25. 例外をうまく処理する

try:
    # 例外を発生させる可能性のあるコード
    risky_operation()
except:
    # ANY の例外が発生した場合に実行されるコード
    print("Something went wrong!")

# 例外処理なし - プログラムがクラッシュする
user_input = "hello"
number = int(user_input)  # ValueError: invalid literal for int() with base 10: 'hello'
print("This line never executes")

# 例外処理あり - プログラムは続行する
user_input = "hello"
 
try:
    number = int(user_input)
    print(f"You entered: {number}")
except:
    print("That's not a valid number!")
    number = 0  # デフォルト値を使う
 
print(f"Using number: {number}")

That's not a valid number!
Using number: 0

# 例外で実行の流れを観察する
print("1. Starting program")
 
try:
    print("2. Entered try block")
    number = int("hello")  # 例外は HERE で発生する
    print("3. After conversion")   # この行は NEVER 実行されない
    result = number * 2            # この行は NEVER 実行されない
    print("4. After calculation")  # この行は NEVER 実行されない
except ValueError:
    print("5. In except block - handling the error")
 
print("6. After try-except block")

1. Starting program
2. Entered try block
5. In except block - handling the error
6. After try-except block

# 例外処理を使った安全な年齢入力
print("Please enter your age:")
user_input = input()
 
try:
    age = int(user_input)
    print(f"You are {age} years old.")
    
    # 生年を計算する（現在の年が 2024 年だと仮定）
    birth_year = 2024 - age
    print(f"You were born around {birth_year}.")
except:
    print("Invalid input! Age must be a number.")
    print("Using default age of 0.")
    age = 0

Please enter your age:
25
You are 25 years old.
You were born around 1999.

Please enter your age:
twenty-five
Invalid input! Age must be a number.
Using default age of 0.

# try ブロックに 2 つの操作
print("Enter a number:")
user_input = input()
 
try:
    number = int(user_input)      # 1つ目の操作 - ValueError を発生させる可能性がある
    result = 100 / number          # 2つ目の操作 - ZeroDivisionError を発生させる可能性がある
    print(f"100 / {number} = {result}")
except:
    print("Something went wrong!")

# try ブロックに複数の操作
print("Enter two numbers to divide:")
numerator_input = input("Numerator: ")
denominator_input = input("Denominator: ")
 
try:
    numerator = int(numerator_input)      # ValueError を発生させる可能性がある
    denominator = int(denominator_input)  # ValueError を発生させる可能性がある
    result = numerator / denominator      # ZeroDivisionError を発生させる可能性がある
    print(f"{numerator} / {denominator} = {result}")
except:
    print("Something went wrong with the calculation!")
    print("Make sure you enter valid numbers and don't divide by zero.")

Enter two numbers to divide:
Numerator: 10
Denominator: 2
10 / 2 = 5.0

Enter two numbers to divide:
Numerator: 10
Denominator: zero
Something went wrong with the calculation!
Make sure you enter valid numbers and don't divide by zero.

Enter two numbers to divide:
Numerator: 10
Denominator: 0
Something went wrong with the calculation!
Make sure you enter valid numbers and don't divide by zero.

# bare except は EVERYTHING を捕捉する - 想定していないものまで
numbers = [10, 20, 30]
 
try:
    index = 5  # index が範囲外なら IndexError を想定する
    value = numbers[index]
    print(f"Value at index {index}: {value}")
except:
    print("Could not access the list element.")

# コードにタイプミスがあったら？
numbers = [10, 20, 30]
 
try:
    index = 2
    value = numbrs[index]  # Typo: 'numbers' の代わりに 'numbrs'
    print(f"Value at index {index}: {value}")
except:
    print("Could not access the list element.")

Could not access the list element.

# 特定の例外型を捕捉する
user_input = "hello"
 
try:
    number = int(user_input)
    print(f"You entered: {number}")
except ValueError:
    print("That's not a valid number!")
    number = 0
 
print(f"Using number: {number}")

That's not a valid number!
Using number: 0

# 間違った例外型を捕捉する
user_input = "hello"
 
try:
    number = int(user_input)  # これは ValueError を発生させる
    print(f"You entered: {number}")
except TypeError:  # しかし TypeError を捕捉している！
    print("That's not a valid number!")
    number = 0
 
print(f"Using number: {number}")

Traceback (most recent call last):
  File "example.py", line 4, in <module>
    number = int(user_input)
ValueError: invalid literal for int() with base 10: 'hello'

# 例外ごとに異なる扱いをする
print("Enter two numbers to divide:")
numerator_input = input("Numerator: ")
denominator_input = input("Denominator: ")
 
try:
    numerator = int(numerator_input)
    denominator = int(denominator_input)
    result = numerator / denominator
    print(f"{numerator} / {denominator} = {result}")
except ValueError:
    print("Error: Both inputs must be valid integers.")
    print("You entered something that isn't a number.")
except ZeroDivisionError:
    print("Error: Cannot divide by zero.")
    print("The denominator must be a non-zero number.")

