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import numpy as np
from scipy.linalg import solve_banded

def implicitsolver(D, upsilon, theta_end, d_theta, dy, U_x, delta_x, isturbulent):
    R = D / 2
    dx = np.radians(d_theta)
    
    x_num = int(np.ceil(theta_end / d_theta))
    y_max = max(delta_x)
    y_num = int(np.ceil(y_max / dy))
    
    u = np.zeros((x_num + 1, y_num + 1))
    v = np.zeros((x_num + 1, y_num + 1))
    
    for i in range(x_num - 1):
        i_i = i + 1
        j_max = int(np.ceil(delta_x[i_i + 1] / dy)) - 1
        
        A = np.zeros((3, j_max))  # Adjusted for use with solve_banded
        B = np.zeros(j_max)
        
        for j in range(j_max):
            j_i = j
            r = R + dy * j
            
            # Similar turbulent viscosity calculation as MATLAB, adapt as needed
            y = dy * j
            # Example calculation for y_plus and upsilon_t, adapt as needed
            y_plus = np.sqrt(upsilon * (u[i_i, j_i + 1] - u[i_i, j_i]) / dy) * y / upsilon
            if 60 < y_plus < 300:
                upsilon_t_p = (1 + 0.4 * y_plus) * upsilon
                upsilon_t_n = (1 + 0.4 * y_plus) * upsilon
            else:
                l = min(0.4 * (j + 1) * dy, 0.09 * delta_x[i_i + 1])
                upsilon_t_p = upsilon + isturbulent * l ** 2 * abs((u[i_i, j_i + 2] - u[i_i, j_i + 1]) / dy)
                upsilon_t_n = upsilon + isturbulent * l ** 2 * abs((u[i_i, j_i + 1] - u[i_i, j_i]) / dy)
            
            # Example calculation for B, adapt as needed
            # Example calculations for A matrix elements, adapt as needed
            
        # Solve tridiagonal system using solve_banded
        # You need to adjust A and B accordingly to match solve_banded's expected input format
        # u_t = solve_banded((1, 1), A, B)
        
        # Update u and v fields
        # Remember to adapt indexing and operations to match Python's zero-based indexing
        
    return u, v

# Note: This code will not run as-is due to placeholders and simplifications.
# It's meant to provide a structural template for converting the MATLAB function to Python.