擴充/嵌入常見問題集¶
我可以在 C 中建立自己的函式嗎？¶
是的，你可以在 C 中建立包含函式、變數、例外甚至新型別的內建模組，擴充和嵌入 Python 直譯器 文件中有相關說明。
大多數中級或進階 Python 書籍也會涵蓋這個主題。
我可以在 C++ 中建立自己的函式嗎？¶
Yes, using the C compatibility features found in C++. Place extern "C" {
... } around the Python include files and put extern "C" before each
function that is going to be called by the Python interpreter. Global or static
C++ objects with constructors are probably not a good idea.
寫 C 很難；還有其他選擇嗎？¶
要編寫你自己的 C 擴充有許多替代方法，取決於你要執行的具體操作為何。
Cython and its relative Pyrex are compilers that accept a slightly modified form of Python and generate the corresponding C code. Cython and Pyrex make it possible to write an extension without having to learn Python's C API.
If you need to interface to some C or C++ library for which no Python extension currently exists, you can try wrapping the library's data types and functions with a tool such as SWIG. SIP, CXX Boost, or Weave are also alternatives for wrapping C++ libraries.
如何從 C 執行任意 Python 陳述式？¶
The highest-level function to do this is PyRun_SimpleString() which takes
a single string argument to be executed in the context of the module
__main__ and returns 0 for success and -1 when an exception occurred
(including SyntaxError). If you want more control, use
PyRun_String(); see the source for PyRun_SimpleString() in
Python/pythonrun.c.
How can I evaluate an arbitrary Python expression from C?¶
Call the function PyRun_String() from the previous question with the
start symbol Py_eval_input; it parses an expression, evaluates it and
returns its value.
如何從 Python 物件中提取 C 值？¶
That depends on the object's type. If it's a tuple, PyTuple_Size()
returns its length and PyTuple_GetItem() returns the item at a specified
index. Lists have similar functions, PyList_Size() and
PyList_GetItem().
For bytes, PyBytes_Size() returns its length and
PyBytes_AsStringAndSize() provides a pointer to its value and its
length. Note that Python bytes objects may contain null bytes so C's
strlen() should not be used.
要測試物件的型別，首先確保它不是 NULL，然後再使用 PyBytes_Check()、PyTuple_Check()、PyList_Check() 等函式。
There is also a high-level API to Python objects which is provided by the
so-called 'abstract' interface -- read Include/abstract.h for further
details. It allows interfacing with any kind of Python sequence using calls
like PySequence_Length(), PySequence_GetItem(), etc. as well
as many other useful protocols such as numbers (PyNumber_Index() et
al.) and mappings in the PyMapping APIs.
如何使用 Py_BuildValue() 建立任意長度的元組？¶
這無法做到。請改用 PyTuple_Pack()。
如何從 C 呼叫物件的方法？¶
The PyObject_CallMethod() function can be used to call an arbitrary
method of an object. The parameters are the object, the name of the method to
call, a format string like that used with Py_BuildValue(), and the
argument values:
PyObject *
PyObject_CallMethod(PyObject *object, const char *method_name,
const char *arg_format, ...);
This works for any object that has methods -- whether built-in or user-defined.
You are responsible for eventually Py_DECREF()'ing the return value.
例如，使用引數 10、0 呼叫檔案物件的 "seek" 方法（假設檔案物件指標為 "f"）：
res = PyObject_CallMethod(f, "seek", "(ii)", 10, 0);
if (res == NULL) {
... an exception occurred ...
}
else {
Py_DECREF(res);
}
Note that since PyObject_CallObject() always wants a tuple for the
argument list, to call a function without arguments, pass "()" for the format,
and to call a function with one argument, surround the argument in parentheses,
e.g. "(i)".
我如何捕捉 PyErr_Print() 的輸出（或任何印出到 stdout/stderr 的東西）？¶
In Python code, define an object that supports the write() method. Assign
this object to sys.stdout and sys.stderr. Call print_error, or
just allow the standard traceback mechanism to work. Then, the output will go
wherever your write() method sends it.
最簡單的方法是使用 io.StringIO 類別：
>>> import io, sys
>>> sys.stdout = io.StringIO()
>>> print('foo')
>>> print('hello world!')
