[Tutor] How to write the __str__ function

[Sydney Shall]

I need some advice that I have been embarrased to ask for, because I 
think that my error is so elementary.

I have written, as advised by the tutors, a complex program in a topic 
that interests me. The program works well and the tests are OK.

Now I want to add a __str__ function, which I thought would be 
straightforward. But I cannot get it right.

The code that I have so far is as folows:

  def __str__(self):
         return("\n"
                "               Output from __str__ of POCWP. "
                "\n"
                "\n After the first turnover, during the "
                "'Population Of Capitals Init' cycle,"
                "\n the productivities were raised from 1.0 "
                "\n to a specific Unit Constant Capital (UCC) "
                "for each specific capital: "
                "\n The input value for the mean of UCC "
                "was %7.5f" % (self.ucc),
                "\n The fractional sigma (FractionalSTD)"
                " of UCC that was input was %7.5f " % (self.fractsigma_ucc))

The error message is:

TypeError: __str__ returned non-string (type tuple)

When I omit either or both of the objects; self.ucc or 
self.fractsigma_ucc, the code works fine.

So, I guess my code has an error. But I cannot detect the error.

Guidance would be much appreciated.

-- 
Sydney