A Python package for reference counting and interop with native pointers
A Python package for reference counting native resources
This package is primarily for managing resources in native libraries, written for instance in C++, from Python. While it boils down to "simply" maintaining a set of counters, it is practically complicated to properly do so and not end up with memory leak or crashes. This package aims to offer structured options for managing external native resources - I could not locate a pypi package doing just what I needed. Other use cases requiring reference counting, aside from native library resources, may benefit from reusing and extending classes in
MIT-derived (see License.txt)
pip install refcount
pip install -r requirements.txt python setup.py install
A canonical illustration of the use of this package, based on one of the unit tests. Say we have a C++ library with objects and a C API:
#define TEST_DOG_PTR testnative::dog* #define TEST_OWNER_PTR testnative::owner* #define TEST_COUNTED_PTR testnative::reference_counter* testnative::dog* create_dog(); testnative::owner* create_owner(testnative::dog* d); void say_walk(testnative::owner* owner); void release(testnative::reference_counter* obj); // etc.
From the outside of the library the API is exported with opaque pointers
void* (C structs pointers and native C99 types could be handled too).
void* create_dog(); void* create_owner(void* d); void say_walk(void* owner); void release(void* obj); // etc.
Starting with the end in mind, from Python we want an API hiding the low level details close to the C API, in particular avoiding managing native memory via
release C API calls, piggybacking the python GC instead.
dog = Dog() owner = DogOwner(dog) owner.say_walk() print(dog.position) dog = None # the "native dog" is still alive though, as the owner incremented the ref count owner = None
This is doable with
refcount and the
cffi package. One possible design is:
ut_ffi = cffi.FFI() ut_ffi.cdef('extern void* create_dog();') ut_ffi.cdef('extern void* create_owner( void* d);') ut_ffi.cdef('extern void say_walk( void* owner);') ut_ffi.cdef('extern void release( void* obj);') # etc. ut_dll = ut_ffi.dlopen('c:/path/to/test_native_library.dll', 1) # Lazy loading class CustomCffiNativeHandle(CffiNativeHandle): def __init__(self, pointer, prior_ref_count = 0): super(CustomCffiNativeHandle, self).__init__(pointer, type_id='', prior_ref_count = prior_ref_count) def _release_handle(self): ut_dll.release(self.get_handle()) return True class Dog(CustomCffiNativeHandle): def __init__(self, pointer = None): if pointer is None: pointer = ut_dll.create_dog() super(Dog, self).__init__(pointer) # etc. class DogOwner(CustomCffiNativeHandle): def __init__(self, dog): super(DogOwner, self).__init__(None) self._set_handle(ut_dll.create_owner(dog.get_handle())) self.dog = dog self.dog.add_ref() # Do note this important reference increment def say_walk(self): ut_dll.say_walk(self.get_handle()) def _release_handle(self): super(DogOwner, self)._release_handle() # super(DogOwner, self)._release_handle() self.dog.release() return True
This python package
refcount actually spawned from prior work for interoperability between C++, R and .NET. The port to Python was also influenced by work authored by Kevin Plastow and undertaken at the Australian Bureau of Meteorology for C/C++/Python interop using
Readers may also want to look at:
- a nuget package dynamic-interop-dll for .NET/native interop.
- A set of mostly c++ software tools for interop with C/C++
- A C# library for generating interop glue code on top of C API glue code.
Other python packages
While this present package was authored in part because no existing prior (Python) work could quite fit the need, there are packages that may better address your particular need:
- infi.pyutils constains a reference counting class.
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