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Mot de passe oublié ?Je m'inscris ! (gratuit) Variable-sized data types Variable-sized data types

ctypes provides some support for variable-sized arrays and structures (this was added in version

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)
>>> resize(short_array, 4)
Traceback (most recent call last):
ValueError: minimum size is 8
>>> resize(short_array, 32)
>>> sizeof(short_array)
>>> sizeof(type(short_array))

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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