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.

 

[88888 Dihedral]

palmeiraæ–¼ 2012å¹´10月6日星期六UTC+8上åˆ11時27分47秒寫é“：

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.









--

View this message in context: http://python.6.n6.nabble.com/write-binary-with-struct-pack-into-tp4991234.html

Sent from the Python - python-list mailing list archive at Nabble.com.

Are you writing and reading files produce by different
languages?

The pickle part is better for OOP and glue logics in python.

The heavy computing part should be done in CYTHON.

 

[88888 Dihedral]

palmeiraæ–¼ 2012å¹´10月6日星期六UTC+8上åˆ11時27分47秒寫é“：

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.









--

View this message in context: http://python.6.n6.nabble.com/write-binary-with-struct-pack-into-tp4991234.html

Sent from the Python - python-list mailing list archive at Nabble.com.

Are you writing and reading files produce by different
languages?

The pickle part is better for OOP and glue logics in python.

The heavy computing part should be done in CYTHON.

 

[Alexander Blinne]

First, you should consider reading the documentation of
struct.unpack_from and struct.pack_into at
http://docs.python.org/library/struct.html quite carefully. It says,
that these commands take a parameter called offset, which names the
location of the data in a buffer (e.g. an opened file).

example:

bloco='>%df' % (252) # Format string (252 floats)
fa = open('testIN.bin', 'rb') # open for reading in binary mode
off = 0 # suppose i want to read block at beginning of file
my_array=struct.unpack_from(bloco, fa, off) #read data


now write them to another file:

fb = open('testOUT.bin', 'r+b') # open for updating in binary mode
off = 0 # suppose i want to write block at beginning of file
struct.pack_into(bloco, fb, off, *my_array) #write data to testOUT