Python Object Conversions and Associated C Data Types for Python Type in Visual Studio .NET

Encode QR in Visual Studio .NET Python Object Conversions and Associated C Data Types for Python Type
Table 263
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Python Object Conversions and Associated C Data Types for Python Type
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Any Any Any String Unicode
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PyArg_Parse*
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Format
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"O" "O!" "O&" "S" "U"
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C Type
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PyObject **r PyTypeObject *type, PyObject **r int (*converter)(PyObject *, void *), void *r PyObject **r PyObject **r
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The "O", "S", and "U" specifiers return raw Python objects of type PyObject * "S" and "U" restrict this object to be a string or Unicode string, respectively The "O!" conversion requires two C arguments: a pointer to a Python type object and a pointer to a PyObject * into which a pointer to the object is placed A TypeError is raised if the type of the object doesn t match the type object For example:
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/* Parse a List Argument */ PyObject *listobj; PyArg_ParseTuple(args,"O!", &PyList_Type, &listobj);
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The following list shows the C type names corresponding to some Python container types that might be commonly used with this conversion
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C Name
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PyList_Type PyDict_Type PySet_Type PyFrozenSet_Type PyTuple_Type PySlice_Type PyByteArray_Type
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Python Type
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list dict set frozen_set tuple slice bytearray
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The "O&" conversion takes two arguments (converter, addr) and uses a function to convert a PyObject * to a C data type converter is a pointer to a function with the prototype int converter(PyObject *obj, void *addr), where obj is the passed Python object and addr is the address supplied as the second argument in PyArg_ParseTuple() converter() should return 1 on success, 0 on failure On error, the converter should also raise an exceptionThis kind of conversion can be used to map Python objects such as lists or tuples into C data structures For example, here is a possible implementation of the distance() wrapper from our earlier code:
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/* Convert a tuple into a Point structure */ int convert_point(PyObject *obj, void *addr) { Point *p = (Point *) addr; return PyArg_ParseTuple(obj,"ii", &p->x, &p->y); } PyObject *py_distance(PyObject *self, PyObject *args) { Point p1, p2; double result; if (!PyArg_ParseTuple(args,"O&O&", convert_point, &p1, convert_point, &p2)) { return NULL; } result = distance(&p1,&p2); return Py_BuildValue("d",result); }
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26 Extending and Embedding Python
Finally, argument format strings can contain a few additional modifiers related to tuple unpacking, documentation, error messages, and default argumentsThe following is a list of these modifiers:
Format String
"(items)" "|" ":" ";"
Description
Unpack a tuple of objects Items consist of format conversions Start of optional arguments End of arguments The remaining text is the function name End of arguments The remaining text is the error message
The "(items)" unpacks values from a Python tupleThis can be a useful way to map tuples into simple C structures For example, here is another possible implementation of the py_distance() wrapper function:
PyObject *py_distance(PyObject *self, PyObject *args) { Point p1, p2; double result; if (!PyArg_ParseTuple(args,"(dd)(dd)", &p1x, &p1y, &p2x, &p2y)) { return NULL; } result = distance(&p1,&p2); return Py_BuildValue("d",result); }
The modifier "|" specifies that all remaining arguments are optionalThis can appear only once in a format specifier and cannot be nestedThe modifier ":" indicates the end of the arguments Any text that follows is used as the function name in any error messagesThe modifier ";" signals the end of the arguments Any following text is used as the error message Note that only one of : and ; should be used Here are some examples:
PyArg_ParseTuple(args,"ii:gcd", &x, &y); PyArg_ParseTuple(args,"ii; gcd requires 2 integers", &x, &y); /* Parse with optional arguments */ PyArg_ParseTuple(args,"s|s", &buffer, &delimiter);
Type Conversion from C to Python
The following C function is used to convert the values contained in C variables to a Python object:
PyObject *Py_BuildValue(char *format, )
This constructs a Python object from a series of C variables format is a string describing the desired conversionThe remaining arguments are the values of C variables to be converted The format specifier is similar to that used with the PyArg_ParseTuple* functions, as shown in Table 264
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Table 264
Format Specifiers for Py_BuildValue() Python Type
None String
Format
"" "s"
C Type
void char *
Description
Nothing Null-terminated string If the C string pointer is NULL, None is returned String and length May contain null bytes If the C string pointer is NULL, None is returned Same as "s" except a byte string is returned Same as "s# except a byte string is returned Same as "s" Same as "s#" Null-terminated Unicode string If the string pointer is NULL, None is returned Unicode string and length Converts a null-terminated C string into a Unicode string Converts a C string into Unicode 8-bit integer 8-bit unsigned integer Short 16-bit integer Unsigned short 16-bit integer Integer Unsigned integer Long integer Unsigned long integer Long long Unsigned long long Python size type Single character Creates a Python string of length 1 Single-precision floating point Double-precision floating point Complex number Any Python object The object is unchanged except for its reference count, which is incremented by 1 If a NULL pointer is given, a NULL pointer is returned This is useful if an error has been signaled elsewhere and you want it to propagate
"s#"