New in version 2.8.0
--------------------

1. Added some utility functions for setting up the log system
   A. Added set_handler function to set the main handler used by logging
      (it will replace all existing handlers on the root logger)
   B. Added capture_output function to capture and divert standard output
      and standard error to the logging system

2. Replaced start_syslog with use_syslog and deprecated the former

3. Renamed IfNotInteractive to WhenNotInteractive

4. Added daemon property on Process to check if it's in daemon mode or not

5. Added module level log function to the log module similar to logging.log

6. The unlink wrapper in application.system now only ignores OSError

7. Refactored how the configuration and runtime settings are specified for
   the process. Along with this, a new configuration directory was introduced.
   While previously there was a system and a local configuration directory,
   now we have system, user and local configuration directories. As for the
   precedence, a local configuration file will override a user configuration
   file which in turn will override a system configuration file.

   The existing attributes, properties and methods that dealt with the
   configuration and runtime settings were organized under two new
   attributes on the process instance:

     process.configuration and process.runtime

   The configuration attribute is an object with the following attributes:
     process.configuration.system_root
     process.configuration.user_root
     process.configuration.local_root
     process.configuration.subdirectory

   these properties:
     process.configuration.system_directory
     process.configuration.user_directory
     process.configuration.local_directory
     process.configuration.directories

   and this method:
     process.configuration.file(name)

   The runtime attribute is an object with the following attributes:
     process.runtime.root
     process.runtime.subdirectory

   this property:
     process.runtime.directory

   and these methods:
     process.runtime.file(name)
     process.runtime.create_directory()

   The directory properties mentioned above if not explicitly set, will
   default to a value that is calculated by combining the corresponding
   root attribute with the subdirectory attribute.

   For example the configuration system_directory if not explicitly set
   will default to the value that results from combining the configuration
   system_root with the configuration subdirectory, while the runtime
   directory if not explicitly set will default to a value that results
   from combining the runtime root with the runtime subdirectory.

   The purpose of the subdirectory attribute of both the configuration and
   runtime settings is to indicate that the application wants to use a
   subdirectory within the corresponding roots for the configuration and
   runtime locations.

   There is one exception to this rule where the configuration local directory
   doesn't use the subdirectory part because of different semantics compared
   to system and user configuration directories.
   This is because the local configuration directory is meant to be used when
   running an application from its source directory where the configuration
   files are either kept in the project root or some directory that is not
   named after the application like with system or user configurations.
   For this reason when the configuration local_directory is not set it will
   simply mirror local_root.

   The default value for the subdirectory attribute is None which means it
   will not be used and files will go directly to the corresponding roots.

   The default values for the system, user and local configuration roots
   are respectively /etc, ~/.config and the application directory.

   The default value for the runtime root is /run or /var/run depending
   on the system.

   The recommended way of setting up the configuration and runtime directories
   is to specify the corresponding subdirectory attribute in case the
   application uses a subdirectory. If a particular directory needs to be
   at some different location, the recommended way is to set that specific
   directory property. The root attributes should only be modified if they
   are in a different location on the system where the application runs, or
   if the root needs to be moved temporarily for testing purposes (like for
   example moving the runtime root from /run to /tmp for testing).

   If a particular configuration directory is not used by the application
   it should be set to None. For example a server application that doesn't
   need user configuration should set

     process.configuration.user_directory = None

   This refactoring introduced a backward incompatible change. In the previous
   version, the runtime directory was created whenever the runtime_directory
   property was set. In the new version this is no longer the case and one has
   to explicitly do it by calling process.runtime.create_directory() after
   setting up the runtime directory. This was done to avoid unexpected hidden
   behaviours when setting the runtime directory.

   The only exception to this rule is if the only file the application writes
   to the runtime directory is the pid file, then it does not need to call
   process.runtime.create_directory() explicitly, because process.daemonize()
   will call it before creating the pid file. For all the other cases where
   the application creates other files in the runtime directory and the runtime
   directory is not guaranteed to exists when the application starts, it has
   to explicitly call process.runtime.create_directory(), either after setting
   up the runtime directory or before writing the runtime files, whichever is
   more appropriate for the application.

   Calling the create_directory function if the directory already exists has
   no ill effects as it only creates the directory if it doesn't exist. In
   addition to creating the directory, it will also check if the application
   has permissions to use the runtime directory, and this check is done
   regardless if the directory was created or not. If the directory cannot be
   created of the application doesn't have permissions to access it it will
   raise ProcessError.

   Because of this incompatible change, it was decided not to keep the old
   properties and functions in a deprecated form for backward compatibility
   and have a partially compatible / partially incompatible change.

