I'me new here, so I wish to say HELLO to all members.
I'm quite new to python, but I manage to do something using python in blender.
Now, I need to save in a binary file raw data (specificaly blocks of 3 float), for a later use. How can I manage this? i tried the write() function, but it need the conversion to string, I need the value! Thanks all!
4  9537 
You may find a suitable solution here.
Thanks for the reply.
I've already take alook at that thread, but, as far I can understand it, they are reading from a file. I need to write the data in it.
I suppose thet I've to use the struct.pack in some way, but I cant figure out how
I use the struct functions all the time for this very thing, though rarely see them talked about here. Probably because I'm missing out on an easy way to do things. I did though write a script as an example for my blog. It may or may not be useful but, goes like this. -
print 'testbinary.py modified 11:00 PM 7/10/2007'
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'''
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Copyright 2007 Dennis Shimer
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Vr Mapping copyright Cardinal Systems, LLC
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No warranties as to safety or usability expressed or implied.
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Free to use, copy, modify, distribute with author credit.
-
-
just a lesson in binary data. Either run from a command line 3 times like
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python testbinary.py
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or open or paste into pyedi and execute 3 times.
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'''
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-
import os,struct
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# We'll use os to test for the existence of the file, an struct to
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# interact with the binary data.
-
-
FormatString='10sddi'
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'''
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The format string tells several parts of the program what we are working
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with in regards to binary data. The format string listed above represents
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a 10 character string, 2 double precision real numbers, and an integer.
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Same as before but I added the int to show by example. Note that it is
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just a string with letters representing different kinds of data.
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'''
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-
ParFileName='testbinary.par'
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# The file by this name will show up in the directory the script is run in.
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print 'Bytes occupied by binary data',struct.calcsize(FormatString),'\n\n'
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'''
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Sometimes you need to know how many bytes to read from a file in order
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to get one set of data when multiple sets exist, a good example goes
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back to the coordinate file mentioned earlier. In order to read and process
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one point at a time you can only read the necessary bytes. the calcsize
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method in struct will read a format string and tell how many bytes are
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represented. You can go into a python interpreter, import struct then
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evaluate some single item format strings like the following for a
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float, a double, an int, and a 1 character string.
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>>> struct.calcsize('f')
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4
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>>> struct.calcsize('d')
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8
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>>> struct.calcsize('i')
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4
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>>> struct.calcsize('s')
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1
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'''
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if not os.path.isfile(ParFileName): # If the file doesn't exist.
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ParFile=open(ParFileName,'wb')
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# Then open it in write (create or erase) mode with a binary modifier
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BinaryData=struct.pack(FormatString,'Word',.2,.0,0)
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'''
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The pack method in the struct module uses the format string to process
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the data that comes next into the bytes which represent them in binary
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form. It is important that the amount and type of data following the
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format string match, notice I am passing in a string (less than 10
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characters allowed, 2 real numbers and an integer.
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'''
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ParFile.write(BinaryData)
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ParFile.close()
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# We have a binary file and binary data so just and write and close it.
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print 'Program run with no existing file'
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print 'Data set to defaults.'
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print 'testbinary.par should now exist.'
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else:
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ParFile=open(ParFileName,'r+b')
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# If it does exist open for reading + writing with binary modifier.
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BinaryData=ParFile.read()
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'''
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This time since the file exists we are going to get the binary data
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right out of the file with a read. Since we want the whole file there
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is no reason to use calcsize to tell it how many bytes to read.
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'''
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DataList=struct.unpack(FormatString,BinaryData)
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# Going the other direction, lets use unpack to use the format string
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# to convert the binary data into a list of values.
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print 'Data from file, note it is a list\n ',DataList
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# and print the list.
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ParFile.seek(0,0)
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'''
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Now assuming that there are new values that need to be used to update
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the parameter file. The first thing we need to do is go back to the
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beginning of the file, read up on seek, but this basically moves you
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0 bytes from the beginning. If in the coordinate file example I had a
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data structure that require 24 bytes and wanted to get the 3rd value
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from the file I could to the 48th byte and read 24. This way you can
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read, write, and overwrite data in place within a file.
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'''
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print 'File position after seek ',ParFile.tell()
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# Every open file has a tell method which gives the current position.
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BinaryData=struct.pack(FormatString,'Updated',1,2,3)
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ParFile.write(BinaryData)
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# Same idea as above, just using the changed data (wherever it came from)
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if DataList[0].split('\x00')[0]=='Updated':
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print 'You have seen it all.'
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print 'Remove testbinary.par to start over'
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'''
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There may be a better way to convert this 10 character string to
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what you normally think of as a string, but it comes back padded
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with the 0 bytes and this works simply enough. Think of it this
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way 'Updated\x00\x00\x00' has 3 extra bytes, if you split on that
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character you get back ['Updated', '', '', ''] of which 'Updated'
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is the first or [0] element. Ugly but I've never really look into
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a better way.
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'''
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else:
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print 'Program run with an existing file'
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print 'Data was updated run again to see change'
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# 2nd run string doesn't equal 'Updated' so print appropriate message.
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ParFile.close()
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# Good form to explicitely close files.
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-
Thanks for the reply.
I've already take alook at that thread, but, as far I can understand it, they are reading from a file. I need to write the data in it.
I suppose thet I've to use the struct.pack in some way, but I cant figure out how
Thanks a lot! It is exactly what I'm looking for! And a very good explanation too.
Thanks again.
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