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\'), and then retrie

Answer 1

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.