String Conversions and Associated C Data Types for PyArg_Parse* Python Type in Visual Studio .NET

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Table 262
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String Conversions and Associated C Data Types for PyArg_Parse* Python Type
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String or byte string of length 1 String String, bytes, or buffer String, bytes, or buffer String or None String, bytes, or None String, bytes, buffer, or None Bytes (null-terminated) Bytes Bytes or buffer String (Unicode) String (Unicode) String
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Format
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"c" "s" "s#" "s*" "z" "z#" "z*" "y" "y#" "y*" "u" "u#" "es"
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C Argument Type
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char *r char **r char **r, int *len Py_buffer *r char **r char **r, int *len Py_buffer *r char **r char **r, int *len Py_buffer *r Py_UNICODE **r Py_UNICODE **r, int *len const char *enc, char **r
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Extension Modules
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Table 262
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Continued Python Type
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String or bytes String or null-terminated bytes String or bytes Read-only buffer Read-write buffer Read-write buffer Read-write buffer
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"es#" "et" "et#" "t#" "w" "w#" "w*"
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C Argument Type
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const char *enc, char **r, int *len const char *enc, char **r, int *len const char *enc, char **r, int *len char **r, int *len char **r char **r, int *len Py_buffer *r
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String handling presents a special problem for C extensions because the char * datatype is used for many different purposes For instance, it might refer to text, a single character, or a buffer of raw binary dataThere is also the issue of what to do with embedded NULL characters ('\x00') that C uses to signal the end of text strings In Table 262, the conversion codes of "s", "z", "u", "es", and "et" should be used if you are passing text For these codes, Python assumes that the input text does not contain any embedded NULLs if so, a TypeError exception is raised However, the resulting string in C can be safely assumed to be NULL-terminated In Python 2, both 8-bit and Unicode strings can be passed, but in Python 3, all conversions except for "et" require the Python str type and do not work with bytesWhen Unicode strings are passed to C, they are always encoded using the default Unicode encoding used by the interpreter (usually UTF-8)The one exception is the "u" conversion code that returns a string using Python s internal Unicode representationThis is an array of Py_UNICODE values where Unicode characters are typically represented by the wchar_t type in C The "es" and "et" codes allow you to specify an alternative encoding for the text For these, you supply an encoding name such as 'utf-8' or 'iso-8859-1', and the text will be encoded into a buffer and returned in that formatThe "et" code differs from "es" in that if a Python byte-string is given, it is assumed to have already been encoded and is passed through unmodified One caution with "es" and "et" conversions is that they dynamically allocate memory for the result and require the user to explicitly release it using PyMem_Free()Thus, code that uses these conversions should look similar to this:
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PyObject *py_wrapper(PyObject *self, PyObject *args) { char *buffer; if (!PyArg_ParseTuple(args,"es","utf-8",&buffer)) { return NULL; } /* Do something */ /* Cleanup and return the result */ PyMem_Free(buffer); return result; }
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26 Extending and Embedding Python
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For handling text or binary data, use the "s#", "z#", "u#", "es#", or "et#" codes These conversions work exactly the same as before except that they additionally return a length Because of this, the restriction on embedded NULL characters is lifted In addition, these conversions add support for byte strings and any other objects that support something known as the buffer interfaceThe buffer interface is a means by which a Python object can expose a raw binary buffer representing its contentsTypically, you find it on strings, bytes, and arrays (eg, the arrays created in the array module support it) In this case, if an object provides a readable buffer interface, a pointer to the buffer and its size is returned Finally, if a non-NULL pointer and length are given to the "es#" and "et#" conversions, it is assumed that these represent a pre-allocated buffer into which the result of the encoding can be placed In this case, the interpreter does not allocate new memory for the result and you don t have to call PyMem_Free() The conversion codes of "s*" and "z*" are similar to "s#" and "z#" except that they populate a Py_buffer structure with information about the received data More information about this can be found in PEP-3118, but this structure minimally has attributes char *buf, int len, and int itemsize that point to the buffer, the buffer length (in bytes), and the size of items held in the buffer In addition, the interpreter places a lock on the buffer that prevents it from being changed by other threads as long as it is held by extension codeThis allows the extension to work with the buffer contents independently, possibly in a different thread than the interpreter It is up to the user to call PyBuffer_Release() on the buffer after all processing is complete The conversion codes of "t#", "w", "w#", and "w*" are just like the "s" family of codes except that they only accept objects implementing the buffer interface "t#" requires the buffer to be readableThe "w" code requires the buffer to be both readable and writable A Python object supporting a writable buffer is assumed to be mutable Thus, it is legal for a C extension to overwrite or modify the buffer contents The conversion codes of "y", "y#", and "y*" are just like the "s" family of codes except that they only accept byte strings Use these to write functions that must only take bytes, not Unicode stringsThe "y" code only accepts byte strings that do not contain embedded NULL characters Table 263 lists conversion codes that are used to accept arbitrary Python objects as input and to leave the result as type PyObject *These are sometimes used for C extensions that need to work with Python objects that are more complicated than simple numbers or strings for example, if you needed a C extension function to accept an instance of a Python class or dictionary