内容
Yes, you can create built-in modules containing functions, variables, exceptions and even new types in C. This is explained in the document 扩展和嵌入 Python 解释器 .
Most intermediate or advanced Python books will also cover this topic.
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.
There are a number of alternatives to writing your own C extensions, depending on what you’re trying to do.
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 ，或 Weave are also alternatives for wrapping C++ libraries.
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__
并返回
0
对于成功和
-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
.
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.
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,
PyListSize()
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.
To test the type of an object, first make sure it isn’t
NULL
, and then use
PyBytes_Check()
,
PyTuple_Check()
,
PyList_Check()
，等。
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.
You can’t. Use
PyTuple_Pack()
代替。
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.
To call, e.g., a file object’s “seek” method with arguments 10, 0 (assuming the file object pointer is “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)”.
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.
The easiest way to do this is to use the
io.StringIO
类：
>>> 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!
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>");
调用
PyObject_SetAttrString()
to assign to variables in the module also works.
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.
For C++ libraries, see Writing C is hard; are there any alternatives? .
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.)
When using GDB with dynamically loaded extensions, you can’t set a breakpoint in your extension until your extension is loaded.
在您的
.gdbinit
file (or interactively), add the command:
br _PyImport_LoadDynamicModule
Then, when you run GDB:
$ 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
Most packaged versions of Python don’t include the
/usr/lib/python2.
x
/config/
directory, which contains various files required for compiling Python extensions.
For Red Hat, install the python-devel RPM to get the necessary files.
For Debian, run
apt-get
安装
python-dev
.
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.
In Python you can use the
codeop
module, which approximates the parser’s behavior sufficiently. IDLE uses this, for example.
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.
However sometimes you have to run the embedded Python interpreter in the same thread as your rest application and you can’t allow the
PyRun_InteractiveLoop()
to stop while waiting for user input. The one solution then is to call
PyParser_ParseString()
and test for
e.error
等于
E_EOF
, which means the input is incomplete. Here’s a sample code fragment, untested, inspired by code from Alex Farber:
#include <Python.h>
#include <node.h>
#include <errcode.h>
#include <grammar.h>
#include <parsetok.h>
#include <compile.h>
int testcomplete(char *code)
/* code should end in \n */
/* return -1 for error, 0 for incomplete, 1 for complete */
{
node *n;
perrdetail e;
n = PyParser_ParseString(code, &_PyParser_Grammar,
Py_file_input, &e);
if (n == NULL) {
if (e.error == E_EOF)
return 0;
return -1;
}
PyNode_Free(n);
return 1;
}
Another solution is trying to compile the received string with
Py_CompileString()
. If it compiles without errors, try to execute the returned code object by calling
PyEval_EvalCode()
. Otherwise save the input for later. If the compilation fails, find out if it’s an error or just more input is required - by extracting the message string from the exception tuple and comparing it to the string “unexpected EOF while parsing”. Here is a complete example using the GNU readline library (you may want to ignore
SIGINT
while calling readline()):
#include <stdio.h>
#include <readline.h>
#include <Python.h>
#include <object.h>
#include <compile.h>
#include <eval.h>
int main (int argc, char* argv[])
{
int i, j, done = 0; /* lengths of line, code */
char ps1[] = ">>> ";
char ps2[] = "... ";
char *prompt = ps1;
char *msg, *line, *code = NULL;
PyObject *src, *glb, *loc;
PyObject *exc, *val, *trb, *obj, *dum;
Py_Initialize ();
loc = PyDict_New ();
glb = PyDict_New ();
PyDict_SetItemString (glb, "__builtins__", PyEval_GetBuiltins ());
while (!done)
{
line = readline (prompt);
if (NULL == line) /* Ctrl-D pressed */
{
done = 1;
}
else
{
i = strlen (line);
if (i > 0)
add_history (line); /* save non-empty lines */
if (NULL == code) /* nothing in code yet */
j = 0;
else
j = strlen (code);
code = realloc (code, i + j + 2);
if (NULL == code) /* out of memory */
exit (1);
if (0 == j) /* code was empty, so */
code[0] = '\0'; /* keep strncat happy */
strncat (code, line, i); /* append line to code */
code[i + j] = '\n'; /* append '\n' to code */
code[i + j + 1] = '\0';
src = Py_CompileString (code, "<stdin>", Py_single_input);
if (NULL != src) /* compiled just fine - */
{
if (ps1 == prompt || /* ">>> " or */
'\n' == code[i + j - 1]) /* "... " and double '\n' */
{ /* so execute it */
dum = PyEval_EvalCode (src, glb, loc);
Py_XDECREF (dum);
Py_XDECREF (src);
free (code);
code = NULL;
if (PyErr_Occurred ())
PyErr_Print ();
prompt = ps1;
}
} /* syntax error or E_EOF? */
else if (PyErr_ExceptionMatches (PyExc_SyntaxError))
{
PyErr_Fetch (&exc, &val, &trb); /* clears exception! */
if (PyArg_ParseTuple (val, "sO", &msg, &obj) &&
!strcmp (msg, "unexpected EOF while parsing")) /* E_EOF */
{
Py_XDECREF (exc);
Py_XDECREF (val);
Py_XDECREF (trb);
prompt = ps2;
}
else /* some other syntax error */
{
PyErr_Restore (exc, val, trb);
PyErr_Print ();
free (code);
code = NULL;
prompt = ps1;
}
}
else /* some non-syntax error */
{
PyErr_Print ();
free (code);
code = NULL;
prompt = ps1;
}
free (line);
}
}
Py_XDECREF(glb);
Py_XDECREF(loc);
Py_Finalize();
exit(0);
}
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
).
Yes, you can inherit from built-in classes such as
int
,
list
,
dict
，等。
The Boost Python Library (BPL, http://www.boost.org/libs/python/doc/index.html ) provides a way of doing this from C++ (i.e. you can inherit from an extension class written in C++ using the BPL).