14.14.1.20 Variable-sized data types
ctypes provides some support for variable-sized arrays and
structures (this was added in version 0.9.9.7).
The resize function can be used to resize the memory buffer of an
existing ctypes object. The function takes the object as first
argument, and the requested size in bytes as the second argument. The
memory block cannot be made smaller than the natural memory block
specified by the objects type, a ValueError is raised if this is
tried:
>>> short_array = (c_short * 4)()
>>> print sizeof(short_array)
8
>>> resize(short_array, 4)
Traceback (most recent call last):
...
ValueError: minimum size is 8
>>> resize(short_array, 32)
>>> sizeof(short_array)
32
>>> sizeof(type(short_array))
8
>>>
This is nice and fine, but how would one access the additional elements contained in this array? Since the type still only knows about 4 elements, we get errors accessing other elements:
>>> short_array[:]
[0, 0, 0, 0]
>>> short_array[7]
Traceback (most recent call last):
...
IndexError: invalid index
>>>
Another way to use variable-sized data types with ctypes is to use
the dynamic nature of Python, and (re-)define the data type after the
required size is already known, on a case by case basis.
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Release 2.5, documentation updated on 19th September, 2006.