Internals for plugins

Many Plugin hooks are passed objects that provide access to internal Datasette functionality. The interface to these objects should not be considered stable with the exception of methods that are documented here.

Request object

The request object is passed to various plugin hooks. It represents an incoming HTTP request. It has the following properties:

.scope - dictionary
The ASGI scope that was used to construct this request, described in the ASGI HTTP connection scope specification.
.method - string
The HTTP method for this request, usually GET or POST.
.url - string
The full URL for this request, e.g. https://latest.datasette.io/fixtures.
.scheme - string
The request scheme - usually https or http.
.headers - dictionary (str -> str)
A dictionary of incoming HTTP request headers.
.host - string
The host header from the incoming request, e.g. latest.datasette.io or localhost.
.path - string
The path of the request, e.g. /fixtures.
.query_string - string
The querystring component of the request, without the ? - e.g. name__contains=sam&age__gt=10.
.args - RequestParameters
An object representing the parsed querystring parameters, see below.

The object also has one awaitable method:

await request.post_vars() - dictionary
Returns a dictionary of form variables that were submitted in the request body via POST.

The RequestParameters class

request.args is a RequestParameters object - a dictionary-like object which provides access to querystring parameters that may have multiple values.

Consider the querystring ?foo=1&foo=2&bar=3 - with two values for foo and one value for bar.

request.args[key] - string
Returns the first value for that key, or raises a KeyError if the key is missing. For the above example request.args["foo"] would return "1".
request.args.get(key) - string or None
Returns the first value for that key, or None if the key is missing. Pass a second argument to specify a different default, e.g. q = request.args.get("q", "").
request.args.getlist(key) - list of strings
Returns the list of strings for that key. request.args.getlist("foo") would return ["1", "2"] in the above example. request.args.getlist("bar") would return ["3"]. If the key is missing an empty list will be returned.
request.args.keys() - list of strings
Returns the list of available keys - for the example this would be ["foo", "bar"].
key in request.args - True or False
You can use if key in request.args to check if a key is present.
for key in request.args - iterator
This lets you loop through every available key.
len(request.args) - integer
Returns the number of keys.

Datasette class

This object is an instance of the Datasette class, passed to many plugin hooks as an argument called datasette.

.plugin_config(plugin_name, database=None, table=None)

plugin_name - string
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 Writing plugins that accept configuration for full details of how this method should be used.

.render_template(template, context=None, request=None)

template - string
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.
context - None or a Python dictionary
The context variables to pass to the template.
request - request object or None
If you pass a Datasette request object here it will be made available to the template.

Renders a Jinja template 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.

.get_database(name)

name - string, optional
The name of the database - optional.

Returns the specified database object. Raises a KeyError if the database does not exist. Call this method without an argument to return the first connected 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:

from datasette.database import Database

datasette.add_database("my-new-database", Database(
    datasette,
    path="path/to/my-new-database.db",
    is_mutable=True
))

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.

.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.

Database class

Instances of the Database class can be used to execute queries against attached SQLite databases, and to run introspection against their schemas.

await db.execute(sql, ...)

Executes a SQL query against the database and returns the resulting rows (see 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.

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. Each of these can be treated as a tuple or can be accessed using row["column"] syntax:

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.

await db.execute_fn(fn)

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.

Example usage:

def get_version(conn):
    return conn.execute(
        "select sqlite_version()"
    ).fetchall()[0][0]

version = await db.execute_fn(get_version)

await db.execute_write(sql, params=None, block=False)

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.

This method can be used to queue up a non-SELECT SQL query to be executed against a single write connection to the database.

You can pass additional SQL parameters as a tuple or dictionary.

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.

await db.execute_write_fn(fn, block=False)

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:

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:

def my_action(conn):
    conn.execute("delete from some_table where id > 5")
    return conn.execute("select count(*) from some_table").fetchone()[0]

try:
    num_rows_left = await database.execute_write_fn(my_action, block=True)
except Exception as e:
    print("An error occurred:", e)

Database introspection

The Database class also provides properties and methods for introspecting the database.

db.name - string
The name of the database - usually the filename without the .db prefix.
db.size - integer
The size of the database file in bytes. 0 for :memory: databases.
db.mtime_ns - integer or None
The last modification time of the database file in nanoseconds since the epoch. None for :memory: databases.
await db.table_exists(table) - boolean
Check if a table called table exists.
await db.table_names() - list of strings
List of names of tables in the database.
await db.view_names() - list of strings
List of names of views in tha database.
await db.table_columns(table) - list of strings
Names of columns in a specific table.
await db.primary_keys(table) - list of strings
Names of the columns that are part of the primary key for this table.
await db.fts_table(table) - string or None
The name of the FTS table associated with this table, if one exists.
await db.label_column_for_table(table) - string or None
The label column that is associated with this table - either automatically detected or using the "label_column" key from Metadata, see Specifying the label column for a table.
await db.foreign_keys_for_table(table) - list of dictionaries
Details of columns in this table which are foreign keys to other tables. A list of dictionaries where each dictionary is shaped like this: {"column": string, "other_table": string, "other_column": string}.
await db.hidden_table_names() - list of strings
List of tables which Datasette "hides" by default - usually these are tables associated with SQLite's full-text search feature, the SpatiaLite extension or tables hidden using the Hiding tables feature.
await db.get_table_definition(table) - string
Returns the SQL definition for the table - the CREATE TABLE statement and any associated CREATE INDEX statements.
await db.get_view_definition(view) - string
Returns the SQL definition of the named view.
await db.get_all_foreign_keys() - dictionary

Dictionary representing both incoming and outgoing foreign keys for this table. It has two keys, "incoming" and "outgoing", each of which is a list of dictionaries with keys "column", "other_table" and "other_column". For example:

{
    "incoming": [],
    "outgoing": [
        {
            "other_table": "attraction_characteristic",
            "column": "characteristic_id",
            "other_column": "pk",
        },
        {
            "other_table": "roadside_attractions",
            "column": "attraction_id",
            "other_column": "pk",
        }
    ]
}