code: def function(self):
          
          if not hasattr(self,'_function'):
              function = self.model.lambdify(self.expression, self.all_variables, **self.lambdify_options)
              self._function = lambda *x: function(*(x + self.fixed_values))
          return self._function
docs: The function passed to the `fit_function` specified in `scipy_data_fitting.Fit.options`,
          and used by `scipy_data_fitting.Fit.pointspace` to generate plots, etc.
  
          Its number of arguments and their order is determined by items 1, 2, and 3
          as listed in `scipy_data_fitting.Fit.all_variables`.
  
          All parameter values will be multiplied by their corresponding prefix before being passed to this function.
  
          By default, it is a functional form of `scipy_data_fitting.Fit.expression` converted
          using `scipy_data_fitting.Model.lambdify`.
  
          See also `scipy_data_fitting.Fit.lambdify_options`.