如何在python十六进制字符串中重新排序字节并转换为long

Question

I have this long hex string 20D788028A4B59FB3C07050E2F30 In python 2.7 I want to extract the first 4 bytes, change their order, convert it to a signed number, divide it by 2^20 and then print it out. In C this would be very easy for me :) but here I'm a little stuck.

For example the correct answer would extract the 4 byte number from the string above as 0x288D720. Then divided by 2^20 would be 40.5525. Mainly I'm having trouble figuring out the right way to do byte manipulation in python. In CI would just grab pointers to each byte and shift them where I wanted them to go and cast into an int or a long.

Answer 1

Python is great in strings, so let's use what we have:

s = "20D788028A4B59FB3C07050E2F30"
t = "".join([s[i-2:i] for i in range(8,0,-2)])
print int(t, 16) * 1.0 / pow(2,20)

But dividing by 2**20 comes a bit strange with bits, so maybe shifting is at least worth a mention too...

print int(t, 16) >> 20

After all, I would

print int(t, 16) * 1.0 / (1 << 20)

Answer 2

For an extraction you can just do foo[:8]

Hex to bytes: hexadecimal string to byte array in python

Rearrange bytes: byte reverse AB CD to CD AB with python

You can use struct for conversion to long

And just do a normal division by (2**20)