How to properly write and read BitArray objects?

Question

I have an issue. I tryed to save my BitArray object into file. After that I want to read it and get the same BitArray object what I saved earlier. But result is not same with input.

from bitarray import bitarray

a = bitarray()
a += bitarray('{0:014b}'.format(15))
print(a.to01(), len(a))
with open('j.j', 'wb') as file:
    a.tofile(file)
b = bitarray()    
with open('j.j', 'rb') as file:
    b.fromfile(file)
print(b.to01(), len(b))

Output:

00000000001111 14
0000000000111100 16

I see my object now is 2-byte representation. But I want to get 14-bit I saved. Do you have any ideas to make it right?

Answer 1

This isn't a great solution, but it does get rid of the 0's on the right.

from bitarray import bitarray

a = bitarray('{0:014b}'.format(15))

print(a.to01(), len(a)) #00000000001111 14

with open('j.j', 'wb') as file:
    a.reverse()
    a.tofile(file)
       
b = bitarray()

with open('j.j', 'rb') as file:
    b.fromfile(file)
    b.reverse()
    
print(b.to01(), len(b)) #0000000000001111 16

You could skip the reversals and just right shift b, but you would have to create a dynamic system that knows exactly how many bits to shift by. Another solution is to simply use bits in multiples of 8 in the first place. What are you saving here by removing 1 to 7 bits? You aren't saving anything in the file. Those bits will be padded regardless.

Answer 2

It's eather not a great solution, but it's a solution)

def encode():
   encoded_bits = bitarray()
   ...
   encoded_bits += bitarray('000') # 48:51 slice
   zeroes_at_the_end = 8 - len(encoded_bits) % 8
   if zeroes_at_the_end < 8:
      encoded_bits[48:51] = bitarray('{0:03b}'.format(zeroes_at_the_end)) 

def decode(bites_sequence):
   zeroes_at_the_end = ba2int(bites_sequence[48:51])
   if zeroes_at_the_end != 0:
        del bites_sequence[-zeroes_at_the_end:] 

I just contain number of zeroes, which will appear after save/read in files and then easy delete those zeroes