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!

Is this Python 2 or 3? Looks like 3 but it's best to be sure, they're different in this case. — Mark Ransom
– Mark Ransom, Commented Jan 22, 2015 at 22:24
unrelated but you can change your while to a for loop for byte in iter(lambda:f.read(1),b"") — Padraic Cunningham
– Padraic Cunningham, Commented Jan 22, 2015 at 22:31
@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. — mbomb007
– mbomb007, Commented Jan 22, 2015 at 22:36

Ignacio Vazquez-Abrams · Accepted Answer · 2015-01-22 22:31:10Z

2

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

answered Jan 22, 2015 at 22:31

Ignacio Vazquez-Abrams

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2 Comments

Kroltan Over a year ago

A promt. The results wouldn't make sense otherwise

Ignacio Vazquez-Abrams Over a year ago

@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 ">>> "

Dunes · Accepted Answer · 2015-01-22 23:30:29Z

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
    ...

answered Jan 22, 2015 at 23:30

Dunes

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4 Comments

jamesb1082 Over a year ago

Thanks, if i wanted to access the bits of a byte, how could i do that?

mbomb007 Over a year ago

byte = 7;index = 7;bit = (byte << index >> 7) % 2;print bit

mbomb007 Over a year ago

Or, if you want the 0th bit to be the bit on the far right, use byte << 7-index >> 7 instead.

mbomb007 Over a year ago

I had to use % 2 because Python doesn't include unsigned right shift, since it doesn't have unsigned numbers.

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