Enter two numbers to divide:
Numerator: 10
Denominator: abc
Error: Both inputs must be valid integers.
You entered something that isn't a number.

Enter two numbers to divide:
Numerator: 10
Denominator: 0
Error: Cannot divide by zero.
The denominator must be a non-zero number.

# 複数の例外型をまとめて捕捉する
print("Enter a number:")
user_input = input()
 
try:
    number = int(user_input)
    result = 100 / number
    print(f"100 divided by {number} is {result}")
except (ValueError, ZeroDivisionError):
    print("Invalid input or division by zero.")
    print("Please enter a non-zero number.")

Enter a number:
hello
Invalid input or division by zero.
Please enter a non-zero number.

Enter a number:
0
Invalid input or division by zero.
Please enter a non-zero number.

# 例外オブジェクトの内容を調べる
try:
    number = int("hello")
except ValueError as error:
    print("Exception caught! Let's examine it:")
    print(f"Type: {type(error)}")
    print(f"String representation: {error}")
    print(f"Args tuple: {error.args}")

Exception caught! Let's examine it:
Type: <class 'ValueError'>
String representation: invalid literal for int() with base 10: 'hello'
Args tuple: ("invalid literal for int() with base 10: 'hello'",)

# 例外によって異なる属性を持つ
numbers = [10, 20, 30]
 
try:
    value = numbers[10]
except IndexError as error:
    print(f"IndexError message: {error}")
    print(f"Exception args: {error.args}")
 
# 次は辞書で試す
grades = {"Alice": 95}
 
try:
    grade = grades["Bob"]
except KeyError as error:
    print(f"KeyError message: {error}")
    print(f"Missing key: {error.args[0]}")

IndexError message: list index out of range
Exception args: ('list index out of range',)
KeyError message: 'Bob'
Missing key: Bob

# 成功時のみ実行するコードに else を使う
print("Enter a number:")
user_input = input()
 
try:
    number = int(user_input)
except ValueError:
    print("That's not a valid number!")
else:
    # int(user_input) が成功した場合のみ実行される
    print(f"Successfully converted: {number}")
    squared = number ** 2
    print(f"The square of {number} is {squared}")

Enter a number:
5
Successfully converted: 5
The square of 5 is 25

Enter a number:
hello
That's not a valid number!

# else が例外スコープに役立つ理由を示す
try:
    number_1 = int(input("Enter a number_1: "))
except ValueError:
    print("Invalid input!")
else:
    # ここでエラーが発生しても、上の except では捕捉されない
    # これにより入力エラーと処理エラーを区別しやすくなる
    number_2 = int(input("Enter a number_2: ")) # ValueError を発生させる可能性がある

# 後始末に finally を使う
print("Enter a number:")
user_input = input()
 
try:
    number = int(user_input)
    result = 100 / number
    print(f"Result: {result}")
except ValueError:
    print("Invalid number!")
except ZeroDivisionError:
    print("Cannot divide by zero!")
finally:
    print("Calculation attempt completed.")

Enter a number:
5
Result: 20.0
Calculation attempt completed.

Enter a number:
hello
Invalid number!
Calculation attempt completed.

Enter a number:
0
Cannot divide by zero!
Calculation attempt completed.

# 完全な例外処理構造
print("Enter a number to calculate its reciprocal:")
user_input = input()
 
try:
    # 危険な操作
    number = int(user_input)
    reciprocal = 1 / number
except ValueError:
    # 変換エラーを処理する
    print("Error: Input must be a valid integer.")
except ZeroDivisionError:
    # ゼロ除算を処理する
    print("Error: Cannot calculate reciprocal of zero.")
else:
    # 成功時のみのコード
    print(f"The reciprocal of {number} is {reciprocal}")
    print(f"Verification: {number} × {reciprocal} = {number * reciprocal}")
finally:
    # 常に実行される後始末コード
    print("Reciprocal calculation completed.")

Enter a number to calculate its reciprocal:
4
The reciprocal of 4 is 0.25
Verification: 4 × 0.25 = 1.0
Reciprocal calculation completed.

Enter a number to calculate its reciprocal:
hello
Error: Input must be a valid integer.
Reciprocal calculation completed.

Enter a number to calculate its reciprocal:
0
Error: Cannot calculate reciprocal of zero.
Reciprocal calculation completed.

