from struct import pack, unpack, error
from pprint import pprint

data = (1, 1, 2, 1, 1)

block = pack('2IB2I', *data)
pprint(block) # '\x01\x00\x00\x00\x01\x00\x00\x00\x02\x00\x00\x00\x01\x00\x00\x00\x01\x00\x00\x00'
print(len(block)) # == 20
print(unpack('2IB2I', block)) # works ok
try:
    print(unpack('=2IB2I', block)) # expects len(block) == 17
except error:
    pass
print(unpack('=2IB2I', block[:17])) # works _but_: (1, 1, 2, 16777216, 16777216)
print(unpack('<2IB2I', block[:17])) # same as above

print('------------------')

block = pack('=2IB2I', *data)
pprint(block) # '\x01\x00\x00\x00\x01\x00\x00\x00\x02\x01\x00\x00\x00\x01\x00\x00\x00'
print(len(block)) # == 17

try:
    print(unpack('2IB2I', block)) # expects len(block) == 29
except error: pass

print(unpack('=2IB2I', block)) # works ok
print(unpack('<2IB2I', block)) # works ok