Take this example:
i = 0x12345678
print("{:08x}".format(i))
# Shows 12345678
i = swap32(i)
print("{:08x}".format(i))
# Should print 78563412
What would be the swap32-function()? Is there a way to byte-swap an int in Python, ideally with built-in tools?
One method is to use the struct module:
def swap32(i):
return struct.unpack("<I", struct.pack(">I", i))[0]
First you pack your integer into a binary format using one endianness, then you unpack it using the other (it doesn't even matter which combination you use, since all you want to do is swap endianness).
{:08x}, it is omitting the 0x. 12345678 != 0x12345678
struct.unpack("<I", struct.pack(">I", i))[0]
Big endian means the layout of a 32 bit int has the most significant byte first,
e.g. 0x12345678 has the memory layout
msb lsb
+------------------+
| 12 | 34 | 56 | 78|
+------------------+
while on little endian, the memory layout is
lsb msb
+------------------+
| 78 | 56 | 34 | 12|
+------------------+
So you can just convert between them with some bit masking and shifting:
def swap32(x):
return (((x << 24) & 0xFF000000) |
((x << 8) & 0x00FF0000) |
((x >> 8) & 0x0000FF00) |
((x >> 24) & 0x000000FF))
From Python 3.2, you can define a function, swap32(), as the following:
def swap32(x):
return int.from_bytes(x.to_bytes(4, byteorder='little'), byteorder='big', signed=False)
It uses an array of bytes to represent the value and reverses the order of the bytes by changing endianness during conversion back to integer.
The array module provides a byteswap() method for fixed sized items. The array module appears to be in versions back to Python 2.7
array.byteswap() “Byteswap” all items of the array. This is only supported for values which are 1, 2, 4, or 8 bytes in size;
Along with the fromfile() and tofile() methods, this module is quite easy to use:
import array
# Open a data file.
input_file = open( 'my_data_file.bin' , 'rb' )
# Create an empty array of unsigned 4-byte integers.
all_data = array.array( 'L' )
# Read all the data from the file.
all_data.fromfile( input_file , 16000 ) # assumes the size of the file
# Swap the bytes in all the data items.
all_data.byteswap( )
# Write all the data to a new file.
output_file = open( filename[:-4] + '.new' , 'wb' ) # assumes a three letter extension
all_data.tofile( output_file )
# Close the files.
input_file.close( )
output_file_close( )
The above code worked for me since I have fixed-size data files. There are more Pythonic ways to handle variable length files.
It may be simpler to use the socket library:
from socket import htonl
swapped = htonl(i)
print(hex(swapped))
That's it. This library also works in the other direction with ntohl.
array.byteswap()method if you first convert the number into a byte-array{ 12 34 56 78 }, it would be{ 78 56 34 12 }.