Using Python How can I read the bits in a byte?

Question

I have a file where the first byte contains encoded information. In Matlab I can read the byte bit by bit with var=fread(file,8, 'ubit1') then retrieve each bit by var(1),var(2), etc.

Is there any equivalent bit reader in python?

Answer 1

The smallest unit you'll be able to work with is a byte. To work at the bit level you need to use bitwise operators.

x = 3
#Check if the 1st bit is set:
x&1 != 0
#Returns True

#Check if the 2nd bit is set:
x&2 != 0
#Returns True

#Check if the 3rd bit is set:
x&4 != 0
#Returns False

Answer 2

The Python wiki explains how.

Answer 3

You won't be able to read each bit one by one - you have to read it byte by byte. You can easily extract the bits out, though:

f = open("myfile", 'rb')
# read one byte
byte = f.read(1)
# convert the byte to an integer representation
byte = ord(byte)
# now convert to string of 1s and 0s
byte = bin(byte)[2:].rjust(8, '0')
# now byte contains a string with 0s and 1s
for bit in byte:
    print bit

Answer 4

There are two possible ways to return the i-th bit of a byte. The "first bit" could refer to the high-order bit or it could refer to the lower order bit.

Here is a function that takes a string and index as parameters and returns the value of the bit at that location. As written, it treats the low-order bit as the first bit. If you want the high order bit first, just uncomment the indicated line.

def bit_from_string(string, index):
       i, j = divmod(index, 8)

       # Uncomment this if you want the high-order bit first
       # j = 8 - j

       if ord(string[i]) & (1 << j):
              return 1
       else:
              return 0

The indexing starts at 0. If you want the indexing to start at 1, you can adjust index in the function before calling divmod.

Example usage:

>>> for i in range(8):
>>>       print i, bit_from_string('\x04', i)
0 0
1 0
2 1
3 0
4 0
5 0
6 0
7 0

Now, for how it works:

A string is composed of 8-bit bytes, so first we use divmod() to break the index into to parts:

• i: the index of the correct byte within the string
• j: the index of the correct bit within that byte

We use the ord() function to convert the character at string[i] into an integer type. Then, (1 << j) computes the value of the j-th bit by left-shifting 1 by j. Finally, we use bitwise-and to test if that bit is set. If so return 1, otherwise return 0.

Answer 5

Read the bits from a file, low bits first.

def bits(f):
    bytes = (ord(b) for b in f.read())
    for b in bytes:
        for i in xrange(8):
            yield (b >> i) & 1

for b in bits(open('binary-file.bin', 'r')):
    print b