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.
.cookies - dictionary (str -> str)
A dictionary of incoming cookies
.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 - MultiParams
An object representing the parsed querystring parameters, see below.
.url_vars - dictionary (str -> str)
Variables extracted from the URL path, if that path was defined using a regular expression. See register_routes().
.actor - dictionary (str -> Any) or None
The currently authenticated actor (see actors), or None if the request is unauthenticated.

The object also has two awaitable methods:

await request.post_vars() - dictionary
Returns a dictionary of form variables that were submitted in the request body via POST. Don't forget to read about CSRF protection!
await request.post_body() - bytes
Returns the un-parsed body of a request submitted by POST - useful for things like incoming JSON data.

The MultiParams class

request.args is a MultiParams 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.

Response class

The Response class can be returned from view functions that have been registered using the register_routes() hook.

The Response() constructor takes the following arguments:

body - string
The body of the response.
status - integer (optional)
The HTTP status - defaults to 200.
headers - dictionary (optional)
A dictionary of extra HTTP headers, e.g. {"x-hello": "world"}.
content_type - string (optional)
The content-type for the response. Defaults to text/plain.

For example:

from datasette.utils.asgi import Response

response = Response(
    "<xml>This is XML</xml>",
    content_type="application/xml; charset=utf-8"
)

The easiest way to create responses is using the Response.text(...), Response.html(...), Response.json(...) or Response.redirect(...) helper methods:

from datasette.utils.asgi import Response

html_response = Response.html("This is HTML")
json_response = Response.json({"this_is": "json"})
text_response = Response.text("This will become utf-8 encoded text")
# Redirects are served as 302, unless you pass status=301:
redirect_response = Response.redirect("https://latest.datasette.io/")

Each of these responses will use the correct corresponding content-type - text/html; charset=utf-8, application/json; charset=utf-8 or text/plain; charset=utf-8 respectively.

Each of the helper methods take optional status= and headers= arguments, documented above.

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.

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

template - string, list of strings or jinja2.Template

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 this is a list of template file names then the first one that exists will be loaded and rendered.

If this is a Jinja Template object it will be used directly.

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.

await .permission_allowed(actor, action, resource=None, default=False)

actor - dictionary
The authenticated actor. This is usually request.actor.
action - string
The name of the action that is being permission checked.
resource - string or tuple, optional
The resource, e.g. the name of the database, or a tuple of two strings containing the name of the database and the name of the table. Only some permissions apply to a resource.
default - optional, True or False
Should this permission check be default allow or default deny.

Check if the given actor has permission to perform the given action on the given resource.

Some permission checks are carried out against rules defined in metadata.json, while other custom permissions may be decided by plugins that implement the permission_allowed(datasette, actor, action, resource) plugin hook.

If neither metadata.json nor any of the plugins provide an answer to the permission query the default argument will be returned.

See Built-in permissions for a full list of permission actions included in Datasette core.

.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.

.sign(value, namespace="default")

value - any serializable type
The value to be signed.
namespace - string, optional
An alternative namespace, see the itsdangerous salt documentation.

Utility method for signing values, such that you can safely pass data to and from an untrusted environment. This is a wrapper around the itsdangerous library.

This method returns a signed string, which can be decoded and verified using .unsign(value, namespace="default").

.unsign(value, namespace="default")

signed - any serializable type
The signed string that was created using .sign(value, namespace="default").
namespace - string, optional
The alternative namespace, if one was used.

Returns the original, decoded object that was passed to .sign(value, namespace="default"). If the signature is not valid this raises a itsdangerous.BadSignature exception.

.add_message(request, message, message_type=datasette.INFO)

request - Request
The current Request object
message - string
The message string
message_type - constant, optional
The message type - datasette.INFO, datasette.WARNING or datasette.ERROR

Datasette's flash messaging mechanism allows you to add a message that will be displayed to the user on the next page that they visit. Messages are persisted in a ds_messages cookie. This method adds a message to that cookie.

You can try out these messages (including the different visual styling of the three message types) using the /-/messages debugging tool.

.absolute_url(request, path)

request - Request
The current Request object
path - string
A path, for example /dbname/table.json

Returns the absolute URL for the given path, including the protocol and host. For example:

absolute_url = datasette.absolute_url(request, "/dbname/table.json")
# Would return "http://localhost:8001/dbname/table.json"

The current request object is used to determine the hostname and protocol that should be used for the returned URL. The force_https_urls configuration setting is taken into account.

datasette.client

Plugins can make internal simulated HTTP requests to the Datasette instance within which they are running. This ensures that all of Datasette's external JSON APIs are also available to plugins, while avoiding the overhead of making an external HTTP call to access those APIs.

