forward() не отрабатывает при отсутствии функции перед собой

commands = {
    'A': 't.forward()',
    'B': 't.forward()',
    '+': 't.rotate(-theta)',
    '-': 't.rotate(theta)',
    }
 
axiom = 'A'
production_rules = {
    'A': 'A-B--B+A++AA+B-',
    'B': '+A-BB--B-A++A+B'
    }
theta = 60
style = {'description_width': 'initial'}
n_iterations = widgets.IntSlider(style=style, min=1, max=7, step=1, description='Number of iterations:', value=3)
display(n_iterations)
 
button = widgets.Button(description="Build a Gosper Curve")
output = widgets.Output()
display(button, output)
def on_fractal_clicked(b):
    with output: 
        plt.clf()
        positions = L_system(commands, axiom, production_rules, theta, n_iterations)
        x0 = 5;
        y0 = 5;
        pointX = x0 + (positions[:, 0] - x0) * cos(60) - (positions[:, 1] - y0) * sin(60);
        pointY = y0 + (positions[:, 1] - y0) * cos(60) + (positions[:, 0] - x0) * sin(60);
        plt.plot(pointX, pointY)
        plt.show()
button.on_click(on_fractal_clicked)

class turtle:
    def __init__(self):
        self._direction = np.array([1, 0]) # 2D direction vector
        self._position = np.array([0, 0]) # 2D position vector
    def forward(self):
        pos = self._position
        dirn = self._direction
        self._position = np.add(pos, dirn)
    def rotate(self, theta):
        (x, y) = self._direction
        current_angle = atan2(y, x)
        new_angle = current_angle + radians(theta)
        self._direction = [cos(new_angle), sin(new_angle)]