I get the next error when trying to use the examples you posted. Seems to be a problem with the function getwebp() when you pass the parameters.
Traceback (most recent call last):
File "C:/Users/Dan/PycharmProjects/projects/webp_convert.py", line 4, in <module>
print(str(webp.dwebp(input_image='random_703.webp',output_image='random_703.png',option='0')))
File "C:\Users\Dan\AppData\Local\Programs\Python\Python37-32\lib\site-packages\webptools\webplib.py", line 29, in dwebp
cmd = getdwebp() + ' ' + input_image + ' ' + option + ' ' + output_image
TypeError: unsupported operand type(s) for +: 'NoneType' and 'str'