解决ValueError: Custom function inv is not defined in `extra_sympy_mappings`.

一、报错问题

ValueError: Custom function inv is not defined in `extra_sympy_mappings`. You can define it with, e.g., `model.set_params(extra_sympy_mappings={'inv': lambda x: 1/x})`, where `lambda x: 1/x` is a valid SymPy function defining the operator. You can also define these at initialization time.

二、解决方式

在model里面inv这个函数是不存在的，需要extra_sympy_mappings 来定义在 SymPy 中定义额外函数或操作符的字典。

报错代码：

unary_operators = [
        "cos",
        "exp",
        "sin",
        "sqrt",
        "inv(x) = 1/x"
    ],

修改为：

extra_sympy_mappings = {'inv':lambda x:1/x},
    unary_operators = [
        "cos",
        "exp",
        "sin",
        "sqrt",
    ],

则可正常运行