[Tutor] truncated dictionary return

[spir]

On 12/01/2013 08:28 PM, richard kappler wrote:
> I have a script that reads sensor values gathered by an Arduino board from
> serial as a dictionary, said values to later be used in the AI for Nav &
> Control. Here's the script:
>
> #!/usr/bin/python
>
> def sensorRead():
>      import serial
>      from time import sleep
>
>      sensors = {}
>      sensors = dict.fromkeys('Sonar1 Sonar2 Sonar3 Sonar4 Dewpoint
> Temperature Humidity Light'.split())
>
>      arduino = serial.Serial('/dev/ttyACM0', 9600)
>      sleep(1)
>      line = arduino.readline().strip()
>      line = line.lstrip('{').rstrip('}').strip()
>
>      d = {}
>      for item in line.split(','):
>          item = item.strip()
>          key, value = item.split(':')
>          key = key.strip()
>          value = value.strip()
>          d[key]=int(value)
>      return d
>
> I hope that comes through okay, I copied it from the text file so
> indentation and such should be fine, if not let me know.
>
> The script works great with one exception. I understand the problem, I'm
> just not sure how to address it. The problem is:
>
> The Arduino runs on a constant loop, it reads each sensor, sends the key
> and the value to the serial bus in format for python to read it as a
> dictionary, lather, rinse, repeat.
>
> Python querries the bus when told. Usually the python script gets the full
> dictionary (all 8 values with keys, brackets etc) but sometimes it doesn't.
> Sometimes it only gets the last few values, sometimes it gets nothing or
> misses a bracket and throws an error. This makes sense. They are not in
> sync.
>
> What I need to figure out how to do is have the python script wait until
> the next round of values as signified by the opening bracket "{" or check
> that it has all 8 values and if not retry or.... something.
>
> Would this be an if/else? try? exception?
>
> I've not yet delved into any of these in my quest to learn python except
> if/else and that doesn't feel right for this, so I'm at a loss as to how to
> proceed.

* What is the point of the 'sensors' dict? (also, you don't need to initialise 
it as an enmpty dict)
* If you know about regexps or another matching utility, use that to decode the 
input line into (key,value) pairs. This is even easier if input has a strict 
format. Would tell us?
* About wrong input (incomplete data), avoid try/except, except if ever most 
cases are ok (it is very costly in case of exception). Anyway, you need to 
decode input, so use that to check for errors/missing stuff.

If you used a matching tool, its absence of result would directly tell about 
wrong input (provided your pattern is correct! ;-) As of now, just place checks 
in your decoding sequence. Possible checks places, at first sight, marked below:

     line = arduino.readline().strip()
     line = line.lstrip('{').rstrip('}').strip()
     # check line not empty
     # (actually big enough for at least {} + one key:val entry)

     d = {}
     # first get items in separate var and check how many:
     for item in line.split(','):
         item = item.strip()          # not needed, for you strip key/val again 
later
         # first get tuple from split() and check its size is 2
         key, value = item.split(':')
         key = key.strip()
         value = value.strip()        # not needed, for int() itself strips
         # maybe one try/except here for checking conversion to int:
         # (else, you need to check yourself it is an int numeral)
         d[key]=int(value)

Each check failure is a sign of wrong input. What do you need to do, then? 
Abandon the whole round?

Denis