Help/example of bit testing

[Mirko Liß]

Bryan Webb wrote:
> I am reading a file (binary)of 1024 char recs (bitmaps) . I can read the
> file , bit im having trouble finding examples of testing bits. I need to
> look at each bit of the above record.

Supposedly, you already use struct.unpack() from the struct-module to
convert
chunks of your binary string data into numbers.

If speed and memory doesn't matter much, you might want to store
the bits in a user-defined list-object, for example:

>>> import UserList
>>> class Bitfield(UserList.UserList):
...   def __init__(self,number):
...       self.data = []
...       while number:
...          self.data.append(number & 1)
...          number = number >> 1
...   def __getitem__(self, key):
...          if key < len(self.data):
...             return self.data[key]
...          else:
...             return 0 
... 
>>> bf=Bitfield(0xc02)
>>> bf
[0, 1, 0, 0, 0, 0, 0, 0, 0, 0, 1, 1]

bf[0] is the least significant bit.
Because of the redefined __getitem__() method, you don't need to worry
about
illegal indices:

>>> len(bf),bf[15],bf[13],bf[12],bf[11]
(12, 0, 0, 0, 1)

You might as well redefine the __getslice__() method if you need to look
at 
various bits at once.


I hope you like this,

Mirko Liss