"types" document needs update to "custom types"

the TypeDecorator class is deprecated and is no longer needed. the "types.myt" document should be updated.

The TypeEngine base class now looks like:

class TypeEngine(object):
    def get_col_spec(self):
    def convert_bind_param(self, value, engine):
    def convert_result_value(self, value, engine):
    def set_impl(self, impltype):
    def get_constructor_args(self):
    def adapt_args(self):

so a user-defined type would want to override usually get_col_spec to provide the "type" used in a CREATE TABLE statement, and convert_bind_param/convert_result_value to provide translation rules for bind parameters and result-set values.

get_constructor_args should return a dictionary consisting of the arguments sent to this TypeEngine object's init method. such as, if the object were constructed like:

    t = MyType(foo=5, blahblah=7)

then get_constructor_args should return:

     {'foo':5, 'blahblah':7}

it is only necessary to override get_constructor_args if you are overriding one of the existing types such as String, Date, Float, etc. AND you are providing new constructor arguments for your type.

so a typical overridden type, like overriding Binary with a Pickle type, looks like:

import cPickle

class PickleType(types.Binary):
      def __init__(self, protocol=cPickle.HIGHEST_PROTOCOL):
           """allows the pickle protocol to be specified"""
           self.protocol = protocol
      def convert_result_value(self, value, engine):
            return cpickle.loads(super(PickleType, self).convert_result_value(value, engine), self.protocol)
      def convert_bind_param(self, value, engine):
            return super(PickleType, self).convert_bind_param(cpickle.dumps(value, self.protocol), engine)
      def get_constructor_args(self):
            return {'protocol':self.protocol}

and a totally custom user type, to store a hypothetical object called a CrazyObject, looks like:

    class MyType(TypeEngine):
       def __init__(param=12):
           self.param = param
       def get_col_spec(self):
            return "MY_CRAZY_TYPE(%d)" % self.param
       def convert_bind_param(self, value, engine):
            return "%s:%d - %s" % (value.param1, value.count, value.param2)
       def convert_result_value(self, value, engine):
            return CrazyObject.parse_from_string(value)
       def set_impl(self, impltype):
            pass

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