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From: eskandari on 31 May 2010 11:56
Hi,
I am a newbie in python. I have an data.pickle file which is
serialized form of an "array of strings", I want to write their
offsets in another binary file, so an C++ program can read and analyse
them.
But when I try to write offset (number) in binary file, it raise
exception below in line "offsetfile.write(offset)"
"TypeError: argument 1 must be string or read-only buffer, not int"

I search the internet, find that all suggest converting number to
string ---with str()---and then write string to file.
But I shouldn't do this. because the above mentioned C++ function,
read file with this assumption that there are numbers in file.
So I want to know, Is there any way to produce an binary file
containing numbers same as the way C++ does?
Can anybody help me?
Thanks a lot.
From: MRAB on 31 May 2010 12:30
eskandari wrote:
> Hi,
> I am a newbie in python. I have an data.pickle file which is
> serialized form of an "array of strings", I want to write their
> offsets in another binary file, so an C++ program can read and analyse
> them.
> But when I try to write offset (number) in binary file, it raise
> exception below in line "offsetfile.write(offset)"
> "TypeError: argument 1 must be string or read-only buffer, not int"
>
> I search the internet, find that all suggest converting number to
> string ---with str()---and then write string to file.
> But I shouldn't do this. because the above mentioned C++ function,
> read file with this assumption that there are numbers in file.
> So I want to know, Is there any way to produce an binary file
> containing numbers same as the way C++ does?
> Can anybody help me?
>
You can't write ints to a file, but you can write bytestrings ('str' in
Python 2, 'bytes' in Python 3).

Use the 'struct' module to convert the int to a bytestring, and remember
to open the file as a binary file.
From: Tim Chase on 31 May 2010 12:42
On 05/31/2010 10:56 AM, eskandari wrote:
> But when I try to write offset (number) in binary file, it raise
> exception below in line "offsetfile.write(offset)"
> "TypeError: argument 1 must be string or read-only buffer, not int"
>
> I search the internet, find that all suggest converting number to
> string ---with str()---and then write string to file.
> But I shouldn't do this. because the above mentioned C++ function,
> read file with this assumption that there are numbers in file.
> So I want to know, Is there any way to produce an binary file
> containing numbers same as the way C++ does?

Well, you have at least two options:

1) use the pack/unpack functions in the "struct" module to
convert a number to a byte representation that you can then write
to a file

2) write the number to the file as a string and then use C++
libraries to parse a number from a string.

In both cases, you have to consider what happens when the number
is outside the bounds of your C++ data-type (you don't mention
what you're using...an int, a long, or "long long"; signed vs.
unsigned). Additionally in the first case, you have to make sure
that your memory-architecture (big-endian vs. little-endian)
matches on both sides; or that you marshal the data through a
pre-defined format (in libraries, commonly called "network"
format). For such reasons, I'd stick with method #2 unless you
have a strong reason not to.

-tkc




From: eskandari on 31 May 2010 12:43
On May 31, 12:30 pm, MRAB <pyt...(a)mrabarnett.plus.com> wrote:
> eskandari wrote:
> > Hi,
> > I am a newbie in python. I have an data.pickle file which is
> > serialized form of an "array of strings", I want to write their
> > offsets in another binary file, so an C++ program can read and analyse
> > them.
> > But when I try to write offset (number) in binary file, it raise
> > exception below in line  "offsetfile.write(offset)"
> > "TypeError: argument 1 must be string or read-only buffer, not int"
>
> > I search the internet, find that all suggest converting number to
> > string ---with str()---and then write string to file.
> > But I shouldn't do this. because the above mentioned C++ function,
> > read file with this assumption that there are numbers in file.
> > So I want to know, Is there any way to produce an binary file
> > containing numbers same as the way C++ does?
> > Can anybody help me?
>
> You can't write ints to a file, but you can write bytestrings ('str' in
> Python 2, 'bytes' in Python 3).
>
> Use the 'struct' module to convert the int to a bytestring, and remember
> to open the file as a binary file.

Thanks alot,
I have an question, if I do so, Will the second program (C++ program)
which process this file, encounter any problem while parsing the file?
It find number of integers by filelen/4 and ..... (It assumes that
file was created as the same way which C++ does)
Thanks in advance
From: Terry Reedy on 31 May 2010 13:23
On 5/31/2010 12:43 PM, eskandari wrote:
> On May 31, 12:30 pm, MRAB<pyt...(a)mrabarnett.plus.com> wrote:
>> eskandari wrote:

>> Use the 'struct' module to convert the int to a bytestring, and remember
>> to open the file as a binary file.
>
> Thanks alot,
> I have an question, if I do so, Will the second program (C++ program)
> which process this file, encounter any problem while parsing the file?
> It find number of integers by filelen/4 and ..... (It assumes that
> file was created as the same way which C++ does)

Tim Chase pretty well answered this. If you are not familiar with the
problem of 'endianess', see
http://en.wikipedia.org/wiki/Endianess

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