Question

I am starting migration process from python to jython. Has anyone done this with ease before?
What are the gotchas? Should I build first inside Jython IDE then deploy or what?

Answer 1

Keep in mind that in jython, while running under Java everything thing is ‘Big-Endian’ no matter what platform you’re on, whereas on a PC/Linux/Mac(x86) platform python is Little Endian. Make sure that when you’re using struct.pack and struct.unpack you use the appropriate prefix

without prefix

write data (enessw.py)

import struct 
f = file('tmp.dat', 'wb') # binary 
f.write(struct.pack('IIII', 1,2,3,4)) # default endianess

read data (enessr.py)

import struct
f = file('tmp.dat', 'rb')
data = f.read()
ints = struct.unpack('IIII', data) # default endianess
print repr(ints)

Results

Write with python, read with jython

C:\Documents and Settings\mat99856\My Documents\tmp>python enessw.py

C:\Documents and Settings\mat99856\My Documents\tmp>python enessr.py
(1, 2, 3, 4)

C:\Documents and Settings\mat99856\My Documents\tmp>jython enessr.py
(16777216L, 33554432L, 50331648L, 67108864L)

Write with jython read with python

C:\Documents and Settings\mat99856\My Documents\tmp>jython enessw.py

C:\Documents and Settings\mat99856\My Documents\tmp>jython enessr.py
(1L, 2L, 3L, 4L)

C:\Documents and Settings\mat99856\My Documents\tmp>python enessr.py
(16777216, 33554432, 50331648, 67108864)

The Fix

Use the < in the format string for pack and unpack. This will instruct pack/unpack that the data is to be specifically little endian in format.

With ‘<IIII’ as pack/unpack format

C:\Documents and Settings\mat99856\My Documents\tmp>python enessw.py

C:\Documents and Settings\mat99856\My Documents\tmp>python enessr.py
(1, 2, 3, 4)

C:\Documents and Settings\mat99856\My Documents\tmp>jython enessr.py
(1L, 2L, 3L, 4L)

C:\Documents and Settings\mat99856\My Documents\tmp>jython enessw.py

C:\Documents and Settings\mat99856\My Documents\tmp>jython enessr.py
(1L, 2L, 3L, 4L)

C:\Documents and Settings\mat99856\My Documents\tmp>python enessw.py

C:\Documents and Settings\mat99856\My Documents\tmp>python enessr.py
(1, 2, 3, 4)

C:\Documents and Settings\mat99856\My Documents\tmp>jython enessr.py
(1L, 2L, 3L, 4L)

References

struct.pack