python-peps/pep-0391.txt

PEP: 391
Title: Dictionary-Based Configuration For Logging
Version: $Revision$
Last-Modified: $Date$
Author: Vinay Sajip <vinay_sajip at red-dove.com>
Status: Final
Type: Standards Track
Content-Type: text/x-rst
Created: 15-Oct-2009
Python-Version: 2.7, 3.2
Post-History:


Abstract
========

This PEP describes a new way of configuring logging using a dictionary
to hold configuration information.


Rationale
=========

The present means for configuring Python's logging package is either
by using the logging API to configure logging programmatically, or
else by means of ConfigParser-based configuration files.

Programmatic configuration, while offering maximal control, fixes the
configuration in Python code.  This does not facilitate changing it
easily at runtime, and, as a result, the ability to flexibly turn the
verbosity of logging up and down for different parts of a using
application is lost.  This limits the usability of logging as an aid
to diagnosing problems - and sometimes, logging is the only diagnostic
aid available in production environments.

The ConfigParser-based configuration system is usable, but does not
allow its users to configure all aspects of the logging package.  For
example, Filters cannot be configured using this system.  Furthermore,
the ConfigParser format appears to engender dislike (sometimes strong
dislike) in some quarters.  Though it was chosen because it was the
only configuration format supported in the Python standard at that
time, many people regard it (or perhaps just the particular schema
chosen for logging's configuration) as 'crufty' or 'ugly', in some
cases apparently on purely aesthetic grounds.

Recent versions of Python include JSON support in the standard
library, and this is also usable as a configuration format.  In other
environments, such as Google App Engine, YAML is used to configure
applications, and usually the configuration of logging would be
considered an integral part of the application configuration.
Although the standard library does not contain YAML support at
present, support for both JSON and YAML can be provided in a common
way because both of these serialization formats allow deserialization
to Python dictionaries.

By providing a way to configure logging by passing the configuration
in a dictionary, logging will be easier to configure not only for
users of JSON and/or YAML, but also for users of custom configuration
methods, by providing a common format in which to describe the desired
configuration.

Another drawback of the current ConfigParser-based configuration
system is that it does not support incremental configuration: a new
configuration completely replaces the existing configuration.
Although full flexibility for incremental configuration is difficult
to provide in a multi-threaded environment, the new configuration
mechanism will allow the provision of limited support for incremental
configuration.


Specification
=============

The specification consists of two parts: the API and the format of the
dictionary used to convey configuration information (i.e. the schema
to which it must conform).


Naming
------

Historically, the logging package has not been PEP 8 conformant [1]_.
At some future time, this will be corrected by changing method and
function names in the package in order to conform with PEP 8.
However, in the interests of uniformity, the proposed additions to the
API use a naming scheme which is consistent with the present scheme
used by logging.


API
---

The logging.config module will have the following addition:

* A function, called ``dictConfig()``, which takes a single argument
  - the dictionary holding the configuration.  Exceptions will be
  raised if there are errors while processing the dictionary.

It will be possible to customize this API - see the section on `API
Customization`_.  `Incremental configuration`_ is covered in its own
section.


Dictionary Schema - Overview
----------------------------

Before describing the schema in detail, it is worth saying a few words
about object connections, support for user-defined objects and access
to external and internal objects.


Object connections
''''''''''''''''''

The schema is intended to describe a set of logging objects - loggers,
handlers, formatters, filters - which are connected to each other in
an object graph.  Thus, the schema needs to represent connections
between the objects.  For example, say that, once configured, a
particular logger has attached to it a particular handler.  For the
purposes of this discussion, we can say that the logger represents the
source, and the handler the destination, of a connection between the
two.  Of course in the configured objects this is represented by the
logger holding a reference to the handler.  In the configuration dict,
this is done by giving each destination object an id which identifies
it unambiguously, and then using the id in the source object's
configuration to indicate that a connection exists between the source
and the destination object with that id.

So, for example, consider the following YAML snippet::

    formatters:
      brief:
        # configuration for formatter with id 'brief' goes here
      precise:
        # configuration for formatter with id 'precise' goes here
    handlers:
      h1: #This is an id
       # configuration of handler with id 'h1' goes here
       formatter: brief
      h2: #This is another id
       # configuration of handler with id 'h2' goes here
       formatter: precise
    loggers:
      foo.bar.baz:
        # other configuration for logger 'foo.bar.baz'
        handlers: [h1, h2]

(Note: YAML will be used in this document as it is a little more
readable than the equivalent Python source form for the dictionary.)

