Displaying signed floats in Python

Question

Until this month, my outdoor DHT22 temperature/humidity monitor has been working fine. However, with the cold weather, and temperatures now sub-zero, I've noticed that my routine is failing to handle negative temperatures - instead they are represented as being positive: the sign has been lost.

I've adjusted my re.search routine to include negative numbers where previously they were excluded,

 # Continuously append data
 while(True):
 # Run the DHT program to get the humidity and temperature readings!
 # DHT22 (Credit to Adafruit)
 output = subprocess.check_output(["/home/pi/scripts/DHT/Adafruit_DHT", "22", "25"]);
 rasp = subprocess.check_output(["vcgencmd", "measure_temp"])
 print output
 matches = re.search("Temp =\s+-([0-9.]+)", output)
 if (not matches):
      time.sleep(3)
      continue
 tempa = float(matches.group(1))
 print tempa

When I

 print output

a string is produced containing the negative temperature.

However, when I

print tempa

it displays as a positive number.

I need to be able to carry the sign into the variable, as temperatures could be either positive or negative (even in a UK winter).

Can anyone help?

Karoly Horvath · Accepted Answer · 2014-12-13 10:16:38Z

2

The negative sign is not captured in the match group because currently it's not part of it.

"Temp =\s+(-[0-9.]+)"
          ^ fix

answered Dec 13, 2014 at 10:16

Karoly Horvath

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Comments

pauliusbau · Accepted Answer · 2015-01-20 07:16:49Z

0

I just tried it, and only "-" sign is not enough.. If you want to measure positive and negative temperatures, regexp string should look like this:

"Temp =\s+([-+]?[0-9.]+)"

answered Jan 20, 2015 at 7:16

pauliusbau

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