Configuration

Configuration in CherryPy is implemented via dictionaries. Keys are strings which name the mapped value; values may be of any type.

In CherryPy 3, you use configuration (files or dicts) to set attributes directly on the engine, server, request, response, and log objects. So the best way to know the full range of what’s available in the config file is to simply import those objects and see what help(obj) tells you.

Architecture

The first thing you need to know about CherryPy 3’s configuration is that it separates global config from application config. If you’re deploying multiple applications at the same site (and more and more people are, as Python web apps are tending to decentralize), you need to be careful to separate the configurations, as well. There’s only ever one “global config”, but there is a separate “app config” for each app you deploy.

CherryPy Requests are part of an Application, which runs in a global context, and configuration data may apply to any of those three scopes. Let’s look at each of those scopes in turn.

Global config

Global config entries apply everywhere, and are stored in cherrypy.config. This flat dict only holds global config data; that is, “site-wide” config entries which affect all mounted applications.

Global config is stored in the cherrypy.config dict, and you therefore update it by calling cherrypy.config.update(conf). The conf argument can be either a filename, an open file, or a dict of config entries. Here’s an example of passing a dict argument:

cherrypy.config.update({'server.socket_host': '64.72.221.48',
                        'server.socket_port': 80,
                       })

The server.socket_host option in this example determines on which network interface CherryPy will listen. The server.socket_port option declares the TCP port on which to listen.

Application config

Application entries apply to a single mounted application, and are stored on each Application object itself as app.config. This is a two-level dict where each top-level key is a path, or “relative URL” (for example, "/" or "/my/page"), and each value is a dict of config entries. The URL’s are relative to the script name (mount point) of the Application. Usually, all this data is provided in the call to tree.mount(root(), script_name='/path/to', config=conf), although you may also use app.merge(conf). The conf argument can be either a filename, an open file, or a dict of config entries.

Configuration file example:

[/]
tools.trailing_slash.on = False
request.dispatch: cherrypy.dispatch.MethodDispatcher()

or, in python code:

config = {'/':
    {
        'request.dispatch': cherrypy.dispatch.MethodDispatcher(),
        'tools.trailing_slash.on': False,
    }
}
cherrypy.tree.mount(Root(), config=config)

CherryPy only uses sections that start with "/" (except [global], see below). That means you can place your own configuration entries in a CherryPy config file by giving them a section name which does not start with "/". For example, you might include database entries like this:

[global]
server.socket_host: "0.0.0.0"

[Databases]
driver: "postgres"
host: "localhost"
port: 5432

[/path]
response.timeout: 6000

Then, in your application code you can read these values during request time via cherrypy.request.app.config['Databases']. For code that is outside the request process, you’ll have to pass a reference to your Application around.

Request config

Each Request object possesses a single request.config dict. Early in the request process, this dict is populated by merging Global config, Application config, and any config acquired while looking up the page handler (see next). This dict contains only those config entries which apply to the given request.

Note

when you do an InternalRedirect, this config attribute is recalculated for the new path.

Declaration

Configuration data may be supplied as a Python dictionary, as a filename, or as an open file object.

Configuration files

When you supply a filename or file, CherryPy uses Python’s builtin ConfigParser; you declare Application config by writing each path as a section header, and each entry as a "key: value" (or "key = value") pair:

[/path/to/my/page]
response.stream: True
tools.trailing_slash.extra = False

Combined Configuration Files

If you are only deploying a single application, you can make a single config file that contains both global and app entries. Just stick the global entries into a config section named [global], and pass the same file to both config.update and tree.mount <cherrypy._cptree.Tree.mount(). If you’re calling cherrypy.quickstart(app root, script name, config), it will pass the config to both places for you. But as soon as you decide to add another application to the same site, you need to separate the two config files/dicts.

Separate Configuration Files

If you’re deploying more than one application in the same process, you need (1) file for global config, plus (1) file for each Application. The global config is applied by calling cherrypy.config.update, and application config is usually passed in a call to cherrypy.tree.mount.

