拉格朗日插值法是scipy库中常用的插值方法。
官网定义如下：
scipy.interpolate.lagrange(x, w)[source]
Return a Lagrange interpolating polynomial.
Given two 1-D arrays x and w, returns the Lagrange interpolating polynomial through the points (x, w).
Warning: This implementation is numerically unstable. Do not expect to be able to use more than about 20 points even if they are chosen optimally.
Parameters:
x : array_like
x represents the x-coordinates of a set of datapoints.
w : array_like
w represents the y-coordinates of a set of datapoints, i.e. f(x).
Returns:
lagrange : numpy.poly1d instance
The Lagrange interpolating polynomial.
其参数为两个一维数组，分别代表坐标的x值和y值，输出是多项式。可以直接用了计算插值。
例如：
x=[100,200,300,400]\
y=[10,20,30,40]\
f=lagrange(x,y)\
求在500处的插值f(500)得到49.999999999999837，符号我们的数据
Warning: This implementation is numerically unstable. Do not expect to be able to use more than about 20 points even if they are chosen optimally.
这方法不是很稳定，所有输入的数据点最好不要超过20个
