Many :ref:`plugin_hooks` are passed objects that provide access to internal Datasette functionality. The interface to these objects should not be considered stable (at least until Datasette 1.0) with the exception of methods that are documented on this page.
.._internals_datasette:
Datasette class
~~~~~~~~~~~~~~~
This object is an instance of the ``Datasette`` class, passed to many plugin hooks as an argument called ``datasette``.
The name of the plugin to look up configuration for. Usually this is something similar to ``datasette-cluster-map``.
``database`` - None or string
The database the user is interacting with.
``table`` - None or string
The table the user is interacting with.
This method lets you read plugin configuration values that were set in ``metadata.json``. See :ref:`plugins_plugin_config` for full details of how this method should be used.
The template file to be rendered, e.g. ``my_plugin.html``. Datasette will search for this file first in the ``--template-dir=`` location, if it was specified - then in the plugin's bundled templates and finally in Datasette's set of default templates.
If you pass a Datasette request object here it will be made available to the template.
Renders a `Jinja template <https://jinja.palletsprojects.com/en/2.11.x/>`__ using Datasette's preconfigured instance of Jinja and returns the resulting string. The template will have access to Datasette's default template functions and any functions that have been made available by other plugins.
.._datasette_add_database:
.add_database(name, db)
-----------------------
``name`` - string
The unique name to use for this database. Also used in the URL.
``db`` - datasette.database.Database instance
The database to be attached.
The ``datasette.add_database(name, db)`` method lets you add a new database to the current Datasette instance. This database will then be served at URL path that matches the ``name`` parameter, e.g. ``/mynewdb/``.
The ``db`` parameter should be an instance of the ``datasette.database.Database`` class. For example:
This will add a mutable database from the provided file path.
The ``Database()`` constructor takes four arguments: the first is the ``datasette`` instance you are attaching to, the second is a ``path=``, then ``is_mutable`` and ``is_memory`` are both optional arguments.
Use ``is_mutable`` if it is possible that updates will be made to that database - otherwise Datasette will open it in immutable mode and any changes could cause undesired behavior.
Use ``is_memory`` if the connection is to an in-memory SQLite database.
.._datasette_remove_database:
.remove_database(name)
----------------------
``name`` - string
The name of the database to be removed.
This removes a database that has been previously added. ``name=`` is the unique name of that database, also used in the URL for it.
.._internals_database:
Database class
~~~~~~~~~~~~~~
Instances of the ``Database`` class can be used to execute queries against attached SQLite databases, and to run introspection against their schemas.
Executes a SQL query against the database and returns the resulting rows (see :ref:`database_results`).
``sql`` - string (required)
The SQL query to execute. This can include ``?`` or ``:named`` parameters.
``params`` - list or dict
A list or dictionary of values to use for the parameters. List for ``?``, dictionary for ``:named``.
``truncate`` - boolean
Should the rows returned by the query be truncated at the maximum page size? Defaults to ``True``, set this to ``False`` to disable truncation.
``custom_time_limit`` - integer ms
A custom time limit for this query. This can be set to a lower value than the Datasette configured default. If a query takes longer than this it will be terminated early and raise a ``dataette.database.QueryInterrupted`` exception.
``page_size`` - integer
Set a custom page size for truncation, over-riding the configured Datasette default.
``log_sql_errors`` - boolean
Should any SQL errors be logged to the console in addition to being raised as an error? Defaults to ``True``.
.._database_results:
Results
-------
The ``db.execute()`` method returns a single ``Results`` object. This can be used to access the rows returned by the query.
Iterating over a ``Results`` object will yield SQLite `Row objects <https://docs.python.org/3/library/sqlite3.html#row-objects>`__. Each of these can be treated as a tuple or can be accessed using ``row["column"]`` syntax:
..code-block:: python
info = []
results = await db.execute("select name from sqlite_master")
for row in results:
info.append(row["name"])
The ``Results`` object also has the following properties and methods:
``.truncated`` - boolean
Indicates if this query was truncated - if it returned more results than the specified ``page_size``. If this is true then the results object will only provide access to the first ``page_size`` rows in the query result. You can disable truncation by passing ``truncate=False`` to the ``db.query()`` method.
``.columns`` - list of strings
A list of column names returned by the query.
``.rows`` - list of sqlite3.Row
This property provides direct access to the list of rows returned by the database. You can access specific rows by index using ``results.rows[0]``.
``.first()`` - row or None
Returns the first row in the results, or ``None`` if no rows were returned.
``.single_value()``
Returns the value of the first column of the first row of results - but only if the query returned a single row with a single column. Raises a ``datasette.database.MultipleValues`` exception otherwise.
``.__len__()``
Calling ``len(results)`` returns the (truncated) number of returned results.
Executes a given callback function against a read-only database connection running in a thread. The function will be passed a SQLite connection, and the return value from the function will be returned by the ``await``.
SQLite only allows one database connection to write at a time. Datasette handles this for you by maintaining a queue of writes to be executed against a given database. Plugins can submit write operations to this queue and they will be executed in the order in which they are received.
By default queries are considered to be "fire and forget" - they will be added to the queue and executed in a separate thread while your code can continue to do other things. The method will return a UUID representing the queued task.
If you pass ``block=True`` this behaviour changes: the method will block until the write operation has completed, and the return value will be the return from calling ``conn.execute(...)`` using the underlying ``sqlite3`` Python library.
This method works like ``.execute_write()``, but instead of a SQL statement you give it a callable Python function. This function will be queued up and then called when the write connection is available, passing that connection as the argument to the function.
The function can then perform multiple actions, safe in the knowledge that it has exclusive access to the single writable connection as long as it is executing.
For example:
..code-block:: python
def my_action(conn):
conn.execute("delete from some_table")
conn.execute("delete from other_table")
await database.execute_write_fn(my_action)
This method is fire-and-forget, queueing your function to be executed and then allowing your code after the call to ``.execute_write_fn()`` to continue running while the underlying thread waits for an opportunity to run your function. A UUID representing the queued task will be returned.
If you pass ``block=True`` your calling code will block until the function has been executed. The return value to the ``await`` will be the return value of your function.
If your function raises an exception and you specified ``block=True``, that exception will be propagated up to the ``await`` line. With ``block=False`` any exceptions will be silently ignored.
Here's an example of ``block=True`` in action:
..code-block:: python
def my_action(conn):
conn.execute("delete from some_table where id > 5")
return conn.execute("select count(*) from some_table").fetchone()[0]
