Add module tutorial

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doc/tutorial/module/default_values.rst

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+.. _tutorial-module-default-values:
+
+Set default values
+==================
+
+Default values are useful to save time for users when entering data.
+On Tryton :ref:`default values <topics-fields_default_value>` are computed on
+server side and they will be set by the client when creating a new record if
+the field is shown on the view.
+If the field is not shown on the view, the server will set this values when
+storing the new records in the database.
+
+In order to define a default value for a field you should define a class method
+named ``default_<field_name>`` that returns the default value.
+For example to add today as the default date of our ``Opportunity`` model the
+following class method is added in :file:`opportunity.py` file:
+
+.. code-block:: python
+
+    from trytond.pool import Pool
+    ...
+    class Opportunity(ModelSQL, ModelView):
+        ...
+        @classmethod
+        def default_start_date(cls):
+            pool = Pool()
+            Date = pool.get('ir.date')
+            return Date.today()
+
+.. _tutorial-module-calling-other-classes:
+
+Call other model methods
+------------------------
+
+In the previous example we called the ``today`` method of the ``ir.date`` model
+from the :class:`~trytond.pool.Pool` instance.
+The :attr:`~trytond.model.Model.__name__` value is used to get the class.
+It is very important to get the class from the pool instead of using a normal
+Python import, because the pool ensures that all of the extensions are applied
+depending on the activated modules.
+For example, if we have the company module also activated the correct timezone
+for the user company will be used for computing the today value.
+
+Great, you have learned how to define default values, and how to call methods
+defined on other classes in the pool.
+Let's continue with :ref:`reacting on user input <tutorial-module-on-change>`.