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1

I updating a script to Python 3 from Python 2 and having trouble with the line below, it decodes and displays the RSSI value of a BLE Beacon:

rssi = struct.unpack("b", packet[packetOffset -1])

I found an alternative method to get the same result, which takes the 2 last characters in the broadcast string and converts them to text. Using an 8 Bit Signed Integer Encoder.

For example c3 becomes -61 (https://cryptii.com/pipes/integer-encoder)

However I am struggling to find a way to do this in python 3!

How do I decode the string?

Any help will be much appreciated.

python python-3.x bluetooth bluetooth-lowenergy

share|improve this question

asked Mar 26 at 23:27

tbowdentbowden

39222 silver badges1818 bronze badges

• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  
  Can you give us tha string?
  
  – Pedro Rodrigues
  Mar 26 at 23:33
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  Did you notice that int('c3', 16) -> 195 and accidentally 195 - 256 = -61?
  
  – Klaus D.
  Mar 26 at 23:34
  

  
  
  
  

add a comment | 

1

I updating a script to Python 3 from Python 2 and having trouble with the line below, it decodes and displays the RSSI value of a BLE Beacon:

rssi = struct.unpack("b", packet[packetOffset -1])

I found an alternative method to get the same result, which takes the 2 last characters in the broadcast string and converts them to text. Using an 8 Bit Signed Integer Encoder.

For example c3 becomes -61 (https://cryptii.com/pipes/integer-encoder)

However I am struggling to find a way to do this in python 3!

How do I decode the string?

Any help will be much appreciated.

python python-3.x bluetooth bluetooth-lowenergy

share|improve this question

asked Mar 26 at 23:27

tbowdentbowden

39222 silver badges1818 bronze badges

• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  
  Can you give us tha string?
  
  – Pedro Rodrigues
  Mar 26 at 23:33
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  Did you notice that int('c3', 16) -> 195 and accidentally 195 - 256 = -61?
  
  – Klaus D.
  Mar 26 at 23:34
  

  
  
  
  

add a comment | 

I updating a script to Python 3 from Python 2 and having trouble with the line below, it decodes and displays the RSSI value of a BLE Beacon:

rssi = struct.unpack("b", packet[packetOffset -1])

I found an alternative method to get the same result, which takes the 2 last characters in the broadcast string and converts them to text. Using an 8 Bit Signed Integer Encoder.

For example c3 becomes -61 (https://cryptii.com/pipes/integer-encoder)

However I am struggling to find a way to do this in python 3!

How do I decode the string?

Any help will be much appreciated.

python python-3.x bluetooth bluetooth-lowenergy

share|improve this question

asked Mar 26 at 23:27

tbowdentbowden

39222 silver badges1818 bronze badges

rssi = struct.unpack("b", packet[packetOffset -1])

share|improve this question

asked Mar 26 at 23:27

tbowdentbowden

39222 silver badges1818 bronze badges

share|improve this question

asked Mar 26 at 23:27

tbowdentbowden

39222 silver badges1818 bronze badges

asked Mar 26 at 23:27

tbowdentbowden

39222 silver badges1818 bronze badges

asked Mar 26 at 23:27

tbowdentbowden

39222 silver badges1818 bronze badges

2 Answers
2

active

oldest

votes

2

Is it what you are looking for?

a = bytes.fromhex('c3')

res = a[0] - 256 if a[0] > 127 else a[0]

bytes.fromhex takes a string and transforms it to a byte object. You take the first byte and make it unsigned by checking whether it's greater than 127.

Alternatively:

res = int.from_bytes(bytes.fromhex('c3'), byteorder='big', signed=True)

share|improve this answer

edited Mar 27 at 0:21

answered Mar 26 at 23:49

Jacques GaudinJacques Gaudin

7,68833 gold badges2828 silver badges5151 bronze badges

• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  Yes! Is there a big difference if I used 'num = int('c3', 16) - 256' instead?
  
  – tbowden
  Mar 26 at 23:53
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  There is. If you try int('0f', 16) - 256 you get -241. You need to check whether the value is >127 or not. Using int is a good idea. Not sure of the consequences. I need to think about it.
  
  – Jacques Gaudin
  Mar 26 at 23:57
  

  
  
  
  

add a comment | 

1

You haven't shown exactly what's in packet, but here's a guess:

import struct


packet = b'x00xc3x00xff'
packetOffset = 2

rssi = struct.unpack("b", packet[packetOffset-1: packetOffset])[0]
print(repr(rssi)) # -> -61

# For a text string result, just do:
rssi = str(rssi)
print(repr(rssi)) # -> '-61'

share|improve this answer

edited Mar 27 at 0:07

answered Mar 27 at 0:01

martineaumartineau

74.3k1111 gold badges101101 silver badges193193 bronze badges

add a comment | 

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2

Is it what you are looking for?

a = bytes.fromhex('c3')

res = a[0] - 256 if a[0] > 127 else a[0]

bytes.fromhex takes a string and transforms it to a byte object. You take the first byte and make it unsigned by checking whether it's greater than 127.

