如何将LMFit参数限制为仅为负值？

import numpy as np
from scipy.optimize import least_squares

# 定义拟合函数
def model_function(params, x):
    return params[0] * np.sin(x) + params[1] * np.cos(x)

# 定义残差函数
def residuals_function(params, x, y):
    return model_function(params, x) - y

# 初始猜测值
initial_guess = [1.0, 1.0]

# 数据点
x_data = np.linspace(0, 2 * np.pi, 100)
y_data = 2 * np.sin(x_data) + 0.5 * np.cos(x_data) + 0.1 * np.random.normal(size=100)

# 设置参数边界（负值）
bounds = [(-np.inf, -np.inf), (-np.inf, -np.inf)]

# 进行拟合
result = least_squares(residuals_function, initial_guess, bounds=bounds, args=(x_data, y_data))

print("拟合参数:", result.x)