Question

In Python I m accessing a binary file by reading it into a string and then using struct.unpack(...). Now I want to write to that string using struct.pack_into(...), but I get the error "Cannot use string as modifiable buffer". What would be a suitable buffer for use with the struct module?

Answer 1

If you aren t trying to pack it into a specific object, just use struct.pack to return a string.

Otherwise, ctypes.create_string_buffer is one way to obtain a mutable buffer.

Answer 2

As noted in another answer, struct_pack is probably all you need and should use. However, objects of type array support the buffer protocol and can be modified:

>>> import array, struct
>>> a = array.array( c ,     * 1000)
>>> c =  a ; i = 1
>>> struct.pack_into( ci , a, -0, c, i)
>>> a
array( c ,  ax00x00x00x01x00x00x00  ...

The original buffer protocol was a bit of a hack primarily for C extensions. It has been deprecated and replaced by a new C-level buffer API and memoryview objects in Python 3 (and in the upcoming 2.7).

Answer 3

Two possibilities leap immediately to mind:

You can use the Python stringio module to make a read/write buffer with file semantics.
You can use the Python array module to get a buffer you can treat like a list, but which will contain just binary bytes.

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