The ids for loggers are the logger names which would be used
programmatically to obtain a reference to those loggers, e.g.
``foo.bar.baz``.  The ids for Formatters and Filters can be any string
value (such as ``brief``, ``precise`` above) and they are transient,
in that they are only meaningful for processing the configuration
dictionary and used to determine connections between objects, and are
not persisted anywhere when the configuration call is complete.

Handler ids are treated specially, see the section on
`Handler Ids`_, below.

The above snippet indicates that logger named ``foo.bar.baz`` should
have two handlers attached to it, which are described by the handler
ids ``h1`` and ``h2``. The formatter for ``h1`` is that described by id
``brief``, and the formatter for ``h2`` is that described by id
``precise``.


User-defined objects
''''''''''''''''''''

The schema should support user-defined objects for handlers, filters
and formatters.  (Loggers do not need to have different types for
different instances, so there is no support - in the configuration -
for user-defined logger classes.)

Objects to be configured will typically be described by dictionaries
which detail their configuration.  In some places, the logging system
will be able to infer from the context how an object is to be
instantiated, but when a user-defined object is to be instantiated,
the system will not know how to do this.  In order to provide complete
flexibility for user-defined object instantiation, the user will need
to provide a 'factory' - a callable which is called with a
configuration dictionary and which returns the instantiated object.
This will be signalled by an absolute import path to the factory being
made available under the special key ``'()'``.  Here's a concrete
example::

    formatters:
      brief:
        format: '%(message)s'
      default:
        format: '%(asctime)s %(levelname)-8s %(name)-15s %(message)s'
        datefmt: '%Y-%m-%d %H:%M:%S'
      custom:
          (): my.package.customFormatterFactory
          bar: baz
          spam: 99.9
          answer: 42

The above YAML snippet defines three formatters.  The first, with id
``brief``, is a standard ``logging.Formatter`` instance with the
specified format string.  The second, with id ``default``, has a
longer format and also defines the time format explicitly, and will
result in a ``logging.Formatter`` initialized with those two format
strings.  Shown in Python source form, the ``brief`` and ``default``
formatters have configuration sub-dictionaries::

    {
      'format' : '%(message)s'
    }

and::

    {
      'format' : '%(asctime)s %(levelname)-8s %(name)-15s %(message)s',
      'datefmt' : '%Y-%m-%d %H:%M:%S'
    }

respectively, and as these dictionaries do not contain the special key
``'()'``, the instantiation is inferred from the context: as a result,
standard ``logging.Formatter`` instances are created.  The
configuration sub-dictionary for the third formatter, with id
``custom``, is::

  {
    '()' : 'my.package.customFormatterFactory',
    'bar' : 'baz',
    'spam' : 99.9,
    'answer' : 42
  }

and this contains the special key ``'()'``, which means that
user-defined instantiation is wanted.  In this case, the specified
factory callable will be used. If it is an actual callable it will be
used directly - otherwise, if you specify a string (as in the example)
the actual callable will be located using normal import mechanisms.
The callable will be called with the *remaining* items in the
configuration sub-dictionary as keyword arguments.  In the above
example, the formatter with id ``custom`` will be assumed to be
returned by the call::

    my.package.customFormatterFactory(bar='baz', spam=99.9, answer=42)

The key ``'()'`` has been used as the special key because it is not a
valid keyword parameter name, and so will not clash with the names of
the keyword arguments used in the call.  The ``'()'`` also serves as a
mnemonic that the corresponding value is a callable.


Access to external objects
''''''''''''''''''''''''''

There are times where a configuration will need to refer to objects
external to the configuration, for example ``sys.stderr``.  If the
configuration dict is constructed using Python code then this is
straightforward, but a problem arises when the configuration is
provided via a text file (e.g. JSON, YAML).  In a text file, there is
no standard way to distinguish ``sys.stderr`` from the literal string
``'sys.stderr'``.  To facilitate this distinction, the configuration
system will look for certain special prefixes in string values and
treat them specially.  For example, if the literal string
``'ext://sys.stderr'`` is provided as a value in the configuration,
then the ``ext://`` will be stripped off and the remainder of the
value processed using normal import mechanisms.

The handling of such prefixes will be done in a way analogous to
protocol handling: there will be a generic mechanism to look for
prefixes which match the regular expression
``^(?P<prefix>[a-z]+)://(?P<suffix>.*)$`` whereby, if the ``prefix``
is recognised, the ``suffix`` is processed in a prefix-dependent
manner and the result of the processing replaces the string value.  If
the prefix is not recognised, then the string value will be left
as-is.

