write binary with struct.pack_into

[palmeira]

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.

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




Regards,

Ronaldo Palmeira.




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