data representations

[kjm1102]

My tenuous grasp of upythons data formats & their representations is causing me grief again. Specifically the lack of symmetry when changing between formats keeps catching me out.

At the moment I can put 2 x 8 bit analog values into a pair of bytes

>>> bytes([129])+bytes([20])
b'\x81\x14'

I can get the integer value

>>> struct.unpack('H',b'\x81\x14')[0]
5249

but when I try to backtrack to those original 2 values of 129 & 20

>>> hex(5249)
'0x1481'

I fall at the first hurdle with an '0x1481 representation instead of b'\x81\x14'

Can someone show me how to get from 5249 back to the 129 & 20 values I started with?

[robert-hh]

If you started with the numbers 129 and 20, you can use arithmetic to get to 5249 as well by 256 * 20 + 129, instead of using bytes() and struct(). From 5249 you can get back to 129 and 20 by 5249 & 0xff and 5249 >> 8 (or 5249 % 256 and 5249 // 256.
Obviously you can as well use a=struct.pack("H", 5249) to put 5249 into a bytes object b"\x81\x14" and use a[0] and a[1] to get the numbers 20 and 129.

[jimmo]

upythons data formats

Just to clarify, this is standard Python behaviour, not MicroPython-specific.

bytes([129])+bytes([20])

This is equivalent to struct.pack('BB', 129, 20) so if you want to unpack it, then you can use:

x = struct.pack('BB', 129, 20) # x = b'\x81\x14'
a, b = struct.unpack('BB', x) # a = 129, b = 20

but you're working with bytes, so you could also just use

x = struct.pack('BB', 129, 20) # x = b'\x81\x14'
a = x[0] # a = 129
b = x[1] # b = 20

[jimmo]

I realise I didn't answer your question about going back from the integer representation (see Robert's answer)

But you can also use struct here:

x = struct.pack('H', 5249)
a, b = struct.unpack('BB', x)

[jimmo]

Specifically the lack of symmetry when changing between formats

@kjm1102 Hopefully this example shows the symmetry:

# encoding side
a = 129
b = 20
x = struct.pack('BB', a, b)
y, = struct.unpack('H', x)

# y is now your 16 bit value

# decoding side
x = struct.pack('H', y)
a, b = struct.unpack('BB', x)

[GitHubsSilverBullet]

Can someone show me how to get from 5249 back to the 129 & 20 values I started with?

>>> x = 5249
>>> divmod(x, 256)
(20, 129)
>>> 

[kjm1102]

Thnx muchly, always possible to get back to the starting point if you're a competent enough programmer.

[peterhinch]

Another - arguably more self-evident - approach is to use a bytearray:

a = bytearray(2)  # Store 2 values
a[0] = 20
a[1] = 129
print(a[0], a[1])