The datasette.client object is a wrapper around the HTTPX Python library, providing an async-friendly API that is similar to the widely used Requests library.

It offers the following methods:

await datasette.client.get(path, **kwargs) - returns HTTPX Response
Execute an internal GET request against that path.
await datasette.client.post(path, **kwargs) - returns HTTPX Respons
Execute an internal POST request. Use data={"name": "value"} to pass form parameters.
await datasette.client.options(path, **kwargs) - returns HTTPX Response
Execute an internal OPTIONS request.
await datasette.client.head(path, **kwargs) - returns HTTPX Respons
Execute an internal HEAD request.
await datasette.client.put(path, **kwargs) - returns HTTPX Response
Execute an internal PUT request.
await datasette.client.patch(path, **kwargs) - returns HTTPX Response
Execute an internal PATCH request.
await datasette.client.delete(path, **kwargs) - returns HTTPX Response
Execute an internal DELETE request.
await datasette.client.request(method, path, **kwargs) - returns HTTPX Response
Execute an internal request with the given HTTP method against that path.

For documentation on available **kwargs options and the shape of the HTTPX Response object refer to the HTTPX Async documentation.

datasette.urls

The datasette.urls object contains methods for building URLs to pages within Datasette. Plugins should use this to link to pages, since these methods take into account any base_url configuration setting that might be in effect.

datasette.urls.instance()
Returns the URL to the Datasette instance root page. This is usually "/"
datasette.urls.path(path)

Takes a path and returns the full path, taking base_url into account.

For example, datasette.urls.path("-/logout") will return the path to the logout page, which will be "/-/logout" by default or /prefix-path/-/logout if base_url is set to /prefix-path/

datasette.urls.logout()
Returns the URL to the logout page, usually "/-/logout"
datasette.urls.static(path)
Returns the URL of one of Datasette's default static assets, for example "/-/static/app.css"
datasette.urls.static_plugins(plugin_name, path)

Returns the URL of one of the static assets belonging to a plugin.

datasette.url.static_plugins("datasette_cluster_map", "datasette-cluster-map.js") would return "/-/static-plugins/datasette_cluster_map/datasette-cluster-map.js"

datasette.urls.static(path)
Returns the URL of one of Datasette's default static assets, for example "/-/static/app.css"
datasette.urls.static_plugins(plugin_name, path)

Returns the URL of one of the static assets belonging to a plugin.

datasette.url.static_plugins("datasette_cluster_map", "datasette-cluster-map.js") would return "/-/static-plugins/datasette_cluster_map/datasette-cluster-map.js"

datasette.urls.database(database_name)
Returns the URL to a database page, for example "/fixtures"
datasette.urls.table(database_name, table_name)
Returns the URL to a table page, for example "/fixtures/facetable"
datasette.urls.query(database_name, query_name)
Returns the URL to a query page, for example "/fixtures/pragma_cache_size"

These functions can be accessed via the {{ urls }} object in Datasette templates, for example:

<a href="{{ urls.instance() }}">Homepage</a>
<a href="{{ urls.database("fixtures") }}">Fixtures database</a>
<a href="{{ urls.table("fixtures", "facetable") }}">facetable table</a>
<a href="{{ urls.query("fixtures", "pragma_cache_size") }}">pragma_cache_size query</a>

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.
db.is_mutable - boolean
Is this database mutable, and allowed to accept writes?
db.is_memory - boolean
Is this database an in-memory database?
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.table_column_details(table) - list of named tuples
Full details of the columns in a specific table. Each column is represented by a Column named tuple with fields cid (integer representing the column position), name (string), type (string, e.g. REAL or VARCHAR(30)), notnull (integer 1 or 0), default_value (string or None), is_pk (integer 1 or 0).
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",
        }
    ]
}

CSRF protection

Datasette uses asgi-csrf to guard against CSRF attacks on form POST submissions. Users receive a ds_csrftoken cookie which is compared against the csrftoken form field (or x-csrftoken HTTP header) for every incoming request.

If your plugin implements a <form method="POST"> anywhere you will need to include that token. You can do so with the following template snippet:

<input type="hidden" name="csrftoken" value="{{ csrftoken() }}">