write binary with struct.pack_into

[Peter Otten]

palmeira wrote:

> Dear pythonists,
> 
> I'm having a problem with read/write binary in python.
> I have a binary file that I need to read information, extract a array,
> modify this array and put these values into file again in same binary
> format.
> I need to use unpack_from and pack_into because sometimes gonna need
> read/write in the middle of file.

Use pack/unpack and file.seek() instead.
 
> Script:
> 
> import struct
> bloco='>%df' %(252)  #Binary format
> 
> # READ
> fa=open('testIN.bin')
> my_array=struct.unpack_from(bloco,fa.read()[0*4:251*4])    # my_aray = 252
> elements array
> ## This read is OK!
> 
> #WRITE
> fb=open('testOUT.bin')
> test=struct.pack_into(bloco,fb.write()[0*4:251*4])  # ERROR in this WRITE

However, I think you have picked the wrong API. So:

# untested

import sys
import array

offset = 0
N = 252

a = array.array("f")
with open("testIN.bin", "rb") as f:
    f.seek(offset)
    a.read(f, N)
if sys.byteorder == "little":
    a.byteswap()

# process a

if sys.byteorder == "little":
    a.byteswap()
with open("testOUT.bin", "wb") as f:
    f.seek(offset)
    a.write(f)