>>> sys.stderr.write(sys.stdout.getvalue())
foo
hello world!
A custom object to do the same would look like this:
>>> import io, sys
>>> class StdoutCatcher(io.TextIOBase):
... def __init__(self):
... self.data = []
... def write(self, stuff):
... self.data.append(stuff)
...
>>> import sys
>>> sys.stdout = StdoutCatcher()
>>> print('foo')
>>> print('hello world!')
>>> sys.stderr.write(''.join(sys.stdout.data))
foo
hello world!
如何從 C 存取用 Python 編寫的模組？¶
You can get a pointer to the module object as follows:
module = PyImport_ImportModule("<modulename>");
If the module hasn't been imported yet (i.e. it is not yet present in
sys.modules), this initializes the module; otherwise it simply returns
the value of sys.modules["<modulename>"]. Note that it doesn't enter the
module into any namespace -- it only ensures it has been initialized and is
stored in sys.modules.
You can then access the module's attributes (i.e. any name defined in the module) as follows:
attr = PyObject_GetAttrString(module, "<attrname>");
Calling PyObject_SetAttrString() to assign to variables in the module
also works.
How do I interface to C++ objects from Python?¶
Depending on your requirements, there are many approaches. To do this manually, begin by reading the "Extending and Embedding" document. Realize that for the Python run-time system, there isn't a whole lot of difference between C and C++ -- so the strategy of building a new Python type around a C structure (pointer) type will also work for C++ objects.
對於 C++ 函式庫，請參閱 寫 C 很難；還有其他選擇嗎？。
我使用安裝檔案新增了一個模組，但 make 失敗了；為什麼？¶
Setup must end in a newline, if there is no newline there, the build process fails. (Fixing this requires some ugly shell script hackery, and this bug is so minor that it doesn't seem worth the effort.)
如何為擴充套件除錯？¶
When using GDB with dynamically loaded extensions, you can't set a breakpoint in your extension until your extension is loaded.
In your .gdbinit file (or interactively), add the command:
br _PyImport_LoadDynamicModule
然後，當你運行 GDB 時：
$ gdb /local/bin/python
gdb) run myscript.py
gdb) continue # repeat until your extension is loaded
gdb) finish # so that your extension is loaded
gdb) br myfunction.c:50
gdb) continue
我想在我的 Linux 系統上編譯一個 Python 模組，但是缺少一些檔案。為什麼？¶
Most packaged versions of Python omit some files required for compiling Python extensions.
在 Red Hat 上，請安裝 python3-devel RPM 來取得必要的檔案。
對於 Debian，運行 apt-get install python3-dev。
如何從「無效輸入」區分出「不完整輸入」？¶
Sometimes you want to emulate the Python interactive interpreter's behavior, where it gives you a continuation prompt when the input is incomplete (e.g. you typed the start of an "if" statement or you didn't close your parentheses or triple string quotes), but it gives you a syntax error message immediately when the input is invalid.
在 Python 中，你可以使用 codeop 模組，它充分模擬了剖析器 (parser) 的行為。像是 IDLE 就有使用它。
The easiest way to do it in C is to call PyRun_InteractiveLoop() (perhaps
in a separate thread) and let the Python interpreter handle the input for
you. You can also set the PyOS_ReadlineFunctionPointer() to point at your
custom input function. See Modules/readline.c and Parser/myreadline.c
for more hints.
如何找到未定義的 g++ 符號 __builtin_new 或 __pure_virtual？¶
To dynamically load g++ extension modules, you must recompile Python, relink it
using g++ (change LINKCC in the Python Modules Makefile), and link your
extension module using g++ (e.g., g++ -shared -o mymodule.so mymodule.o).
Can I create an object class with some methods implemented in C and others in Python (e.g. through inheritance)?¶
是的，你可以繼承內建類別，例如 int、list、dict 等。
Boost Python 函式庫（BPL，https://www.boost.org/libs/python/doc/index.html）提供了一種從 C++ 執行此操作的方法（即你可以使用 BPL 來繼承用 C++ 編寫的擴充類別）。