我正在使用boost.python来包装一个C ++类'A'，它将一个字符串作为构造函数。然后我有一个'fun（A＆amp; arg）'函数，它引用'A'作为参数。我想有一个'fun'的python包装器，如果我传递一个对python字符串的引用的变量，这个引用首先会自动转换为对'A'的引用。
一个例子可能有所帮助。在python方面，我希望能够做到这一点：
a = 'some string'
fun(a)
然后让'a'实际上是（引用）'A'，而不是（引用）原始字符串。我想这样做是因为我希望能够避免将其写为
a = A('some string')
fun(a)
（你可能有充分的理由怀疑这是一个相关的救星，但我们只是假设它对我很重要。）
这样的事情可能吗？如果不使用boost.python，也许直接使用Python-C API？
注意：我知道如果我写的话
fun('some string')
无法将对字符串的引用转换为对其他类型的引用。
答案 0 :(得分：3)
这是可能的，但解决方案可能依赖于Python实现。
例如，在Python 2.7中，inspect模块和sys.settrace()可用于修改特定帧上的locals()。这也可以在Python / C API中完成，但在Python中操作Python框架通常更易于管理。
在下面example.py中，update_locals()函数将更新给定框架中的locals()：
import inspect
import sys
def _force_locals(old_frame, new_locals):
''' Force a new set of locals on a given frame.
:param old_frame: The frame to which locals will be applied.
:type old_frame: frame.
:param new_locals: What locals() should return for old_frame.
:type new_locals: dict.
.. note:: This function will force a custom trace function onto
the old_frame. Within the context of a trace function
(often used for debugging), a frame's f_locals is
modifiable. In most execution paths, f_locals is
writable but not modifiable.
'''
# Force tracing by setting the global tracing function to
# any non-None function.
if not sys.gettrace():
sys.settrace(lambda *args, **keys: None)
# Custom trace function that will force locals.
def trace(frame, event, arg):
# Update the frame's locals.
frame.f_locals.update(new_locals)
# Set frame to use default trace, preventing this trace
# function from resetting the locals on each trace call.
del frame.f_trace
# Set the old frame's trace function to the custom trace.
old_frame.f_trace = trace
def update_locals(frame, *refs):
''' Modifies a frame's locals based on the provided references.
:param frame: The frame from which a locals will be updated.
:type frame: frame.
:param refs: Iterable pair of (old_ref, new_ref) tuples.
:type refs: Iterable type of pairs.
'''
new_locals = frame.f_locals.copy()
has_changes = False
# If a frame's local has an identity patch with a provided
# reference, then update new_locals with the new reference.
for key, ref in new_locals.iteritems():
for old_ref, new_ref in refs:
if ref is old_ref:
new_locals[key] = new_ref
has_changes = True
# If new_locals has changes, then force them onto the frame.
if has_changes:
_force_locals(frame, new_locals)
交互式使用：
>>> import example
>>> import inspect
>>> x = 42
>>> x
42
>>> example.update_locals(inspect.currentframe(), (x, '3.14'))
>>> x
'3.14'
x变量引用了int(42)对象，但example.update_locals()函数已将x更改为引用str('3.14')对象。
由于能够修改调用者的框架，下一步是在Python中修补C ++ fun()以构造A的实例，如果参数是str的实例，然后委托C ++ fun(A&)函数并更新调用者的框架。
在下面的示例中，C ++ Spam类型和fun(Spam&)函数向_example Python模块公开。
#include <iostream>
#include <string>
#include <boost/python.hpp>
/// @brief Mockup type.
class Spam
{
public:
explicit Spam(std::string str)
: str_(str)
{}
void action()
{
std::cout << "Spam::action(): " << str_ << std::endl;
}
private:
std::string str_;
};
/// @brief Mockup function.
void fun(Spam& spam)
{
std::cout << "fun() -> ";
spam.action();
}
BOOST_PYTHON_MODULE(_example)
{
namespace python = boost::python;
python::class_<Spam>("Spam", python::init<std::string>());
python::def("fun", &fun);
}
如果提供给example的参数是_example.fun()的实例并进行操作，则更高级Spam模块将使补丁fun()构建str对象呼叫者的框架与上述类似的方式：
from _example import *
import inspect
import sys
def _force_locals(old_frame, new_locals):
''' Force a new set of locals on a given frame.
:param old_frame: The frame to which locals will be applied.
:type old_frame: frame.
:param new_locals: What locals() should return for old_frame.
:type new_locals: dict.
.. note:: This function will force a custom trace function onto
the old_frame. Within the context of a trace function
(often used for debugging), a frame's f_locals is
modifiable. In most execution paths, f_locals is
writable but not modifiable.
'''
# Force tracing by setting the global tracing function to
# any non-None function.
if not sys.gettrace():
sys.settrace(lambda *args, **keys: None)
# Custom trace function that will force locals.
def trace(frame, event, arg):
# Update the frame's locals.
frame.f_locals.update(new_locals)
# Set frame to use default trace, preventing this trace
# function from resetting the locals on each trace call.
del frame.f_trace
# Set the old frame's trace function to the custom trace.
old_frame.f_trace = trace
def _update_locals(frame, *refs):
''' Modifies a frame's locals based on the provided references.
:param frame: The frame from which a locals will be updated.
:type frame: frame.
:param refs: Iterable pair of (old_ref, new_ref) tuples.
:type refs: Iterable type of pairs.
'''
new_locals = frame.f_locals.copy()
has_changes = False
# If a frame's local has an identity patch with a provided
# reference, then update new_locals with the new reference.
for key, ref in new_locals.iteritems():
for old_ref, new_ref in refs:
if ref is old_ref:
new_locals[key] = new_ref
has_changes = True
# If new_locals has changes, then force them onto the frame.
if has_changes:
_force_locals(frame, new_locals)
def _patch_fun():
old_fun = fun
# Create a function that will perform custom operations then
# delegate to the original function.
def patch(spam, *args, **kwargs):
if isinstance(spam, str):
old_spam, spam = spam, Spam(spam)
# In the caller's frame, force the variables that reference
# old_spam to now reference spam.
_update_locals(
inspect.currentframe(1), # Caller's frame.
(old_spam, spam))
return old_fun(spam, *args, **kwargs)
return patch
fun = _patch_fun()
交互式使用：
>>> import example
>>> s1 = example.Spam('abc')
>>> type(s1)
<class '_example.Spam'>
>>> example.fun(s1)
fun() -> Spam::action(): abc
>>> type(s1) # s1's type has not changed.
<class '_example.Spam'>
>>> s2 = 'def'
>>> type(s2)
<type 'str'>
>>> example.fun(s2)
fun() -> Spam::action(): def
>>> type(s2) # s2's type has changed.
<class '_example.Spam'>
>>> example.fun('ghi')
fun() -> Spam::action(): ghi
请注意，上例中的帧操作仅修改了fun()的调用者的帧，而不是整个堆栈。
答案 1 :(得分：0)
要使其工作fun(a)，必须修改原始的a对象引用。 fun(a)实际获得的是对象引用a的本地副本，而不是作为参数传递的原始a对象引用。
换句话说，Python不能像那样工作，你需要将其称为a = fun(a)才能更改引用a（而不是它引用的对象）。