(Could be a numpy bug)
##Just run the script, then uncomment Line 14 and run again
The result of "supertrend" is different, depending on whether I print "LB" first or not.
If I don't print it first, the result is wrong.

It also does'nt matter if I print it to stdout or stderr, it only works this way.

(sry if I did something wrong)

This is likely not a bug in python. You might have better luck asking on a numpy support list.

That said, what results do you get, and what do you expect to get? We don't know what error you're seeing.

You might replace:
print("LB: ", LB)

with:
str(LB)

and see what happens. That's the only operation that's called on LB when you're printing.

Please try to provide a minimal reproducible example:

https://stackoverflow.com/help/minimal-reproducible-example

http://sscce.org/

At the very least, you need to provide three pieces of information:

1. What you tried.
2. What you expected to happen (and why, if it isn't obvious).
3. What actually happened instead.


Ideally you should be able to get your example down to using one or two values, not three lists with 30 values each (90 values).

When you do:

    FINUB = np.empty(len(close))
    FINLB = np.empty(len(close))

you're creating two *uninitialised* arrays of values. (See the NumPy documentation at https://numpy.org/doc/stable/reference/generated/numpy.empty.html.)

When you then do 

    FINUB[i] = UB[i] if UB[i] < FINUB[i-1] \
            	and close[i-1] > FINUB[i] else FINUB[i-1]

on the first iteration of the loop (i = 1), you make use of the (undefined) value in FINUB[0] to compute FINUB[1].

In other words, this is a bug in your code, rather than in Python or NumPy.