extractor interface which can be used to extract C++ types from Python objects.

We have discussed two main use cases for extractions

1. In user code which needs to retrieve C++ objects from their corresponding Python objects:

1. Getting value:

   •     void f(object x)
         {
             int y = extract<int>(x); // retrieve an int from x
         }

2. Users may also want to explicitly check for convertibility:

   •     int g(object x)
         {
             extract<int> get_int(x);
             if (get_int.check())
                 return get_int();
             else
                 return 0;
         }

2. In the implementation of sequence from_python converters (e.g. Python tuple/list -> std::vector)