 / 0

Traceback (most recent call last):
  File "example.py", line 2, in <module>
    result = 10 / 0
ZeroDivisionError: division by zero

try:
    result = 10 / 0
except ZeroDivisionError:
    print("Cannot divide by zero.")

Cannot divide by zero.

try:
    result = 10 / 0
except ZeroDivisionError as e:
    print(f"Error: {e}")

Error: division by zero

try:
    number = int("not_a_number")
except ValueError as e:
    print(f"Invalid value: {e}")

Invalid value: invalid literal for int() with base 10: 'not_a_number'

try:
    result = "hello" + 42
except TypeError as e:
    print(f"Type error: {e}")

Type error: can only concatenate str (not "int") to str

try:
    with open("nonexistent_file.txt", "r") as f:
        content = f.read()
except FileNotFoundError:
    print("File not found. Check the file path.")

data = {"name": "Alice", "age": 30}
 
try:
    email = data["email"]
except KeyError as e:
    print(f"Missing key: {e}")

Missing key: 'email'

items = [10, 20, 30]
 
try:
    value = items[5]
except IndexError:
    print("Index out of range.")

def parse_config(raw_value):
    try:
        parts = raw_value.split(":")
        key = parts[0]
        value = int(parts[1])
        return {key: value}
    except IndexError:
        print("Config format error: missing colon separator.")
    except ValueError:
        print("Config format error: value is not a number.")
    except AttributeError:
        print("Config format error: input is not a string.")
 
parse_config("timeout:30")    # Returns {'timeout': 30}
parse_config("timeout")       # Config format error: missing colon separator.
parse_config("timeout:abc")   # Config format error: value is not a number.
parse_config(12345)            # Config format error: input is not a string.

try:
    value = int(input("Enter a number: "))
    result = 100 / value
except (ValueError, ZeroDivisionError) as e:
    print(f"Invalid input: {e}")

try:
    number = int("42")
except ValueError:
    print("That is not a valid number.")
else:
    print(f"Successfully parsed: {number}")

Successfully parsed: 42

filename = "data.txt"
 
try:
    f = open(filename, "r")
except FileNotFoundError:
    print(f"File '{filename}' does not exist.")
else:
    content = f.read()
    f.close()
    print(f"Read {len(content)} characters.")

f = None
try:
    f = open("data.txt", "r")
    content = f.read()
except FileNotFoundError:
    print("File not found.")
finally:
    if f is not None:
        f.close()
        print("File handle closed.")

def divide(a, b):
    try:
        return a / b
    except ZeroDivisionError:
        return None
    finally:
        print("Division attempted.")
 
result = divide(10, 3)
# Output: Division attempted.
# result = 3.3333333333333335
 
result = divide(10, 0)
# Output: Division attempted.
# result = None

import json
 
def load_config(filepath):
    """Load and parse a JSON config file."""
    f = None
    try:
        f = open(filepath, "r")
        data = json.load(f)
    except FileNotFoundError:
        print(f"Config file '{filepath}' not found.")
        return {}
    except json.JSONDecodeError as e:
        print(f"Invalid JSON in '{filepath}': {e}")
        return {}
    else:
        print(f"Config loaded successfully with {len(data)} keys.")
        return data
    finally:
        if f is not None:
            f.close()
            print("File handle released.")
 
config = load_config("settings.json")

def set_age(age):
    if not isinstance(age, int):
        raise TypeError(f"Age must be an integer, got {type(age).__name__}")
    if age < 0 or age > 150:
        raise ValueError(f"Age must be between 0 and 150, got {age}")
    return age
 
# Valid usage
print(set_age(25))  # 25
 
# Invalid usage
try:
    set_age(-5)
except ValueError as e:
    print(e)  # Age must be between 0 and 150, got -5
 
try:
    set_age("thirty")
except TypeError as e:
    print(e)  # Age must be an integer, got str

import logging
 
try:
    result = 10 / 0
except ZeroDivisionError:
    logging.error("Division by zero encountered")
    raise  # re-raises the original ZeroDivisionError

class ValidationError(Exception):
    """Raised when input data fails validation."""
    def __init__(self, field, message):
        self.field = field
        self.message = message
        super().__init__(f"Validation failed on '{field}': {message}")
 
class DatabaseConnectionError(Exception):
    """Raised when the database connection fails."""
    pass
 