   This means that before it can work with the new version, in addition to
   explicitly creating the runtime directory, code needs to apply the
   following translations:

     process.runtime_directory        -> process.runtime.directory
     process.runtime_file(name)       -> process.runtime.file(name)

     process.system_config_directory  -> process.configuration.system_directory
     process.local_config_directory   -> process.configuration.local_directory
     process.get_config_directories() -> process.configuration.directories
     process.config_file(name)        -> process.configuration.file(name)


New in version 2.7.0
--------------------

Changed the subclasses generator in application.python into a function that
returns a list. This significantly improved speed and also fixed duplicate
entries in the output that happened in certain cases with the old version.


New in version 2.6.0
--------------------

Allow process to wait for network to become available before starting


New in version 2.5.0
--------------------

1. Added subclasses generator in application.python. The generator receives
   a class as the argument an it will recursively find and yield all the
   subclasses for it.

2. The logging system was refactored and has seen the following changes:

   A. Aligned the signature of the logging functions with their counterparts
      from the python logging module. This means that context is no longer
      passed on as keyword arguments to the functions, but instead it has
      to be provided through the extra keyword argument and it has to be
      applied by the formatter or a contextual logger (see below).

   B. The exception logging function was enhanced to be callable without a
      message, in which case it will only log the exception backtrace.
      In addition it also accepts an exc_info argument that contains an
      exception info tuple that can be used to log that given traceback.
      This enhanced exception logging function is fully backwards compatible
      with the original version and to maintain consistency it is also made
      available on the logging.Logger class.

   C. Deprecated log.msg and log.err in favor of log.info and log.exception
      respectively. These functions emit a DeprecationWarning now and they
      will be removed in the future (run your code with python -W default
      to see the deprecation warning).

   D. Provide the ability to capture and log python warnings. The feature is
      turned on by default but can be disabled with capture_warnings(False).
      With this feature turned on, the warnings generated by the warnings
      module in python, will be redirected to the logging system and logged
      by the 'python' logger at the WARNING level.

   E. Replaced log.Formatter with an enhanced version which can be customized
      to obtain a desired logging format without the need to replace it. In
      addition a mechanism to set the default formatter is provided in case
      the formatter customization level is not enough for the application.
      This Formatter can be customized by overwriting the prefix_format and
      prefix_length class attributes or it can be subclassed if more advanced
      formatting requirements are needed.

   F. Provide a ContextualLogger abstract class that can be subclassed to
      implement loggers that can modify the logged messages using additional
      context that is specific to that particular logger, without the need to
      modify the formatter.

   G. Added the ability to create additional named log levels and to modify
      the name of the existing ones. For example to create a new log level
      with the name FOOBAR and level 37, a NamedLevel should be attached
      to log.level like this:

        log.level.FOOBAR = log.NamedLevel(37)

      after which it can be either set as the log level or used to log
      messages at that particular level:

        log.level.current = log.level.FOOBAR   # set log level to FOOBAR

        logger = log.get_logger()
        logger.log(log.level.FOOBAR, message)  # log message at FOOBAR level

      The name of the level can be changed by modifying its name property:

        log.level.FOOBAR.name = 'FOO'

      The list of all the named levels (both predefined and the ones defined
      by the application) is available by accessing log.level.named_levels

   H. Renamed prefix to name in SyslogHandler and start_syslog arguments


New in version 2.0.0
--------------------

1. Refactored weakobjectmap and based it on MutableMapping instead of dict.
   As a result weakobjectmap is now slightly faster. A side effect of this
   change is that if __missing__ is defined by a subclass is no longer
   called with objectid(key), but with the key itself (as it should have
   been). While this is a backward incompatible change, it is also for the
   better as calling __missing__ with objectid(key) was just a necessary
   hack that resulted from the particular details of how the class was
   implemented on top of dict, but it was non-standard. Now __missing__ is
   called with the missing key as the argument as expected.

2. Added defaultweakobjectmap to application.python.weakref that is to
   weakobjectmap what defaultdict is to dict.

3. Refactored the configuration code. As part of this process, the metaclass
   was renamed from ConfigSectionMeta to ConfigSectionType and all the
   methods were moved from ConfigSection to its metaclass. The alternative
   methods __read__, __set__ and __reset__ were removed as now read, set
   and reset cannot be overwritten anymore by config section attributes.
   The __tracing__ attribute and the related functionality was removed.

