How can I convert a byte array to a double in python?

Question

I am using Java to convert a double into a byte array. Like this:

public static byte[] toByteArray(double value) {
    byte[] bytes = new byte[8];
    ByteBuffer.wrap(bytes).putDouble(value);
    return bytes;
}

Now, I would like to convert this byte array back into a double. In Java I would do it like this:

public static double toDouble(byte[] bytes) {
    return ByteBuffer.wrap(bytes).getDouble();
}

Now, how can I write the toDouble() method in Python?

Answer 1

You want the struct module:

>>> d = 1.234
>>> b = struct.pack('d', d)
>>> b
b'X9\xb4\xc8v\xbe\xf3?'
>>> d2, = struct.unpack('d', b)
>>> d2
1.234

The pack method gives you a bytes in Python 3.x, or a str in Python 2.x. This type isn't mutable like a Java byte[] , and in 2.x it also acts like a sequence of single-character strings, not a sequence of numbers from 0-255. If you need to fix either of those, just convert it to bytearray .

Also, note that—in both Java and Python—you probably want to specify an explicit endianness more often than not, especially if you're planning to save the bytes to a file or send them over the network. See Format Strings for details.

So:

>>> b = bytearray(struct.pack('!d', d))
>>> b
bytearray(b'?\xf3\xbev\xc8\xb49X')
>>> b[0]
63

Answer 2

Python has the struct module to convert bytes back to float values:

import struct

value = struct.unpack('d', bytes)[0]

Here 'd' signifies that a double value is expected (in native endianess, as 8 bytes). See the module documentation for more options, including specifying endianess.

Another option is to turn your bytes value into an array object ; you'd use this is if you had a homogenous sequence of doubles:

import array

doubles_sequence = array.array('d', bytes)

where every 8 bytes is interpreted as a double value, making doubles_sequence a sequence of doubles, addressable by index. To support a different endianess, you can swap the byte order with doubles_sequence.byteswap() .

Answer 3

This provides a Python implementation of the Java longBitsToDouble and doubleToLongBits operations. Convert the longs to/from byte sequences, eg., as:

def doubleToLongBits(d) : 
  bts = struct.pack('d',d)
  return MathLib.bytes2integer(bts)

def longBitsToDouble(x) : 
  bts = MathLib.integer2bytes(x)
  d, = struct.unpack('d',bytes(bts))
  return d