CODEWITHSUNDEEP
pythonstructtypes
Zoroshino
Zoroshino

Reputation: 111

Python len() not returning whole numbers

I am currently working modifying some code I found on Github. It is for a spectrum analyzer using ath9k-based wireless cards.

Anyways, the issue is with this bit of code here:

sdata = struct.unpack_from("56B", data, pos)
pos += 56

# calculate power in dBm
sumsq_sample = 0
samples = []
for raw_sample in sdata:
    if raw_sample == 0:
        sample = 1
    else:
        sample = raw_sample << max_exp
    sumsq_sample += sample*sample
    samples.append(sample)

print (len(samples))

I expect the last print statement to always produce the value "56", however I have run into some strange behavior. Here is some example output:

56
56
56
56
56
56
56S 15.952354 
56
56
56
56
56
56
56
56
56
56
56
56
56S 15.689883 
56
56S 16.510071 
56
56S 17.591084 
56S 18.358299 
56
56
56
56
56
56
56
56
56
56
56
56
56
56
56
56
56
56
56

This makes no sense to me. I have checked the documentation for len() and struct.unpack_from(), but haven't gotten anywhere. Samples should contain 56 one-byte integers but occasionally I get odd values like "56S 10.237633"

Github repo of project I am modifying can be found at https://github.com/bcopeland/speccy Code sample above is from spectrum_file.py starting at line 87

Any ideas? Thanks

Upvotes: 1

Views: 98

Answers (1)

Daniel Corin
Daniel Corin

Reputation: 2067

As mentioned by @kchomski, your issue is due to another part of your program writing to stderr.

I found the following line of code in speccy.py, which I believe is causing the unexpected output:

sys.stderr.write('FPS %f \r' % (sum(self.fps) / len(self.fps)))

An MVCE would be the following:

import sys

sys.stderr.write('TOSTDERR\r')
print('54')

which outputs:

54STDERR

The carriage return character \r is moving your cursor back to the beginning of the line and then the 54 is printed on top of what was previously written to stderr.

You can resolve the issue by switching \r to \n which will put a new line at the end of the output to stderr and avoid this issue of overwriting.

Upvotes: 2

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