The implementation will provide for a set of standard prefixes such as
``ext://`` but it will be possible to disable the mechanism completely
or provide additional or different prefixes for special handling.


Access to internal objects
''''''''''''''''''''''''''

As well as external objects, there is sometimes also a need to refer
to objects in the configuration.  This will be done implicitly by the
configuration system for things that it knows about.  For example, the
string value ``'DEBUG'`` for a ``level`` in a logger or handler will
automatically be converted to the value ``logging.DEBUG``, and the
``handlers``, ``filters`` and ``formatter`` entries will take an
object id and resolve to the appropriate destination object.

However, a more generic mechanism needs to be provided for the case
of user-defined objects which are not known to logging.  For example,
take the instance of ``logging.handlers.MemoryHandler``, which takes
a ``target`` which is another handler to delegate to. Since the system
already knows about this class, then in the configuration, the given
``target`` just needs to be the object id of the relevant target
handler, and the system will resolve to the handler from the id.  If,
however, a user defines a ``my.package.MyHandler`` which has a
``alternate`` handler, the configuration system would not know that
the ``alternate`` referred to a handler.  To cater for this, a
generic resolution system will be provided which allows the user to
specify::

    handlers:
      file:
        # configuration of file handler goes here

      custom:
        (): my.package.MyHandler
        alternate: cfg://handlers.file

The literal string ``'cfg://handlers.file'`` will be resolved in an
analogous way to the strings with the ``ext://`` prefix, but looking
in the configuration itself rather than the import namespace.  The
mechanism will allow access by dot or by index, in a similar way to
that provided by ``str.format``.  Thus, given the following snippet::

    handlers:
      email:
        class: logging.handlers.SMTPHandler
        mailhost: localhost
        fromaddr: my_app@domain.tld
        toaddrs:
          - support_team@domain.tld
          - dev_team@domain.tld
        subject: Houston, we have a problem.

in the configuration, the string ``'cfg://handlers'`` would resolve to
the dict with key ``handlers``, the string ``'cfg://handlers.email``
would resolve to the dict with key ``email`` in the ``handlers`` dict,
and so on.  The string ``'cfg://handlers.email.toaddrs[1]`` would
resolve to ``'dev_team.domain.tld'`` and the string
``'cfg://handlers.email.toaddrs[0]'`` would resolve to the value
``'support_team@domain.tld'``. The ``subject`` value could be accessed
using either ``'cfg://handlers.email.subject'`` or, equivalently,
``'cfg://handlers.email[subject]'``.  The latter form only needs to be
used if the key contains spaces or non-alphanumeric characters.  If an
index value consists only of decimal digits, access will be attempted
using the corresponding integer value, falling back to the string
value if needed.

Given a string ``cfg://handlers.myhandler.mykey.123``, this will
resolve to ``config_dict['handlers']['myhandler']['mykey']['123']``.
If the string is specified as ``cfg://handlers.myhandler.mykey[123]``,
the system will attempt to retrieve the value from
``config_dict['handlers']['myhandler']['mykey'][123]``, and fall back
to ``config_dict['handlers']['myhandler']['mykey']['123']`` if that
fails.


Handler Ids
'''''''''''

Some specific logging configurations require the use of handler levels
to achieve the desired effect.  However, unlike loggers which can
always be identified by their names, handlers have no persistent
handles whereby levels can be changed via an incremental configuration
call.

Therefore, this PEP proposes to add an optional ``name`` property to
handlers. If used, this will add an entry in a dictionary which maps
the name to the handler.  (The entry will be removed when the handler
is closed.)  When an incremental configuration call is made, handlers
will be looked up in this dictionary to set the handler level
according to the value in the configuration.  See the section on
`incremental configuration`_ for more details.

In theory, such a "persistent name" facility could also be provided
for Filters and Formatters.  However, there is not a strong case to be
made for being able to configure these incrementally.  On the basis
that practicality beats purity, only Handlers will be given this new
``name`` property.  The id of a handler in the configuration will
become its ``name``.

The handler name lookup dictionary is for configuration use only and
will not become part of the public API for the package.


Dictionary Schema - Detail
--------------------------

The dictionary passed to ``dictConfig()`` must contain the following
keys:

* `version` - to be set to an integer value representing the schema
  version.  The only valid value at present is 1, but having this key
  allows the schema to evolve while still preserving backwards
  compatibility.

All other keys are optional, but if present they will be interpreted
as described below.  In all cases below where a 'configuring dict' is
mentioned, it will be checked for the special ``'()'`` key to see if a
custom instantiation is required.  If so, the mechanism described
above is used to instantiate; otherwise, the context is used to
determine how to instantiate.

