Have DEFAULT_PADDR
as a constant; but paddr
as a variable with DEFAULT_PADDR
as its default value. Do not change a "constant", even if it is just a matter of style - your code becomes cryptic.
I.e. for now, do this:
DEFAULT_PADDR = '/dev/ttyUSB0'
# ...
paddr = DEFAULT_PADDR
then you can change paddr
when you add argument parsing.