Associating a name with each bit in a bitmask

Question

I have the following code:

dic = {}
bin_rep = [1 if digit == '1' else 0 for digit in bin(n)[2:]]
if len(bin_rep) < size_bin:
    bin_rep += [0] * (size_bin - len(bin_rep))
# turn this into dictionnary - might all be done better?
for i in range(0, size_bin):
    dic[self.p.get_agents()[i]] = bin_rep[i]
return dic

Essentially, I'm taking an number n, turning it into a list (its binary representation, potentially padded with 0 up to size_bin), say:

[0 1 1 1 1 0 0 0 0 0 0]

From that padded list, I create a dictionary, where key==agent name and value==1/0, say:

{'a1':0, 'a2':1 ... 'an':0}

(The array as a whole corresponds to a team, the 0/1 represents the inclusion of an agent into the team. So each team corresponds to a binary array which can be represented as an int.)

The code works, using list and dictionary comprehension. However, would someone have a cleaner/smarter way to perform this? It's good if it's optimal, though not crucial, and would like to keep it readable.

I absolutely need to have the ultimate result into a dictionary because other parts of the code use that representation.

EDIT: using python 3.6

Answer 1

Suggestions

• enumerate is built-in function useful here
• dict comprehensions can be used to create dictionaries from arbitrary key and value expressions
• imho '{:b}'.format(n) is easier to read than bin(n)[2:]
• in additional you may use OrderedDict if you often iterate over agents 1, 2, 3, 4, ...

Final code (n = 10, size_bin = 6)

{'a{}'.format(k): int(v) for k, v in enumerate('{:b}'.format(10).ljust(6, '0'), start=1)}

and with full utilize format syntax

{'a{}'.format(k): int(v) for k, v in enumerate('{:<0{}b}'.format(10, 6), start=1)}

Answer 2

As far as I can tell, you're wanting a test to determine if certain bits are set in a number, and you want to use named constants for the individual bits.

This is fine, except that I think you'll come back to not wanting to use named constants later. So you'll want some kind of programmatic access.

For that, I strongly recommend you check out the documentation of the various enum classes - I think either Flag (recommended) or IntFlag will do what you need.

If you have a particular set of name:value mappings in mind, go ahead and spell them out. You don't say what version of Python you are using, and it matters in this case. For 3.6 you might try something like:

import enum

class Agent(enum.Flag):
    HILTON   = 0b01000000
    SMART    = 0b00100000
    BOND     = 0b00010000
    LEITER   = 0b00001000
    POWERS   = 0b00000100
    SHAGWELL = 0b00000010

def active_agents(n):
    active = [agent.name for agent in Agent if n & agent]
    return active

For < 3.6, there's no Flag enum, so you'll have to use IntEnum something like:

def active_agents(n):
    active = [agent.name 
        for agent in Agent.__members__.values()   # <-- awkward!
        if n & agent]
    return active

Answer 3

This isn't really much shorter, but to me, converting from int to str and back to int in order to extract bits seems... well, weird. So, here:

def bitmask_to_dict(mask, mlength=16, prefix='a'):
    return {
        '%s%d' % (prefix, i): (mask >> (mlength - i)) & 1
        for i in range(mlength)
    }