I am trying to specify a standard binary format length from a variable but for some reason it never works. Am I doing the formatting wrong or the variable inclusion?
comp.write("{0:%ib}".format(I) % num_bits)
ValueError: Invalid conversion specification
Firstly, it's in the wrong order:
("{0:%ib}" % num_bits).format(I)
Secondly, this isn't the way to do it! Mixing up types of formatting operator implies you don't know it can be done together. You want:
"{:{}b}".format(I, num_bits)
and if you really want to do it in two steps:
"{{:{}b}}".format(num_bits).format(I)
The {{ and }} are escaped, so are transformed to single braces, after the first .format.
You're doing the interpolation the wrong way round. You'll need to resolve the %i before passing it to format. This would work:
comp.write(("{0:%ib}" % num_bits).format(I))
but is pretty horrible, you probably want to split it into two:
fmt = "{0:%ib}" % num_bits
comp.write(fmt.format(I))
% operates on the result of the format call, rather than being passed to it.