salt/doc/topics/development/dunder_dictionaries.rst

===================
Dunder Dictionaries
===================

Salt provides several special "dunder" dictionaries as a convenience for Salt
development.  These include ``__opts__``, ``__context__``, ``__salt__``, and
others. This document will describe each dictionary and detail where they exist
and what information and/or functionality they provide.


__opts__
--------

Available in
~~~~~~~~~~~~

- All loader modules

The ``__opts__`` dictionary contains all of the options passed in the
configuration file for the master or minion.

.. note::

    In many places in salt, instead of pulling raw data from the __opts__
    dict, configuration data should be pulled from the salt `get` functions
    such as config.get, aka - __salt__['config.get']('foo:bar')
    The `get` functions also allow for dict traversal via the *:* delimiter.
    Consider using get functions whenever using __opts__ or __pillar__ and
    __grains__ (when using grains for configuration data)

The configuration file data made available in the ``__opts__`` dictionary is the
configuration data relative to the running daemon. If the modules are loaded and
executed by the master, then the master configuration data is available, if the
modules are executed by the minion, then the minion configuration is
available. Any additional information passed into the respective configuration
files is made available

__salt__
--------

Available in
~~~~~~~~~~~~

- Execution Modules
- State Modules
- Returners
- Runners

``__salt__`` contains the execution module functions. This allows for all
functions to be called as they have been set up by the salt loader.

.. code-block:: python

    __salt__['cmd.run']('fdisk -l')
    __salt__['network.ip_addrs']()

.. note::

    When used in runners, ``__salt__`` references other runner modules, and not
    execution modules.

__grains__
----------

Available in
~~~~~~~~~~~~

- Execution Modules
- State Modules
- Returners
- External Pillar

The ``__grains__`` dictionary contains the grains data generated by the minion
that is currently being worked with. In execution modules, state modules and
returners this is the grains of the minion running the calls, when generating
the external pillar the ``__grains__`` is the grains data from the minion that
the pillar is being generated for.

__pillar__
-----------

Available in
~~~~~~~~~~~~

- Execution Modules
- State Modules
- Returners

The ``__pillar__`` dictionary contains the pillar for the respective minion.

__context__
-----------

``__context__`` exists in state modules and execution modules.

During a state run the ``__context__`` dictionary persists across all states
that are run and then is destroyed when the state ends.

When running an execution module ``__context__`` persists across all module
executions until the modules are refreshed; such as when
:py:func:`saltutil.sync_all <salt.modules.saltutil.sync_all>` or
:py:func:`state.apply <salt.modules.state.apply_>` are executed.

A great place to see how to use ``__context__`` is in the cp.py module in
salt/modules/cp.py. The fileclient authenticates with the master when it is
instantiated and then is used to copy files to the minion. Rather than create a
new fileclient for each file that is to be copied down, one instance of the
fileclient is instantiated in the ``__context__`` dictionary and is reused for
each file. Here is an example from salt/modules/cp.py:

.. code-block:: python

    if not 'cp.fileclient' in __context__:
        __context__['cp.fileclient'] = salt.fileclient.get_file_client(__opts__)


.. note:: Because __context__ may or may not have been destroyed, always be
          sure to check for the existence of the key in __context__ and
          generate the key before using it.