code: def lambdify(self, expression, symbols, **kwargs):
          
          if isinstance(expression, str):
              expression = self.expressions[expression]
  
          if hasattr(symbols, '__iter__'):
              variables = []
              for s in symbols:
                  if isinstance(s, str):
                      variables.append(self.symbol(s))
                  else:
                      variables.append(s)
          else:
              if isinstance(symbols, str):
                  variables = (self.symbol(symbols), )
              else:
                  variables = (symbols, )
  
          return sympy.lambdify(tuple(variables), expression, **kwargs)
docs: Converts a SymPy expression into a function using [`sympy.lambdify`][1].
  
          `expression` can be a SymPy expression or the name of an expression
          in `scipy_data_fitting.Model.expressions`.
  
          `symbols` can be any of the following,
          or a list of any combination of the following:
  
          - A SymPy symbol.
          - The name of a symbol in `scipy_data_fitting.Model.symbols`.
  
          Additional keyword arguments are passed to [`sympy.lambdify`][1].
  
          [1]: http://docs.sympy.org/latest/modules/utilities/lambdify.html#sympy.utilities.lambdify.lambdify