I have been trying to figure this out for some time now and i get nowhere. I have two unsigned integers. This means four bytes.
3A BC 00 00
These represent a float value. I want to convert them to float using python.
I can do this with python using struct.unpack.
Specifically:
values = [0x3A, 0xBC, 0x00, 0x00]
struct.unpack('>f', bytes(values))
Can someone explain this particular unpack command?
I know that the > part reverses the bits (big endian implementation). So they will be 00 00 BC 3A.
What does f does? According to the docs it means float... What does this mean? I tried omitting it but i got struct.error: unpack requires a buffer of 0 bytes
Finally, why the casting to bytes happen?
The struct module deals with conversion between python objects and a contiguous sequence of bytes. [0x3A, 0xBC, 0x00, 0x00] is a list of python integers (that is, references to int objects), not a contiguous sequence of bytes. A bytes object is a contiguous sequence of bytes and can be initialized with an iterable of integers between 0 and 255, so that's the first step.
Now, how should python interpret these bytes? Are they an integer, a bunch of floats, a bunch of characters? That's what the format string is for. "f" means "grab me a 4 byte float". The ">" is big-endian as you note, but it's a bit more subtle. Python will grab 4 bytes, then deal with the endianness, and finally do the float conversion. Suppose the input was [0x3A, 0xBC, 0x00, 0x00, 0x11, 0x22, 0x33, 0x44]? The format ">ff" would return 2 floats.
Finally, that's not "two integers". It's a sign bit, 8 bit exponent and 23 bit mantissa which is a standard format for single precision floats.