# Sanic Application

# Instance

The most basic building block is the Sanic() instance. It is not required, but the custom is to instantiate this in a file called server.py.

# /path/to/server.py
from sanic import Sanic
app = Sanic("My Hello, world app")

# Application context

NEW in v21.3

Most applications will have the need to share/reuse data or objects across different parts of the code base. The most common example is DB connections.

In versions of Sanic prior to v21.3, this was commonly done by attaching an attribute to the application instance

# Raises a warning as deprecated feature in 21.3
app = Sanic("MyApp")
app.db = Database()

Because this can create potential problems with name conflicts, and to be consistent with request context objects, v21.3 introduces application level context object.

# Correct way to attach objects to the application
app = Sanic("MyApp")
app.ctx.db = Database()

# App Registry

When you instantiate a Sanic instance, that can be retrieved at a later time from the Sanic app registry. This can be useful, for example, if you need to access your Sanic instance from a location where it is not otherwise accessible.

# ./path/to/server.py
from sanic import Sanic
app = Sanic("my_awesome_server")
# ./path/to/somewhere_else.py
from sanic import Sanic
app = Sanic.get_app("my_awesome_server")

If you call Sanic.get_app("non-existing") on an app that does not exist, it will raise SanicException by default. You can, instead, force the method to return a new instance of Sanic with that name.

app = Sanic.get_app(
    "non-existing",
    force_create=True,
)

NEW in v21.3

If there is only one Sanic instance registered, then calling Sanic.get_app() with no arguments will return that instance

Sanic("My only app")
app = Sanic.get_app()

# Configuration

Sanic holds the configuration in the config attribute of the Sanic instance. Configuration can be modified either using dot-notation OR like a dictionary.

app = Sanic('myapp')
app.config.DB_NAME = 'appdb'
app.config['DB_USER'] = 'appuser'
db_settings = {
    'DB_HOST': 'localhost',
    'DB_NAME': 'appdb',
    'DB_USER': 'appuser'
}
app.config.update(db_settings)

Heads up

Config keys should be uppercase. But, this is mainly by convention, and lowercase will work most of the time.

app.config.GOOD = "yay!"
app.config.bad = "boo"

There is much more detail about configuration later on.

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