formations/algo/poo/cours/annexes/symbol.txt

Symbols
=======

Using sets
-----------

There are different ways of defining symbols.
You can do it very simply with sets::

  >>> s = set("abc", "def", "ghi")
  Traceback (most recent call last):
    File "<stdin>", line 1, in <module>
  TypeError: set expected at most 1 arguments, got 3
  >>> s = set(["abc", "def", "ghi"])
  >>> s
  set(['abc', 'ghi', 'def'])
  >>> "abc" in s
  True
  >>>


If you are using python 3.X, the notation::

  Python 3.5.2 (default, Sep 10 2016, 08:21:44)
  [GCC 5.4.0 20160609] on linux
  Type "help", "copyright", "credits" or "license" for more information.
  >>> 1 in {1,2,3}
  True
  >>>

is very effective for working with symbols.
Use immutable sets.

Using namespaces
-----------------

Here is a real world example from the tiramisu library

.. code-block:: python

    # ____________________________________________________________
    class _NameSpace(object):
        """convenient class that emulates a module
        and builds constants (that is, unique names)
        when attribute is added, we cannot delete it
        """

        def __setattr__(self, name, value):
            if name in self.__dict__:  # pragma: optional cover
                raise ConstError(_("can't rebind {0}").format(name))
            self.__dict__[name] = value

        def __delattr__(self, name):  # pragma: optional cover
            if name in self.__dict__:
                raise ConstError(_("can't unbind {0}").format(name))
            raise ValueError(name)


    class GroupModule(_NameSpace):
        "emulates a module to manage unique group (OptionDescription) names"
        class GroupType(str):
            """allowed normal group (OptionDescription) names
            *normal* means : groups that are not master
            """
            pass

        class DefaultGroupType(GroupType):
            """groups that are default (typically 'default')"""
            pass

        class MasterGroupType(GroupType):
            """allowed normal group (OptionDescription) names
            *master* means : groups that have the 'master' attribute set
            """
            pass


    class OwnerModule(_NameSpace):
        """emulates a module to manage unique owner names.

        owners are living in `Config._cfgimpl_value_owners`
        """
        class Owner(str):
            """allowed owner names
            """
            pass

        class DefaultOwner(Owner):
            """groups that are default (typically 'default')"""
            pass


    class MultiTypeModule(_NameSpace):
        "namespace for the master/slaves"
        class MultiType(str):
            pass

        class DefaultMultiType(MultiType):
            pass

        class MasterMultiType(MultiType):
            pass

        class SlaveMultiType(MultiType):
            pass


    # ____________________________________________________________
    def populate_groups():
        """populates the available groups in the appropriate namespaces

        groups.default
            default group set when creating a new optiondescription

        groups.master
            master group is a special optiondescription, all suboptions should be
            multi option and all values should have same length, to find master's
            option, the optiondescription's name should be same than de master's
            option

        groups.family
            example of group, no special behavior with this group's type
        """
        groups.default = groups.DefaultGroupType('default')
        groups.master = groups.MasterGroupType('master')
        groups.family = groups.GroupType('family')


    def populate_owners():
        """populates the available owners in the appropriate namespaces

        default
            is the config owner after init time

        user
            is the generic is the generic owner
        """
        setattr(owners, 'default', owners.DefaultOwner('default'))
        setattr(owners, 'user', owners.Owner('user'))

        def addowner(name):
            """
            :param name: the name of the new owner
            """
            setattr(owners, name, owners.Owner(name))
        setattr(owners, 'addowner', addowner)

    # ____________________________________________________________
    # populate groups and owners with default attributes
    groups = GroupModule()
    populate_groups()
    owners = OwnerModule()
    populate_owners()