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write binary with struct.pack_into

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  write binary with struct.pack_into palmeira <palmeira@gmail.com> - 2012-10-05 20:27 -0700
    Re: write binary with struct.pack_into 88888 Dihedral <dihedral88888@googlemail.com> - 2012-10-05 21:39 -0700
    Re: write binary with struct.pack_into 88888 Dihedral <dihedral88888@googlemail.com> - 2012-10-05 21:39 -0700
    Re: write binary with struct.pack_into Alexander Blinne <news@blinne.net> - 2012-10-06 17:55 +0200

#30862 — write binary with struct.pack_into

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.

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#30864

palmeira於 2012年10月6日星期六UTC+8上午11時27分47秒寫道：
> 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.

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#30865

palmeira於 2012年10月6日星期六UTC+8上午11時27分47秒寫道：
> 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.

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#30892

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

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