You are an expert Python programmer. Your task is to implement a Python method for a class based on the provided class definition, method header, and docstring. Follow these rules:
1. Write clean, maintainable, and efficient Python code.
2. Do not include import statements in the method. Assume required imports are handled externally.
3. Ensure the method follows the principle of single responsibility.
4. Use attributes from the class constructor (`__init__`) wherever applicable.
5. The method must include inline comments where necessary.

## Input Placeholders:
- {class_definition}: The complete class definition with docstrings, including the `__init__` method implementation.
- {method_header}: The header and docstring for the method to be implemented.
- {extra_info}: Additional information or examples to clarify the implementation.

## Output:
- Python method implementation without import statements.

---

### Example 1: General Python Programming
class_definition:
class ShoppingCart:
    """
    A class to represent a shopping cart.

    Attributes:
        items (list): List of items in the cart.
    """

    def __init__(self):
        """
        Initializes the shopping cart with an empty list of items.
        """
        self.items = []
method_header:
def add_item(self, item):
    """
    Adds an item to the shopping cart.

    Args:
        item (str): The item to add.
    """
extra_info:

---

### Implementation:
def add_item(self, item):
    """
    Adds an item to the shopping cart.

    Args:
        item (str): The item to add.
    """
    # Append the item to the list of items
    self.items.append(item)

---

### Example 2: Machine Learning with Scikit-learn
class_definition:
class MLModel:
    """
    A class to represent a machine learning model.

    Attributes:
        model (object): The ML model instance.
        trained (bool): Whether the model is trained.
    """

    def __init__(self, model):
        """
        Initializes the MLModel with the given model.

        Args:
            model (object): An instance of an ML model.
        """
        self.model = model
        self.trained = False
method_header:
def train(self, X, y):
    """
    Trains the model using the provided data.

    Args:
        X (numpy.ndarray): Feature matrix.
        y (numpy.ndarray): Target labels.
    """
extra_info:

---

### Implementation:
def train(self, X, y):
    """
    Trains the model using the provided data.

    Args:
        X (numpy.ndarray): Feature matrix.
        y (numpy.ndarray): Target labels.
    """
    # Fit the model and mark it as trained
    self.model.fit(X, y)
    self.trained = True

---

### Example 3: Web Programming with Reflex
class_definition:
class SignupForm:
    """
    A class to represent a signup form in a web application.

    Attributes:
        username (str): The username entered by the user.
        password (str): The password entered by the user.
    """

    def __init__(self):
        """
        Initializes the SignupForm with empty fields.
        """
        self.username = ""
        self.password = ""
method_header:
def validate(self):
    """
    Validates the username and password.

    Returns:
        bool: True if both fields are non-empty, False otherwise.
    """
extra_info:

---

### Implementation:
def validate(self):
    """
    Validates the username and password.

    Returns:
        bool: True if both fields are non-empty, False otherwise.
    """
    # Check that both username and password are not empty
    return bool(self.username and self.password)
