# 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

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,
)

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.

# Customization

The Sanic application instance can be customized for your application needs in a variety of ways at instantiation.

# Custom configuration

NEW in v21.6

This simplest form of custom configuration would be to pass your own object directly into that Sanic application instance

If you create a custom configuration object, it is highly recommended that you subclass the Sanic Config option to inherit its behavior. You could use this option for adding properties, or your own set of custom logic.

from sanic.config import Config
class MyConfig(Config):
    FOO = "bar"
app = Sanic(..., config=MyConfig())

A useful example of this feature would be if you wanted to use a config file in a form that differs from what is supported.

from sanic import Sanic, text
from sanic.config import Config
class TomlConfig(Config):
    def __init__(self, *args, path: str, **kwargs):
        super().__init__(*args, **kwargs)
        with open(path, "r") as f:
            self.apply(toml.load(f))
    def apply(self, config):
        self.update(self._to_uppercase(config))
    def _to_uppercase(self, obj: Dict[str, Any]) -> Dict[str, Any]:
        retval: Dict[str, Any] = {}
        for key, value in obj.items():
            upper_key = key.upper()
            if isinstance(value, list):
                retval[upper_key] = [
                    self._to_uppercase(item) for item in value
                ]
            elif isinstance(value, dict):
                retval[upper_key] = self._to_uppercase(value)
            else:
                retval[upper_key] = value
        return retval
toml_config = TomlConfig(path="/path/to/config.toml")
app = Sanic(toml_config.APP_NAME, config=toml_config)

# Custom context

NEW in v21.6

By default, the application context is a SimpleNamespace() (opens new window) that allows you to set any properties you want on it. However, you also have the option of passing any object whatsoever instead.

app = Sanic(..., ctx=1)

app = Sanic(..., ctx={})

class MyContext:
    ...
app = Sanic(..., ctx=MyContext())

# Custom requests

It is sometimes helpful to have your own Request class, and tell Sanic to use that instead of the default. One example is if you wanted to modify the default request.id generator.

Important

It is important to remember that you are passing the class not an instance of the class.

import time
from sanic import Request, Sanic, text
class NanoSecondRequest(Request):
    @classmethod
    def generate_id(*_):
        return time.time_ns()
app = Sanic(..., request_class=NanoSecondRequest)
@app.get("/")
async def handler(request):
    return text(str(request.id))

# Custom error handler

See exception handling for more

from sanic.handlers import ErrorHandler
class CustomErrorHandler(ErrorHandler):
    def default(self, request, exception):
        ''' handles errors that have no error handlers assigned '''
        # You custom error handling logic...
        return super().default(request, exception)
app = Sanic(..., error_handler=CustomErrorHandler())

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