* `formatters` - the corresponding value will be a dict in which each
  key is a formatter id and each value is a dict describing how to
  configure the corresponding Formatter instance.

  The configuring dict is searched for keys ``format`` and ``datefmt``
  (with defaults of ``None``) and these are used to construct a
  ``logging.Formatter`` instance.

* `filters` - the corresponding value will be a dict in which each key
  is a filter id and each value is a dict describing how to configure
  the corresponding Filter instance.

  The configuring dict is searched for key ``name`` (defaulting to the
  empty string) and this is used to construct a ``logging.Filter``
  instance.

* `handlers` - the corresponding value will be a dict in which each
  key is a handler id and each value is a dict describing how to
  configure the corresponding Handler instance.

  The configuring dict is searched for the following keys:

  * ``class`` (mandatory).  This is the fully qualified name of the
    handler class.

  * ``level`` (optional).  The level of the handler.

  * ``formatter`` (optional).  The id of the formatter for this
    handler.

  * ``filters`` (optional).  A list of ids of the filters for this
    handler.

  All *other* keys are passed through as keyword arguments to the
  handler's constructor.  For example, given the snippet::

      handlers:
        console:
          class : logging.StreamHandler
          formatter: brief
          level   : INFO
          filters: [allow_foo]
          stream  : ext://sys.stdout
        file:
          class : logging.handlers.RotatingFileHandler
          formatter: precise
          filename: logconfig.log
          maxBytes: 1024
          backupCount: 3

  the handler with id ``console`` is instantiated as a
  ``logging.StreamHandler``, using ``sys.stdout`` as the underlying
  stream.  The handler with id ``file`` is instantiated as a
  ``logging.handlers.RotatingFileHandler`` with the keyword arguments
  ``filename='logconfig.log', maxBytes=1024, backupCount=3``.

* `loggers` - the corresponding value will be a dict in which each key
  is a logger name and each value is a dict describing how to
  configure the corresponding Logger instance.

  The configuring dict is searched for the following keys:

  * ``level`` (optional).  The level of the logger.

  * ``propagate`` (optional).  The propagation setting of the logger.

  * ``filters`` (optional).  A list of ids of the filters for this
    logger.

  * ``handlers`` (optional).  A list of ids of the handlers for this
    logger.

  The specified loggers will be configured according to the level,
  propagation, filters and handlers specified.

* `root` - this will be the configuration for the root logger.
  Processing of the configuration will be as for any logger, except
  that the ``propagate`` setting will not be applicable.

* `incremental` - whether the configuration is to be interpreted as
  incremental to the existing configuration.  This value defaults to
  ``False``, which means that the specified configuration replaces the
  existing configuration with the same semantics as used by the
  existing ``fileConfig()`` API.

  If the specified value is ``True``, the configuration is processed
  as described in the section on `Incremental Configuration`_, below.

* `disable_existing_loggers` - whether any existing loggers are to be
  disabled. This setting mirrors the parameter of the same name in
  ``fileConfig()``. If absent, this parameter defaults to ``True``.
  This value is ignored if `incremental` is ``True``.

A Working Example
-----------------

The following is an actual working configuration in YAML format
(except that the email addresses are bogus)::

    formatters:
      brief:
        format: '%(levelname)-8s: %(name)-15s: %(message)s'
      precise:
        format: '%(asctime)s %(name)-15s %(levelname)-8s %(message)s'
    filters:
      allow_foo:
        name: foo
    handlers:
      console:
        class : logging.StreamHandler
        formatter: brief
        level   : INFO
        stream  : ext://sys.stdout
        filters: [allow_foo]
      file:
        class : logging.handlers.RotatingFileHandler
        formatter: precise
        filename: logconfig.log
        maxBytes: 1024
        backupCount: 3
      debugfile:
        class : logging.FileHandler
        formatter: precise
        filename: logconfig-detail.log
        mode: a
      email:
        class: logging.handlers.SMTPHandler
        mailhost: localhost
        fromaddr: my_app@domain.tld
        toaddrs:
          - support_team@domain.tld
          - dev_team@domain.tld
        subject: Houston, we have a problem.
    loggers:
      foo:
        level : ERROR
        handlers: [debugfile]
      spam:
        level : CRITICAL
        handlers: [debugfile]
        propagate: no
      bar.baz:
        level: WARNING
    root:
      level     : DEBUG
      handlers  : [console, file]


Incremental Configuration
=========================

It is difficult to provide complete flexibility for incremental
configuration.  For example, because objects such as filters
and formatters are anonymous, once a configuration is set up, it is
not possible to refer to such anonymous objects when augmenting a
configuration.

