How is int.from_bytes() calculated?

Question

I am trying to understand what from_bytes() actually does.

The documentation mention this:

The byteorder argument determines the byte order used to represent the integer. If byteorder is "big", the most significant byte is at the beginning of the byte array. If byteorder is "little", the most significant byte is at the end of the byte array. To request the native byte order of the host system, use sys.byteorder as the byte order value.

But this does not really tell me how the bytes values are actually calculated. For example I have this set of bytes:

In [1]: byte = b'\xe6\x04\x00\x00'

In [2]: int.from_bytes(byte, 'little')
Out[2]: 1254

In [3]: int.from_bytes(byte, 'big')
Out[3]: 3859021824

In [4]:

I tried ord() and it returns this:

In [4]: ord(b'\xe6')
Out[4]: 230

In [5]: ord(b'\x04')
Out[5]: 4

In [6]: ord(b'\x00')
Out[6]: 0

In [7]:

I don't see how either 1254 or 3859021824 was calculated from the values above.

I also found this question but it does not seem to explain exactly how it works.

So how is it calculated?

Answer 1

Big byte-order is like the usual decimal notation, but in base 256:

230 * 256**3 + 4 * 256**2 + 0 * 256**1 + 0 * 256**0 = 3859021824

just like

1234 = 1 * 10**3 + 2 * 10**2 + 3 * 10**1 + 4 * 10**0

For little byte-order, the order is reversed:

0 * 256**3 + 0 * 256**2 + 4 * 256**1 + 230 = 1254