Alternatively:

res = int.from_bytes(bytes.fromhex('c3'), byteorder='big', signed=True)

share|improve this answer

edited Mar 27 at 0:21

answered Mar 26 at 23:49

Jacques GaudinJacques Gaudin

7,68833 gold badges2828 silver badges5151 bronze badges

• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  Yes! Is there a big difference if I used 'num = int('c3', 16) - 256' instead?
  
  – tbowden
  Mar 26 at 23:53
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  There is. If you try int('0f', 16) - 256 you get -241. You need to check whether the value is >127 or not. Using int is a good idea. Not sure of the consequences. I need to think about it.
  
  – Jacques Gaudin
  Mar 26 at 23:57
  

  
  
  
  

add a comment | 

2

Is it what you are looking for?

a = bytes.fromhex('c3')

res = a[0] - 256 if a[0] > 127 else a[0]

bytes.fromhex takes a string and transforms it to a byte object. You take the first byte and make it unsigned by checking whether it's greater than 127.

Alternatively:

res = int.from_bytes(bytes.fromhex('c3'), byteorder='big', signed=True)

share|improve this answer

edited Mar 27 at 0:21

answered Mar 26 at 23:49

Jacques GaudinJacques Gaudin

7,68833 gold badges2828 silver badges5151 bronze badges

• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  Yes! Is there a big difference if I used 'num = int('c3', 16) - 256' instead?
  
  – tbowden
  Mar 26 at 23:53
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  There is. If you try int('0f', 16) - 256 you get -241. You need to check whether the value is >127 or not. Using int is a good idea. Not sure of the consequences. I need to think about it.
  
  – Jacques Gaudin
  Mar 26 at 23:57
  

  
  
  
  

add a comment | 

Is it what you are looking for?

a = bytes.fromhex('c3')

res = a[0] - 256 if a[0] > 127 else a[0]

bytes.fromhex takes a string and transforms it to a byte object. You take the first byte and make it unsigned by checking whether it's greater than 127.

Alternatively:

res = int.from_bytes(bytes.fromhex('c3'), byteorder='big', signed=True)

share|improve this answer

edited Mar 27 at 0:21

answered Mar 26 at 23:49

Jacques GaudinJacques Gaudin

7,68833 gold badges2828 silver badges5151 bronze badges

a = bytes.fromhex('c3')

res = a[0] - 256 if a[0] > 127 else a[0]

res = int.from_bytes(bytes.fromhex('c3'), byteorder='big', signed=True)

share|improve this answer

edited Mar 27 at 0:21

answered Mar 26 at 23:49

Jacques GaudinJacques Gaudin

7,68833 gold badges2828 silver badges5151 bronze badges

answered Mar 26 at 23:49

Jacques GaudinJacques Gaudin

7,68833 gold badges2828 silver badges5151 bronze badges

answered Mar 26 at 23:49

Jacques GaudinJacques Gaudin

7,68833 gold badges2828 silver badges5151 bronze badges

1

You haven't shown exactly what's in packet, but here's a guess:

import struct


packet = b'x00xc3x00xff'
packetOffset = 2

rssi = struct.unpack("b", packet[packetOffset-1: packetOffset])[0]
print(repr(rssi)) # -> -61

# For a text string result, just do:
rssi = str(rssi)
print(repr(rssi)) # -> '-61'

share|improve this answer

edited Mar 27 at 0:07

answered Mar 27 at 0:01

martineaumartineau

74.3k1111 gold badges101101 silver badges193193 bronze badges

add a comment | 

1

You haven't shown exactly what's in packet, but here's a guess:

import struct


packet = b'x00xc3x00xff'
packetOffset = 2

rssi = struct.unpack("b", packet[packetOffset-1: packetOffset])[0]
print(repr(rssi)) # -> -61

# For a text string result, just do:
rssi = str(rssi)
print(repr(rssi)) # -> '-61'

share|improve this answer

edited Mar 27 at 0:07

answered Mar 27 at 0:01

martineaumartineau

74.3k1111 gold badges101101 silver badges193193 bronze badges

add a comment | 

You haven't shown exactly what's in packet, but here's a guess:

import struct


packet = b'x00xc3x00xff'
packetOffset = 2

rssi = struct.unpack("b", packet[packetOffset-1: packetOffset])[0]
print(repr(rssi)) # -> -61

# For a text string result, just do:
rssi = str(rssi)
print(repr(rssi)) # -> '-61'

share|improve this answer

edited Mar 27 at 0:07

answered Mar 27 at 0:01

martineaumartineau

74.3k1111 gold badges101101 silver badges193193 bronze badges

import struct


packet = b'x00xc3x00xff'
packetOffset = 2

rssi = struct.unpack("b", packet[packetOffset-1: packetOffset])[0]
print(repr(rssi)) # -> -61

# For a text string result, just do:
rssi = str(rssi)
print(repr(rssi)) # -> '-61'

share|improve this answer

edited Mar 27 at 0:07

answered Mar 27 at 0:01

martineaumartineau

74.3k1111 gold badges101101 silver badges193193 bronze badges

answered Mar 27 at 0:01

martineaumartineau

74.3k1111 gold badges101101 silver badges193193 bronze badges

answered Mar 27 at 0:01

martineaumartineau

74.3k1111 gold badges101101 silver badges193193 bronze badges