import json

class SaveError(Exception):
    def __init__(self, message):
        super().__init__(f"SaveError: {message}")

class LoadError(Exception):
    def __init__(self, message):
        super().__init__(f"LoadError: {message}")

class Osoba:
    def __init__(self, name, last_name, age):
        self.name = name
        self.last_name = last_name
        self.age = age

    def get_name(self):
        return self.name

    def get_last_name(self):
        return self.last_name

    def get_age(self):
        return self.age

    def to_dict(self):
        return {
            'name': self.name,
            'last_name': self.last_name,
            'age': self.age
        }

class Note:
    def __init__(self, grades):
        self.grades = grades

    def get_grades(self):
        return self.grades

    def get_grade(self, subject):
        return self.grades.get(subject, None)

    def to_dict(self):
        return self.grades

class Student(Osoba):
    def __init__(self, name, last_name, age, index_nr, grades):
        super().__init__(name, last_name, age)
        self.index_nr = index_nr
        self.notes = Note(grades)

    def get_index_nr(self):
        return self.index_nr

    def get_notes(self):
        return self.notes.get_grades()

    def to_dict(self):
        data = super().to_dict()
        data.update({
            'index_nr': self.index_nr,
            'grades': self.notes.to_dict()
        })
        return data

class Group:
    def __init__(self):
        self.students = []

    def add_student(self, student):
        self.students.append(student)

    def get_students(self):
        return self.students

    def display_students(self):
        for student in self.students:
            print(f"Student: {student.get_name()} {student.get_last_name()}, Age: {student.get_age()}, Index: {student.get_index_nr()}")
            print(f"Grades: {student.get_notes()}")

    def save_to_file(self, filename):
        try:
            with open(filename, 'w') as file:
                json.dump([student.to_dict() for student in self.students], file, indent=4)
            print(f"Data successfully saved to {filename}")
        except IOError as e:
            raise SaveError(f"An error occurred while writing to the file: {e}")

    def load_from_file(self, filename):
        try:
            with open(filename, 'r') as file:
                data = json.load(file)
                self.students = []
                for student_data in data:
                    student = Student(
                        student_data['name'],
                        student_data['last_name'],
                        student_data['age'],
                        student_data['index_nr'],
                        student_data['grades']
                    )
                    self.add_student(student)
            print(f"Data successfully loaded from {filename}")
        except IOError as e:
            raise LoadError(f"An error occurred while reading the file: {e}")
        except json.JSONDecodeError as e:
            raise LoadError(f"An error occurred while decoding JSON data: {e}")

# Przykładowa inicjalizacja studentów
grades_student1 = {
    "Mathematics": 5,
    "Physics": 4,
    "Chemistry": 3
}

grades_student2 = {
    "Mathematics": 4,
    "Physics": 5,
    "Chemistry": 4
}

student1 = Student("Anna", "Nowak", 22, "2021001", grades_student1)
student2 = Student("Piotr", "Kowalski", 23, "2021002", grades_student2)

# Przykładowa inicjalizacja grupy z dowolnymi studentami
group = Group()
group.add_student(student1)
group.add_student(student2)

# Zapis grupy studentów do pliku
try:
    group.save_to_file("students_group.json")
except SaveError as e:
    print(e)

# Wyświetlenie studentów w grupie przed odczytem z pliku
print("\nGroup before loading from file:")
group.display_students()

# Symulacja błędu odczytu - zmiana nazwy pliku na nieistniejący
print("\nSimulating file read error:")
try:
    group.load_from_file("nonexistent_file.json")
except LoadError as e:
    print(e)

# Odczyt grupy studentów z pliku
try:
    group.load_from_file("students_group.json")
except LoadError as e:
    print(e)

# Wyświetlenie studentów w grupie po odczycie z pliku
print("\nGroup after loading from file:")
group.display_students()