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114 lines
3.4 KiB
Python
114 lines
3.4 KiB
Python
"""
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Conversion from ctypes to dtype.
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In an ideal world, we could acheive this through the PEP3118 buffer protocol,
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something like::
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def dtype_from_ctypes_type(t):
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# needed to ensure that the shape of `t` is within memoryview.format
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class DummyStruct(ctypes.Structure):
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_fields_ = [('a', t)]
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# empty to avoid memory allocation
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ctype_0 = (DummyStruct * 0)()
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mv = memoryview(ctype_0)
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# convert the struct, and slice back out the field
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return _dtype_from_pep3118(mv.format)['a']
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Unfortunately, this fails because:
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* ctypes cannot handle length-0 arrays with PEP3118 (bpo-32782)
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* PEP3118 cannot represent unions, but both numpy and ctypes can
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* ctypes cannot handle big-endian structs with PEP3118 (bpo-32780)
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"""
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import _ctypes
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import ctypes
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import numpy as np
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def _from_ctypes_array(t):
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return np.dtype((dtype_from_ctypes_type(t._type_), (t._length_,)))
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def _from_ctypes_structure(t):
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for item in t._fields_:
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if len(item) > 2:
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raise TypeError(
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"ctypes bitfields have no dtype equivalent")
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if hasattr(t, "_pack_"):
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formats = []
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offsets = []
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names = []
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current_offset = 0
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for fname, ftyp in t._fields_:
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names.append(fname)
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formats.append(dtype_from_ctypes_type(ftyp))
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# Each type has a default offset, this is platform dependent for some types.
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effective_pack = min(t._pack_, ctypes.alignment(ftyp))
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current_offset = ((current_offset + effective_pack - 1) // effective_pack) * effective_pack
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offsets.append(current_offset)
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current_offset += ctypes.sizeof(ftyp)
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return np.dtype(dict(
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formats=formats,
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offsets=offsets,
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names=names,
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itemsize=ctypes.sizeof(t)))
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else:
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fields = []
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for fname, ftyp in t._fields_:
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fields.append((fname, dtype_from_ctypes_type(ftyp)))
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# by default, ctypes structs are aligned
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return np.dtype(fields, align=True)
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def _from_ctypes_scalar(t):
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"""
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Return the dtype type with endianness included if it's the case
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"""
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if getattr(t, '__ctype_be__', None) is t:
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return np.dtype('>' + t._type_)
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elif getattr(t, '__ctype_le__', None) is t:
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return np.dtype('<' + t._type_)
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else:
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return np.dtype(t._type_)
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def _from_ctypes_union(t):
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formats = []
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offsets = []
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names = []
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for fname, ftyp in t._fields_:
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names.append(fname)
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formats.append(dtype_from_ctypes_type(ftyp))
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offsets.append(0) # Union fields are offset to 0
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return np.dtype(dict(
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formats=formats,
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offsets=offsets,
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names=names,
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itemsize=ctypes.sizeof(t)))
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def dtype_from_ctypes_type(t):
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"""
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Construct a dtype object from a ctypes type
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"""
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if issubclass(t, _ctypes.Array):
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return _from_ctypes_array(t)
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elif issubclass(t, _ctypes._Pointer):
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raise TypeError("ctypes pointers have no dtype equivalent")
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elif issubclass(t, _ctypes.Structure):
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return _from_ctypes_structure(t)
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elif issubclass(t, _ctypes.Union):
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return _from_ctypes_union(t)
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elif isinstance(getattr(t, '_type_', None), str):
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return _from_ctypes_scalar(t)
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else:
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raise NotImplementedError(
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"Unknown ctypes type {}".format(t.__name__))
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