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- [section Pickle support]
- [section Introduction]
- Pickle is a Python module for object serialization, also known as persistence, marshalling, or flattening.
- It is often necessary to save and restore the contents of an object to a file. One approach to this problem is to write a pair of functions that read and write data from a file in a special format. A powerful alternative approach is to use Python's pickle module. Exploiting Python's ability for introspection, the pickle module recursively converts nearly arbitrary Python objects into a stream of bytes that can be written to a file.
- The Boost Python Library supports the pickle module through the interface as described in detail in the [@https://docs.python.org/2/library/pickle.html Python Library Reference for pickle]. This interface involves the special methods `__getinitargs__`, `__getstate__` and `__setstate__` as described in the following. Note that `Boost.Python` is also fully compatible with Python's cPickle module.
- [endsect]
- [section The Pickle Interface]
- At the user level, the Boost.Python pickle interface involves three special methods:
- [variablelist
- [[__getinitargs__][When an instance of a Boost.Python extension class is pickled, the pickler tests if the instance has a `__getinitargs__` method. This method must return a Python `tuple` (it is most convenient to use a [link object_wrappers.boost_python_tuple_hpp.class_tuple `boost::python::tuple`]). When the instance is restored by the unpickler, the contents of this tuple are used as the arguments for the class constructor.
- If `__getinitargs__` is not defined, `pickle.load` will call the constructor (`__init__`) without arguments; i.e., the object must be default-constructible.]]
- [[__getstate__][When an instance of a `Boost.Python` extension class is pickled, the pickler tests if the instance has a `__getstate__` method. This method should return a Python object representing the state of the instance.]]
- [[__setstate__][When an instance of a `Boost.Python` extension class is restored by the unpickler (`pickle.load`), it is first constructed using the result of `__getinitargs__` as arguments (see above). Subsequently the unpickler tests if the new instance has a `__setstate__` method. If so, this method is called with the result of `__getstate__` (a Python object) as the argument.]]
- ]
- The three special methods described above may be `.def()`\ 'ed individually by the user. However, `Boost.Python` provides an easy to use high-level interface via the `boost::python::pickle_suite` class that also enforces consistency: `__getstate__` and `__setstate__` must be defined as pairs. Use of this interface is demonstrated by the following examples.
- [endsect]
- [section Example]
- There are three files in `python/test` that show how to provide pickle support.
- [section pickle1.cpp]
- The C++ class in this example can be fully restored by passing the appropriate argument to the constructor. Therefore it is sufficient to define the pickle interface method `__getinitargs__`. This is done in the following way:
- Definition of the C++ pickle function:
- ``
- struct world_pickle_suite : boost::python::pickle_suite
- {
- static
- boost::python::tuple
- getinitargs(world const& w)
- {
- return boost::python::make_tuple(w.get_country());
- }
- };
- ``
- Establishing the Python binding:
- ``
- class_<world>("world", args<const std::string&>())
- // ...
- .def_pickle(world_pickle_suite())
- // ...
- ``
- [endsect]
- [section pickle2.cpp]
- The C++ class in this example contains member data that cannot be restored by any of the constructors. Therefore it is necessary to provide the `__getstate__`/`__setstate__` pair of pickle interface methods:
- Definition of the C++ pickle functions:
- ``
- struct world_pickle_suite : boost::python::pickle_suite
- {
- static
- boost::python::tuple
- getinitargs(const world& w)
- {
- // ...
- }
- static
- boost::python::tuple
- getstate(const world& w)
- {
- // ...
- }
- static
- void
- setstate(world& w, boost::python::tuple state)
- {
- // ...
- }
- };
- ``
- Establishing the Python bindings for the entire suite:
- ``
- class_<world>("world", args<const std::string&>())
- // ...
- .def_pickle(world_pickle_suite())
- // ...
- ``
- For simplicity, the `__dict__` is not included in the result of `__getstate__`. This is not generally recommended, but a valid approach if it is anticipated that the object's `__dict__` will always be empty. Note that the safety guard described below will catch the cases where this assumption is violated.
- [endsect]
- [section pickle3.cpp]
- This example is similar to pickle2.cpp. However, the object's `__dict__` is included in the result of `__getstate__`. This requires a little more code but is unavoidable if the object's `__dict__` is not always empty.
- [endsect]
- [endsect]
- [section Pitfall and Safety Guard]
- The pickle protocol described above has an important pitfall that the end user of a Boost.Python extension module might not be aware of:
- [*`__getstate__` is defined and the instance's `__dict__` is not empty.]
- The author of a `Boost.Python` extension class might provide a `__getstate__` method without considering the possibilities that:
- * his class is used in Python as a base class. Most likely the `__dict__` of instances of the derived class needs to be pickled in order to restore the instances correctly.
- * the user adds items to the instance's `__dict__` directly. Again, the `__dict__` of the instance then needs to be pickled.
- To alert the user to this highly unobvious problem, a safety guard is provided. If `__getstate__` is defined and the instance's `__dict__` is not empty, `Boost.Python` tests if the class has an attribute `__getstate_manages_dict__`. An exception is raised if this attribute is not defined:
- ``
- RuntimeError: Incomplete pickle support (__getstate_manages_dict__ not set)
- ``
- To resolve this problem, it should first be established that the `__getstate__` and `__setstate__` methods manage the instances's `__dict__` correctly. Note that this can be done either at the C++ or the Python level. Finally, the safety guard should intentionally be overridden. E.g. in C++ (from pickle3.cpp):
- ``
- struct world_pickle_suite : boost::python::pickle_suite
- {
- // ...
- static bool getstate_manages_dict() { return true; }
- };
- ``
- Alternatively in Python:
- ``
- import your_bpl_module
- class your_class(your_bpl_module.your_class):
- __getstate_manages_dict__ = 1
- def __getstate__(self):
- # your code here
- def __setstate__(self, state):
- # your code here
- ``
- [endsect]
- [section Practical Advice]
- * In `Boost.Python` extension modules with many extension classes, providing complete pickle support for all classes would be a significant overhead. In general complete pickle support should only be implemented for extension classes that will eventually be pickled.
- * Avoid using `__getstate__` if the instance can also be reconstructed by way of `__getinitargs__`. This automatically avoids the pitfall described above.
- * If `__getstate__` is required, include the instance's `__dict__` in the Python object that is returned.
- [endsect]
- [section Light-weight alternative: pickle support implemented in Python]
- The pickle4.cpp example demonstrates an alternative technique for implementing pickle support. First we direct Boost.Python via the class_::enable_pickling() member function to define only the basic attributes required for pickling:
- ``
- class_<world>("world", args<const std::string&>())
- // ...
- .enable_pickling()
- // ...
- ``
- This enables the standard Python pickle interface as described in the Python documentation. By "injecting" a `__getinitargs__` method into the definition of the wrapped class we make all instances pickleable:
- ``
- # import the wrapped world class
- from pickle4_ext import world
- # definition of __getinitargs__
- def world_getinitargs(self):
- return (self.get_country(),)
- # now inject __getinitargs__ (Python is a dynamic language!)
- world.__getinitargs__ = world_getinitargs
- ``
- See also the tutorial section on injecting additional methods from Python.
- [endsect]
- [endsect]
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