byte to bit manipulation in python

Question

I have a bmp file that I read in my Python program. Once I have read in the bytes, I want to do bit-wise operations on each byte I read in. My program is:

with open("ship.bmp", "rb") as f:
 byte = f.read(1)
 while byte != b"":
 # Do stuff with byte.
 byte = f.read(1)
 print(byte)

output:

b'\xfe' 

I was wondering how I can do manipulation on that? I.e convert it to bits. Some general pointers would be good. I lack experience with Python, so any help would be appreciated!

• Mark Ransom: Is this Python 2 or 3? Looks like 3 but it's best to be sure, they're different in this case.
• Padraic Cunningham: unrelated but you can change your while to a for loop for byte in iter(lambda:f.read(1),b"")
• mbomb007: @PadraicCunningham Maybe he wants readability.
• Padraic Cunningham: @mbomb007, unless you are stevie wonder that is readable
• mbomb007: @PadraicCunningham It's definitely not as readable. Tons of languages have while loops. Not many have a function called iter or identical lambda syntax. Using a while is inherently easier to read and understand.

Answer 1

bytes objects yield integers from 0 through 255 inclusive when indexed. So, just perform the bit manipulation on the result of indexing.

3>> b'\xfe'[0]
254
3>> b'\xfe'[0] ^ 0x55
171

• Kroltan: A promt. The results wouldn't make sense otherwise
• Ignacio Vazquez-Abrams: @Dunes: I modified my Python 3.x prompt so that I can easily differentiate between 3.x in one terminal and 2.x in another. The normal prompt in both is ">>> "

Answer 2

file.read(1) constructs a length 1 bytes objects, which is a bit overkill when you want the byte as an integer. To access each byte as an integer the following would be more succinct, and have the benefit of using a for loop.

with open("ship.bmp", "rb") as f:
 byte_data = f.read()
for byte in byte_data:
 # do stuff with byte. eg.
 result = byte & 0x2
 ...

• jamesb1082: Thanks, if i wanted to access the bits of a byte, how could i do that?
• mbomb007: byte = 7;index = 7;bit = (byte << index >> 7) % 2;print bit
• mbomb007: Or, if you want the 0th bit to be the bit on the far right, use byte << 7-index >> 7 instead.
• mbomb007: I had to use % 2 because Python doesn't include unsigned right shift, since it doesn't have unsigned numbers.