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Python Programming On Win32 by Mark Hammond, Andy Robinson

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Passing and Obtaining Python Objects from COM

COM supports a variety of argument types, using the VARIANT data structure. The types that can be passed to COM functions include integers and floats of various sizes, strings, date/time values, COM objects, or arrays of any of these types.

In many cases, PythonCOM can translate between Python objects and VARIANT structures seamlessly. When you call a COM object and pass a Python object, PythonCOM automatically creates a VARIANT of the right type and passes the VARIANT to COM. In the absence of any hints, PythonCOM performs the translations as listed in Table 12.1 and Table 12.2. In Table 12.1, for example, you can see that a Python integer is automatically converted to a VARIANT type VT_I4.

Table 12.1. Default Python Object to VARIANT Translation

Python Object Type

VARIANT Type

Integer

VT_I4

Long Integer

VT_I4 if the value is less than 232, or VT_I8 if greater

String/Unicode

VT_BSTR

Float

VT_R8

PyTrue/PyFalse

VT_BOOL

None

VT_NULL

win32com.client.Dispatch instance

VT_DISPATCH

PyIDispatch

VT_DISPATCH

All other PyI* PythonCOM objects

VT_UNKNOWN

Pywintypes PyTIME object

VT_DATE

Any other Python sequence

An array of VARIANTs; each element of the sequence is translated using this table

Table 12.2. Default Python Object to VARIANT Translation

VARIANT Type

Python Object

VT_BOOL

VT_I2

VT_I4

VT_ERROR

Integer

VT_R4

VT_R8

Float

VT_DISPATCH

PyIDispatch

VT_UNKNOWN

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