In general, you should set global config first, and then mount each application with its own config. Among other benefits, this allows you to set up global logging so that, if something goes wrong while trying to mount an application, you’ll see the tracebacks. In other words, use this order:

# global config
cherrypy.config.update({'environment': 'production',
                        'log.error_file': 'site.log',
                        # ...
                        })

# Mount each app and pass it its own config
cherrypy.tree.mount(root1, "", appconf1)
cherrypy.tree.mount(root2, "/forum", appconf2)
cherrypy.tree.mount(root3, "/blog", appconf3)

if hasattr(cherrypy.engine, 'block'):
    # 3.1 syntax
    cherrypy.engine.start()
    cherrypy.engine.block()
else:
    # 3.0 syntax
    cherrypy.server.quickstart()
    cherrypy.engine.start()

Values in config files use Python syntax

Config entries are always a key/value pair, like server.socket_port = 8080. The key is always a name, and the value is always a Python object. That is, if the value you are setting is an int (or other number), it needs to look like a Python int; for example, 8080. If the value is a string, it needs to be quoted, just like a Python string. Arbitrary objects can also be created, just like in Python code (assuming they can be found/imported). Here’s an extended example, showing you some of the different types:

[global]
log.error_file: "/home/fumanchu/myapp.log"
environment = 'production'
server.max_request_body_size: 1200

[/myapp]
tools.trailing_slash.on = False
request.dispatch: cherrypy.dispatch.MethodDispatcher()

_cp_config: attaching config to handlers

Config files have a severe limitation: values are always keyed by URL. For example:

[/path/to/page]
methods_with_bodies = ("POST", "PUT", "PROPPATCH")

It’s obvious that the extra method is the norm for that path; in fact, the code could be considered broken without it. In CherryPy, you can attach that bit of config directly on the page handler:

def page(self):
    return "Hello, world!"
page.exposed = True
page._cp_config = {"request.methods_with_bodies": ("POST", "PUT", "PROPPATCH")}

_cp_config is a reserved attribute which the dispatcher looks for at each node in the object tree. The _cp_config attribute must be a CherryPy config dictionary. If the dispatcher finds a _cp_config attribute, it merges that dictionary into the rest of the config. The entire merged config dictionary is placed in cherrypy.request.config.

This can be done at any point in the tree of objects; for example, we could have attached that config to a class which contains the page method:

class SetOPages:

    _cp_config = {"request.methods_with_bodies": ("POST", "PUT", "PROPPATCH")}

    def page(self):
        return "Hullo, Werld!"
    page.exposed = True

Note

This behavior is only guaranteed for the default dispatcher. Other dispatchers may have different restrictions on where you can attach _cp_config attributes.

This technique allows you to:

  • Put config near where it’s used for improved readability and maintainability.
  • Attach config to objects instead of URL’s. This allows multiple URL’s to point to the same object, yet you only need to define the config once.
  • Provide defaults which are still overridable in a config file.

Namespaces

Because config entries usually just set attributes on objects, they’re almost all of the form: object.attribute. A few are of the form: object.subobject.attribute. They look like normal Python attribute chains, because they work like them. We call the first name in the chain the “config namespace”. When you provide a config entry, it is bound as early as possible to the actual object referenced by the namespace; for example, the entry response.stream actually sets the stream attribute of cherrypy.response! In this way, you can easily determine the default value by firing up a python interpreter and typing:

>>> import cherrypy
>>> cherrypy.response.stream
False

Each config namespace has its own handler; for example, the “request” namespace has a handler which takes your config entry and sets that value on the appropriate “request” attribute. There are a few namespaces, however, which don’t work like normal attributes behind the scenes; however, they still use dotted keys and are considered to “have a namespace”.

Builtin namespaces

Entries from each namespace may be allowed in the global, application root ("/") or per-path config, or a combination:

Scope Global Application Root App Path
engine X    
hooks X X X
log X X  
request X X X
response X X X
server X    
tools X X X

engine

Entries in this namespace controls the ‘application engine’. These can only be declared in the global config. Any attribute of cherrypy.engine may be set in config; however, there are a few extra entries available in config:

  • Plugin attributes. Many of the Engine Plugins are themselves attributes of cherrypy.engine. You can set any attribute of an attached plugin by simply naming it. For example, there is an instance of the Autoreloader class at engine.autoreload; you can set its “frequency” attribute via the config entry engine.autoreload.frequency = 60. In addition, you can turn such plugins on and off by setting engine.autoreload.on = True or False.
  • engine.SIGHUP/SIGTERM: These entries can be used to set the list of listeners for the given channel. Mostly, this is used to turn off the signal handling one gets automatically via cherrypy.quickstart().

hooks

Declares additional request-processing functions. Use this to append your own Hook functions to the request. For example, to add my_hook_func to the before_handler hookpoint:

