# bf.py -- Terry Jan Reedy, 2013 Oct 8
"bytes version of str.format, as function"
import re

def bf(form, obs):
    "Return bytes-formated objects interpolated into bytes form."
    fields = re.split(b'{:?([^}]*)}', form)
    print(fields)
    if len(fields) != 2*len(obs)+1:
        raise ValueError('Number of replacement fields not same as len(obs)')
    j = 1 # index into fields
    for ob in obs:
        field = fields[j]
        fields[j] = format(ob, field.decode()).encode() if field else ob
        j += 2
    return b''.join(fields)

# test code
form = b"bytes: {} and {:}; int: {:5d}; float: {:7.2f}; end"
obs = (b'somebytes', b'morebytes', 123, 12.3)
exp = form.decode().format(*( (ob.decode()  if isinstance(ob, bytes) else ob)
                                                for ob in obs)).encode()
res = bf(form, obs)
print(exp)
print(res)
assert res == exp