Furthermore, there is not a compelling case for arbitrarily altering
the object graph of loggers, handlers, filters, formatters at
run-time, once a configuration is set up; the verbosity of loggers and
handlers can be controlled just by setting levels (and, in the case of
loggers, propagation flags).  Changing the object graph arbitrarily in
a safe way is problematic in a multi-threaded environment; while not
impossible, the benefits are not worth the complexity it adds to the
implementation.

Thus, when the ``incremental`` key of a configuration dict is present
and is ``True``, the system will ignore any ``formatters`` and
``filters`` entries completely, and process only the ``level``
settings in the ``handlers`` entries, and the ``level`` and
``propagate`` settings in the ``loggers`` and ``root`` entries.

It's certainly possible to provide incremental configuration by other
means, for example making ``dictConfig()`` take an ``incremental``
keyword argument which defaults to ``False``. The reason for
suggesting that a value in the configuration dict be used is that it
allows for configurations to be sent over the wire as pickled dicts
to a socket listener. Thus, the logging verbosity of a long-running
application can be altered over time with no need to stop and
restart the application.

Note: Feedback on incremental configuration needs based on your
practical experience will be particularly welcome.


API Customization
=================

The bare-bones ``dictConfig()`` API will not be sufficient for all
use cases.  Provision for customization of the API will be made by
providing the following:

* A class, called ``DictConfigurator``, whose constructor is passed
  the dictionary used for configuration, and which has a
  ``configure()`` method.

* A callable, called ``dictConfigClass``, which will (by default) be
  set to ``DictConfigurator``.  This is provided so that if desired,
  ``DictConfigurator`` can be replaced with a suitable user-defined
  implementation.

The ``dictConfig()`` function will call ``dictConfigClass`` passing
the specified dictionary, and then call the ``configure()`` method on
the returned object to actually put the configuration into effect::

      def dictConfig(config):
          dictConfigClass(config).configure()

This should cater to all customization needs. For example, a subclass
of ``DictConfigurator`` could call ``DictConfigurator.__init__()`` in
its own ``__init__()``, then set up custom prefixes which would be
usable in the subsequent ``configure() call``. The ``dictConfigClass``
would be bound to the subclass, and then ``dictConfig()`` could be
called exactly as in the default, uncustomized state.

Change to Socket Listener Implementation
========================================

The existing socket listener implementation will be modified as
follows: when a configuration message is received, an attempt will be
made to deserialize to a dictionary using the json module. If this
step fails, the message will be assumed to be in the fileConfig format
and processed as before. If deserialization is successful, then
``dictConfig()`` will be called to process the resulting dictionary.


Configuration Errors
====================

If an error is encountered during configuration, the system will raise
a ``ValueError``, ``TypeError``, ``AttributeError`` or ``ImportError``
with a suitably descriptive message.  The following is a (possibly
incomplete) list of conditions which will raise an error:

* A ``level`` which is not a string or which is a string not
  corresponding to an actual logging level

* A ``propagate`` value which is not a boolean

* An id which does not have a corresponding destination

* A non-existent handler id found during an incremental call

* An invalid logger name

* Inability to resolve to an internal or external object


Discussion in the community
===========================

The PEP has been announced on python-dev and python-list. While there
hasn't been a huge amount of discussion, this is perhaps to be
expected for a niche topic.

Discussion threads on python-dev:

http://mail.python.org/pipermail/python-dev/2009-October/092695.html
http://mail.python.org/pipermail/python-dev/2009-October/092782.html
http://mail.python.org/pipermail/python-dev/2009-October/093062.html

And on python-list:

http://mail.python.org/pipermail/python-list/2009-October/1223658.html
http://mail.python.org/pipermail/python-list/2009-October/1224228.html

There have been some comments in favour of the proposal, no
objections to the proposal as a whole, and some questions and
objections about specific details. These are believed by the author
to have been addressed by making changes to the PEP.


Reference implementation
========================

A reference implementation of the changes is available as a module
dictconfig.py with accompanying unit tests in test_dictconfig.py, at:

http://bitbucket.org/vinay.sajip/dictconfig

This incorporates all features other than the socket listener change.


References
==========

.. [1] PEP 8, Style Guide for Python Code, van Rossum, Warsaw
   (http://www.python.org/dev/peps/pep-0008)


Copyright
=========

This document has been placed in the public domain.



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