data coming from arduino to python (raspberryPi) invalid literal error

Question

I have seen this invalid literal error meaning the data in the int() on the python side isn't a base 10 digit like its expecting. I get that, but on the arduino side I send it across serial with:

void loop()
{
  temperature = bmp085GetTemperature(bmp085ReadUT());
  pressure = bmp085GetPressure(bmp085ReadUP());
  Serial.println(temperature, DEC);  //fails on python side, outputs a 240 usually
  delay(50);
  Serial.println(pressure, DEC); //pa
  delay(1000);
}

and on the python side I pick it up with this:

while True:
     if(serialFromArduino.inWaiting() > 0):
          input = serialFromArduino.readline().rstrip()
          print(input)
          inputAsInteger =int(input)   #FAILS
          print("done")

Even did an rstrip to make sure i get rid of any \t\r\n etc.

So I am puzzled as to why the constant

pi@raspberrypi ~/pythoncode $ python serialtest.py
Running
240
Traceback (most recent call last):
 File "serialtest.py", line 18, in <module>
   inputAsInteger =int(input)
 ValueError: invalid literal for int() with base 10: ''

Just not sure what I am missing? New to python though so its feasible its something incredibly stupid I am doing.

Just so you know exactly what you're dealing with, on the debug print, try this: print(input.encode('hex')) — Jonathon Reinhart
– Jonathon Reinhart, Commented May 20, 2013 at 5:31
awesome! okay didnt know I had that option, ran again got a bazillion 0's and it ends in: 0323430 is this an ascii value? before the previous number is a count of 2951 0s — Codejoy
– Codejoy, Commented May 20, 2013 at 5:35
Yes, 00 32 34 30 is (null-byte) '2' '4' '0'. So your data is being prepended with a bunch of null characters. See my answer for how to remove them. I'm not sure why it's happening though. What is serialFromArduino? — Jonathon Reinhart
– Jonathon Reinhart, Commented May 20, 2013 at 5:41

Jonathon Reinhart · Accepted Answer · 2013-05-20 05:42:56Z

1

Sounds like your data has a bunch of null bytes (ASCII code 0x00). You'll want to remove them with strip('\x00'). By default strip() only removes whitspace, which doesn't include null bytes. So we specify what bytes to remove manually.

 if(serialFromArduino.inWaiting() > 0):
      input = serialFromArduino.readline().strip().strip('\x00')      # <--
      print(input)
      inputAsInteger = int(input)
      print("done")

Perhaps the better solution is to identify where the null bytes are coming from and prevent this huge waste of serial bandwidth from happening.

PS. The reason the error message was so unhelpful here:

ValueError: invalid literal for int() with base 10: ''

Is probably because they are string formatting the invalid literal into their error message. However, some library call along the way is interpreting the null byte as a null-terminator and not printing the rest of the string.

edited May 20, 2013 at 5:42

answered May 20, 2013 at 5:36

Jonathon Reinhart

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4 Comments

Codejoy Over a year ago

If i had to guess its in the calls to the library im using i didn't write: ` temperature = bmp085GetTemperature(bmp085ReadUT()); pressure = bmp085GetPressure(bmp085ReadUP());`

Codejoy Over a year ago

p.s. stripped out the bytes using that strip \x00 and it printed same thing in the hex call

Jonathon Reinhart Over a year ago

I think you realize it, but strip() returns the modified string; it doesn't modify it in-place.

Codejoy Over a year ago

ha so didn't realize, its working now :) woo hoo thanks! now my pachube library is woefully old it seems, thats for tomorrow. awesome guys thanks!

Jakub M. · Accepted Answer · 2013-05-20 05:34:51Z

1

The error means that your input received an empty line '' and you try to convert it to an integer.

You should cautiously process the external input at input = serialFromArduino.readline(). If it is not a proper integer (like, empty line or some noise) you program will crash. You should, for example:

try:
    input = serialFromArduino.readline().rstrip()
except ValueError:
    raise InputError('got bad value {}'.format(input))

answered May 20, 2013 at 5:34

Jakub M.

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