# 意図的に例外を発生させる
age = -5
 
if age < 0:
    raise ValueError("Age cannot be negative!")
 
print(f"Age: {age}")  # この行は実行されない

Traceback (most recent call last):
  File "example.py", line 5, in <module>
    raise ValueError("Age cannot be negative!")
ValueError: Age cannot be negative!

# 例外を発生させて入力を検証する関数
def calculate_discount(price, discount_percent):
    """割引後価格を計算する。
    
    Args:
        price: 元の価格（正の値である必要がある）
        discount_percent: 割引率（0-100 である必要がある）
    
    Returns:
        割引後価格
    
    Raises:
        ValueError: 入力が不正な場合
    """
    if price < 0:
        raise ValueError("Price cannot be negative!")
    
    if discount_percent < 0 or discount_percent > 100:
        raise ValueError("Discount must be between 0 and 100!")
    
    discount_amount = price * (discount_percent / 100)
    return price - discount_amount
 
# 関数を使う
try:
    final_price = calculate_discount(100, 20)
    print(f"Final price: ${final_price}")
except ValueError as error:
    print(f"Error: {error}")

Final price: $80.0

# 不正な価格
try:
    final_price = calculate_discount(-50, 20)
    print(f"Final price: ${final_price}")
except ValueError as error:
    print(f"Error: {error}")

Error: Price cannot be negative!

# 不正な割引率
try:
    final_price = calculate_discount(100, 150)
    print(f"Final price: ${final_price}")
except ValueError as error:
    print(f"Error: {error}")

Error: Discount must be between 0 and 100!

# 適切な例外型を使う
def get_student_grade(grades, student_name):
    """grades 辞書から生徒の成績を取得する。
    
    Args:
        grades: 生徒名から成績への対応を持つ辞書
        student_name: 生徒名
    
    Returns:
        生徒の成績
    
    Raises:
        TypeError: grades が辞書でない場合
        KeyError: student_name が grades に存在しない場合
        ValueError: 成績が不正な場合
    """
    if not isinstance(grades, dict):
        raise TypeError("Grades must be a dictionary!")
    
    if student_name not in grades:
        raise KeyError(f"Student '{student_name}' not found!")
    
    grade = grades[student_name]
    
    if not (0 <= grade <= 100):
        raise ValueError(f"Invalid grade: {grade} (must be 0-100)")
    
    return grade
 
# 正しいデータでテスト
grades = {"Alice": 95, "Bob": 87, "Carol": 92}
 
try:
    grade = get_student_grade(grades, "Alice")
    print(f"Alice's grade: {grade}")
except (TypeError, KeyError, ValueError) as error:
    print(f"Error: {error}")

Alice's grade: 95

# 存在しない生徒でテスト
try:
    grade = get_student_grade(grades, "David")
    print(f"David's grade: {grade}")
except (TypeError, KeyError, ValueError) as error:
    print(f"Error: {error}")

Error: Student 'David' not found!

# 型が間違っている場合のテスト
try:
    grade = get_student_grade("not a dict", "Alice")
    print(f"Alice's grade: {grade}")
except (TypeError, KeyError, ValueError) as error:
    print(f"Error: {error}")

Error: Grades must be a dictionary!

# ログ出力の後に例外を再送出する
def divide_numbers(a, b):
    """2つの数を割り算し、エラーログを出す。"""
    try:
        result = a / b
        return result
    except ZeroDivisionError:
        print("ERROR LOG: Division by zero attempted")
        print(f"  Numerator: {a}, Denominator: {b}")
        raise  # 同じ例外を再送出する
 
# 関数を使う
try:
    result = divide_numbers(10, 0)
    print(f"Result: {result}")
except ZeroDivisionError:
    print("Cannot divide by zero!")

ERROR LOG: Division by zero attempted
  Numerator: 10, Denominator: 0
Cannot divide by zero!

# 既存の例外から新しい例外を発生させる
def load_config(config_dict, key):
    """辞書から設定値を読み込む。"""
    try:
        config_value = config_dict[key]
        
        # 整数としてパースを試みる
        parsed_value = int(config_value)
        return parsed_value
        
    except KeyError as error:
        raise RuntimeError(f"Configuration key missing: {key}") from error
    except ValueError as error:
        raise RuntimeError(f"Invalid configuration format for {key}") from error
 
# 関数を使う
config = {"timeout": "30", "retries": "5"}
 
try:
    value = load_config(config, "timeout")
    print(f"Config value: {value}")
except RuntimeError as error:
    print(f"Configuration error: {error}")
    print(f"Original cause: {error.__cause__}")

Config value: 30

try:
    value = load_config(config, "missing_key")
    print(f"Config value: {value}")
except RuntimeError as error:
    print(f"Configuration error: {error}")
    print(f"Original cause: {error.__cause__}")

Configuration error: Configuration key missing: missing_key
Original cause: 'missing_key'