# Usage
def validate_email(email):
    if "@" not in email:
        raise ValidationError("email", "Must contain @ symbol")
    if "." not in email.split("@")[1]:
        raise ValidationError("email", "Domain must contain a dot")
    return email
 
try:
    validate_email("userexample.com")
except ValidationError as e:
    print(e)          # Validation failed on 'email': Must contain @ symbol
    print(e.field)    # email
    print(e.message)  # Must contain @ symbol

class AppError(Exception):
    """Base exception for this application."""
    pass
 
class ConfigError(AppError):
    pass
 
class NetworkError(AppError):
    pass
 
# Caller can catch all app errors or specific ones
try:
    raise NetworkError("Connection timed out")
except AppError as e:
    print(f"Application error: {e}")

# BAD - catches everything, hides real bugs
try:
    do_something()
except:
    pass
 
# GOOD - catches specific exceptions
try:
    do_something()
except ValueError as e:
    logging.warning(f"Invalid value: {e}")

# BAD - too much code in try
try:
    data = load_data()
    cleaned = clean_data(data)
    result = analyze(cleaned)
    save_results(result)
except Exception as e:
    print(f"Something failed: {e}")
 
# GOOD - narrow try blocks
data = load_data()
cleaned = clean_data(data)
 
try:
    result = analyze(cleaned)
except ValueError as e:
    print(f"Analysis failed: {e}")
    result = default_result()
 
save_results(result)

import logging
 
try:
    process_record(record)
except ValueError as e:
    logging.error(f"Failed to process record {record['id']}: {e}")

# Prefer context managers for resource cleanup
with open("data.txt", "r") as f:
    content = f.read()
# File is automatically closed, even if an exception occurs

import json
 
def read_json_config(filepath):
    """Read a JSON configuration file with proper error handling."""
    try:
        with open(filepath, "r") as f:
            config = json.load(f)
    except FileNotFoundError:
        print(f"Config file not found: {filepath}")
        return None
    except PermissionError:
        print(f"No permission to read: {filepath}")
        return None
    except json.JSONDecodeError as e:
        print(f"Invalid JSON at line {e.lineno}, column {e.colno}: {e.msg}")
        return None
    else:
        print(f"Loaded config with keys: {list(config.keys())}")
        return config
 
config = read_json_config("app_config.json")
if config:
    db_host = config.get("database_host", "localhost")

import urllib.request
import urllib.error
import json
 
def fetch_user(user_id):
    """Fetch user data from an API with retry logic."""
    url = f"https://jsonplaceholder.typicode.com/users/{user_id}"
    max_retries = 3
 
    for attempt in range(1, max_retries + 1):
        try:
            with urllib.request.urlopen(url, timeout=5) as response:
                data = json.loads(response.read().decode())
            return data
        except urllib.error.HTTPError as e:
            if e.code == 404:
                print(f"User {user_id} not found.")
                return None
            print(f"HTTP error {e.code} on attempt {attempt}")
        except urllib.error.URLError as e:
            print(f"Network error on attempt {attempt}: {e.reason}")
        except json.JSONDecodeError:
            print("API returned invalid JSON.")
            return None
 
    print(f"All {max_retries} attempts failed.")
    return None
 
user = fetch_user(1)
if user:
    print(f"Found user: {user['name']}")

import csv
 
def process_sales_data(filepath):
    """Process a CSV file with robust error handling."""
    results = []
 
    try:
        with open(filepath, "r", newline="") as f:
            reader = csv.DictReader(f)
            for row_num, row in enumerate(reader, start=2):
                try:
                    amount = float(row["amount"])
                    quantity = int(row["quantity"])
                    results.append({
                        "product": row["product"],
                        "total": amount * quantity,
                    })
                except KeyError as e:
                    print(f"Row {row_num}: Missing column {e}")
                except ValueError as e:
                    print(f"Row {row_num}: Invalid number - {e}")
    except FileNotFoundError:
        print(f"File not found: {filepath}")
    except PermissionError:
        print(f"Cannot read file: {filepath}")
 
    return results

# LBYL (Look Before You Leap) - using if/else
if "email" in user_data:
    email = user_data["email"]
else:
    email = "unknown"
 
# EAFP (Easier to Ask Forgiveness) - using try/except
try:
    email = user_data["email"]
except KeyError:
    email = "unknown"