In Python, convert an array of bytes to integer

Question

I am retrieving bytes of data from an IC2 device using the read_i2c_block_data function. Within this block of data are signed 16-bit values that I want to display. I am able to print out the value correctly using the following:

print ("PC12  : %d mW" % int.from_bytes((return_data[5],return_data[6]), byteorder='little', signed=True))

Though, I have been trying print out the value using the following, which doesn't work and I don't know why. I am curious to why I can't get it to work.

print ("PC12  : %d mW" % int.from_bytes(return_data[5:6], byteorder='little', signed=True))

Does anyone know what I am doing wrong? I thought I could specify the range in the from_bytes function.

Thanks, Mark

What exactly happens, and how does that differ from what you expected to happen? What type is return_data? — user2357112
– user2357112, Commented May 1, 2015 at 3:38
The conversion was not correct and only converting just one of the bytes, but I couldn't figure out why. vituat enlightened me to what was happening. — lordhog
– lordhog, Commented May 1, 2015 at 3:56

vitaut · Accepted Answer · 2015-05-01 03:56:02Z

2

return_data[5:6] returns an array consisting of a single element at index 5:

>>> return_data = b'\00\01\02\03\04\05\06'
>>> return_data[5:6]
b'\x05'

Since you want to convert a 16-bit integer, you need to use return_data[5:7].

edited May 1, 2015 at 3:56

answered May 1, 2015 at 3:50

vitaut

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lordhog Over a year ago

vitaut, brilliant. That worked like a charm. It appears I really didn't understand the indexing/range syntax. With that, I was able to get the selection to work and fix a few other bugs I had. Thanks!!!!

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