[/]
hooks.before_handler = myapp.my_hook_func

log

Configures logging. These can only be declared in the global config (for global logging) or [/] config (for each application). See LogManager for the list of configurable attributes. Typically, the “access_file”, “error_file”, and “screen” attributes are the most commonly configured.

request

Sets attributes on each Request. See the Request class for a complete list.

response

Sets attributes on each Response. See the Response class for a complete list.

server

Controls the default HTTP server via cherrypy.server (see that class for a complete list of configurable attributes). These can only be declared in the global config.

tools

Enables and configures additional request-processing packages. See the Tools overview for more information.

wsgi

Adds WSGI middleware to an Application’s “pipeline”. These can only be declared in the app’s root config (“/”).

  • wsgi.pipeline: Appends to the WSGi pipeline. The value must be a list of (name, app factory) pairs. Each app factory must be a WSGI callable class (or callable that returns a WSGI callable); it must take an initial ‘nextapp’ argument, plus any optional keyword arguments. The optional arguments may be configured via wsgi.<name>.<arg>.
  • wsgi.response_class: Overrides the default Response class.

checker

Controls the “checker”, which looks for common errors in app state (including config) when the engine starts. You can turn off individual checks by setting them to False in config. See cherrypy._cpchecker.Checker for a complete list. Global config only.

Custom config namespaces

You can define your own namespaces if you like, and they can do far more than simply set attributes. The test/test_config module, for example, shows an example of a custom namespace that coerces incoming params and outgoing body content. The cherrypy._cpwsgi module includes an additional, builtin namespace for invoking WSGI middleware.

In essence, a config namespace handler is just a function, that gets passed any config entries in its namespace. You add it to a namespaces registry (a dict), where keys are namespace names and values are handler functions. When a config entry for your namespace is encountered, the corresponding handler function will be called, passing the config key and value; that is, namespaces[namespace](k, v). For example, if you write:

def db_namespace(k, v):
    if k == 'connstring':
        orm.connect(v)
cherrypy.config.namespaces['db'] = db_namespace

then cherrypy.config.update({"db.connstring": "Oracle:host=1.10.100.200;sid=TEST"}) will call db_namespace('connstring', 'Oracle:host=1.10.100.200;sid=TEST').

The point at which your namespace handler is called depends on where you add it:

Scope Namespace dict Handler is called in
Global cherrypy.config.namespaces cherrypy.config.update
Application app.namespaces Application.merge (which is called by cherrypy.tree.mount)
Request app.request_class.namespaces Request.configure (called for each request, after the handler is looked up)

The name can be any string, and the handler must be either a callable or a (Python 2.5 style) context manager.

If you need additional code to run when all your namespace keys are collected, you can supply a callable context manager in place of a normal function for the handler. Context managers are defined in PEP 343.

Environments

The only key that does not exist in a namespace is the “environment” entry. It only applies to the global config, and only when you use cherrypy.config.update. This special entry imports other config entries from the following template stored in cherrypy._cpconfig.environments[environment].

Config.environments = environments = {
    "staging": {
        'engine.autoreload.on': False,
        'checker.on': False,
        'tools.log_headers.on': False,
        'request.show_tracebacks': False,
        'request.show_mismatched_params': False,
    },
    "production": {
        'engine.autoreload.on': False,
        'checker.on': False,
        'tools.log_headers.on': False,
        'request.show_tracebacks': False,
        'request.show_mismatched_params': False,
        'log.screen': False,
    },
    "embedded": {
        # For use with CherryPy embedded in another deployment stack.
        'engine.autoreload.on': False,
        'checker.on': False,
        'tools.log_headers.on': False,
        'request.show_tracebacks': False,
        'request.show_mismatched_params': False,
        'log.screen': False,
        'engine.SIGHUP': None,
        'engine.SIGTERM': None,
    },
    "test_suite": {
        'engine.autoreload.on': False,
        'checker.on': False,
        'tools.log_headers.on': False,
        'request.show_tracebacks': True,
        'request.show_mismatched_params': True,
        'log.screen': False,
    },
}

If you find the set of existing environments (production, staging, etc) too limiting or just plain wrong, feel free to extend them or add new environments:

cherrypy._cpconfig.environments['staging']['log.screen'] = False

cherrypy._cpconfig.environments['Greek'] = {
    'tools.encode.encoding': 'ISO-8859-7',
    'tools.decode.encoding': 'ISO-8859-7',
    }