simple python module in gnuradio fails "Expression None is invalid for type 'real'"

Question

I have a simple python module to iterate over a few frequencies in a list. For me this should be simple but it's not working within grc.

Expression None is invalid for type 'real'

If I run the module manually it returns f as expected. Within grc the flow fails. If I simply return a frequency, it also works.

What did I miss here?

# this module will be imported in the into your flowgraph
fc = [438100000, 438200000]
fs = len(fc)

i = 1
f = fc[i]

def sweeper(prob_lvl):
    global fc, fs, i
    if prob_lvl:
        f = fc[i]
        i += 1
    if i == fs:
        i = 1
    # return int('438000000') ### this works
    return int(f)  ### this doesnt (block says: Expression None is invalid for type 'real')

Answer 1

Lesson learned: don't use globals

If you use globals, don't forget to include everything needed, "f" in this case.

running gnuradio-companion --log debug helps