@Ted.
Try this:

slow_stoch = SimpleMovingAverage(inputs=[stoch], window_length=4)  

Vladimir:
Thanks so much for answering my question. Adding the brackets around the stoch variable did the trick.

I am guessing that the brackets make it a pointer. Is that true?

Just another rookie,
Ted

In this case, the brackets mean the parameter is a list (see the python docs). The 'inputs' parameter can include more than one dataset or factor. They are passed as a list to accommodate this. A subtle clue is that the parameter name is plural (ie inputs).

One often sees an error message like TypeError: 'int' object is not iterable. This typically implies a list is expected rather than a single value. Putting brackets around the variable will usually fix it. Unfortunately, in this case the error message was rather cryptic (ValueError: FastStochasticOscillator does not have multiple outputs.).

Dan:
Thanks for the explanation.

Ted

One more question: this is about calculating the slow stochastic %K.
How can one determine the slow stochastic %D?

I thought this would be simply something like this:
fast_stoch = FastStochasticOscillator(window_length=14)
slow_stoch_k = SimpleMovingAverage(inputs=[fast_stoch], window_length=3)
slow_stoch_d = SimpleMovingAverage(inputs=[slow_stoch_k], window_length=3)

Determining slow_stoch_k works fine but determining slow_stoch_d gives the error:
NonWindowSafeInput: Can't compute windowed expression SimpleMovingAverage([SimpleMovingAverage(...)], 3) with windowed input SimpleMovingAverage([FastStochasticOscillator(...)], 3).

I suppose this cannot be solved by using the built in function SimpleMovingAverage?
For this you have to write your own simple moving average?

Ber:
I believe you are correct. You will have to calculate the slow_stoch_d by calculating multiple slow_stoch_k's, This what I did. There may be a better way but I did not find it.

Ted