How do I write a long integer as binary in Python?

Question

In Python, long integers have unlimited precision. I would like to write a 16 byte (128 bit) integer to a file. struct from the standard library supports only u

Answer 1

You could pickle the object to binary, use protocol buffers (I don't know if they allow you to serialize unlimited precision integers though) or BSON if you do not want to write code.

But writing a function that dumps 16 byte integers by shifting it should not be so hard to do if it's not time critical.

Answer 2

The PyPi bitarray module in combination with the builtin bin() function seems like a good combination for a solution that is simple and flexible.

bytes = bitarray(bin(my_long)[2:]).tobytes()

The endianness can be controlled with a few more lines of code. You'll have to evaluate the efficiency.

Answer 3

This may be a little late, but I don't see why you can't use struct:

bigint = 0xFEDCBA9876543210FEDCBA9876543210L
print bigint,hex(bigint).upper()

cbi = struct.pack("!QQ",bigint&0xFFFFFFFFFFFFFFFF,(bigint>>64)&0xFFFFFFFFFFFFFFFF)

print len(cbi)

The bigint by itself is rejected, but if you mask it with &0xFFFFFFFFFFFFFFFF you can reduce it to an 8 byte int instead of 16. Then the upper part is shifted and masked as well. You may have to play with byte ordering a bit. I used the ! mark to tell it to produce a network endian byte order. Also, the msb and lsb (upper and lower bytes) may need to be reversed. I will leave that as an exercise for the user to determine. I would say saving things as network endian would be safer so you always know what the endianess of your data is.

No, don't ask me if network endian is big or little endian...