Python 2 (unlike py3) is doing a signed 32-bit CRC.
Those sites are doing an unsigned 32-bit CRC.
The values are the same otherwise, as you can see from this:
>>> 0x100000000 - 0xb1d4025b == 0x4e2bfda5
True
One quick way to convert from 32-bit signed to 32-bit unsigned is:*
>>> -1311505829 % (1<<32)
2983461467
Or, in hex:
>>> hex(-1311505829 % (1<<32))
'0xb1d4025b'
& 0xFFFFFFFF or % 0x100000000 or & (2**32-1) or % (2**32) and so on are all equivalent ways to do the same bit-twiddling; it just comes down to which one you find most readable.
* This only works in languages that do floored integer division, like Python (-3 // 2 == -2); in languages that do truncated integer division, like Java (-3 / 2 == -1), you’ll still end up with a negative number. And in languages that don’t even require that division and mod go together properly, like C, all bets are off—but in C, you’d just cast the bytes to the type you want…