   Added the ability to take snapshots of configuration sections by calling
   saved_state = SaveState(config_section), which can later be restored by
   calling config_section.reset(state=saved_state). This method is the
   recommended one and it overrides the previous way of doing it by using
   a dictionary like this: saved_state = dict(config_section) and later
   config_section.set(**saved_state). The new method is not only faster
   as it doesn't reevaluate each setting, but is also protected against
   applying saved states from other config sections. While the old method
   still works is not recommended to be used as it unnecessarily reevaluates
   every setting before reapplying it. The set method should only be used
   to set multiple settings at once atomically. Calling reset without any
   arguments still works as before and it will reset the section to its
   hardcoded defaults.

4. Improved the timer class by turning it into a context manager. Now code
   can be timed by simply surrounding it with a with statement like this:

   from application.debug.timing import timer

   with timer("timed code description"):
       your_timed_code_here

   The timer context manager only works at the script/module level (i.e. it
   will not work inside a function), but the timed code can be any python
   code.

   The timer is smart enough to determine how many loops it needs to perform
   so specifying the loop count is not necessary, unless one has a particular
   need to do so. It accepts description, loops, repeat and time_function as
   arguments (all optional). For a complete overview of what can be specified
   and their default values look at the class implementation.

   In addition the timer has code to measure and compensate for the iteration
   overhead, resulting in more accurate results.

   In addition a new context manager called time_probe which can measure the
   time a code section takes to execute. Unlike timer this one doesn't run
   the timed code in a loop and computes the average time, but only measures
   the surrounded code by taking a time measurement when entering the section
   and one when leaving it and computing the difference. This can be used to
   measure live code in-place to determine the time spent in critical certain
   sections, with minimal changes to the code (putting a with context manager
   around the measured section) and with no functional interference to the
   measured code. The time_probe context manager works everywhere, including
   inside functions.
   Because measuring very short code sections that take very little time can
   be inaccurate, the time_probe will show the level of uncertainty for the
   measurement as a percent. The higher the uncertainty, the less reliable
   the measured value is.
   A companion for the time_probe context manager is the measure_time
   decorator that can be used to decorate functions in order to time them.
   The decorator uses the time_probe in its implementation, being just an
   easier way of using the time probe to measure whole functions.

5. Added openfile function in application.system. This function behaves the
   same as python's open, but in addition it understands 'x' mode (create
   a file in exclusive mode, i.e. only if it doesn't already exist) and it
   also accepts a permissions argument which allows one to create files with
   certain permissions in a single step to prevent race conditions and
   security issues. The permissions argument is ignored unless the file is
   created as a result of this openfile() call. In addition, if the 'x'
   mode is used and the file already exists, it will raise FileExistsError
   (which is a subclass of IOError).

   Another improvement over python's open is that openfile will consistently
   seek to the file end if the file is opened in append mode regardless of
   the platform (in python2, when a file is opened in append mode, open will
   not seek to the end until the first write on Microsoft Windows).

6. The MarkerType from application.python.types was modified to create
   markers that behave like a boolean False value by default (the markers
   used to behave like boolean True before). This was changed because it
   is more useful for markers to behave like a boolean False value than
   the other way around. This can be overwritten by adding:

   __boolean__ = True

   to the marker definition to make it have a True value in boolean
   expressions.

7. The dependency module was removed.


New in version 1.5.0
--------------------

Added a generic thread pool in application.python.threadpool which can be
used to run certain operations that need to run in the background, in a
collection of threads that can be defined using the ThreadPool class.
After a threadpool is created, it can be used either directly by calling
pool.run(func, *args, **kw) or by decorating the functions that need to
run in this threadpool with @run_in_threadpool(pool) (the decorator is
provided in the threadpool module as well).


New in version 1.4.0
--------------------

1. Dropped compatibility with python2.5

2. Added the MarkerType type that can be used to define marker entities

3. Added the application.python.weakref module containing the weakobjectmap
   class, which offers the same functionality as WeakKeyDictionary from the
   standard python weakref module, with a few notable improvements:

   - it works even with objects (keys) that are not hashable
   - subclasses can implement __missing__ to define defaultdict like behavior
   - it is faster as it directly subclasses dict instead of using a UserDict
   - it is thread safe, as all it's operations are atomic, in the sense that
     they are the dict's methods executing in C while being protected by the
     GIL
   - iterating it as well as the iterating methods (iterkeys, itervalues
     and iteritems) are safe from changes to the mapping while iterating
   - it provides a __repr__ implementation that makes it display similar
     to a dict which provides an easy way to inspect it

4. Added timestamp and center attributes on posted notifications. Timestamp
   is a POSIX timestamp of when the notification was posted. Center is a
   reference to the NotificationCenter instance that posted the notification.
   In addition 2 properties are also available: datetime and utcdatetime
   which are provided for convenience. As their name suggests, they will
   give a datetime object with the local respectively the UTC datetime that
   correspond to the timestamp value. They are lazily computed and cached.

5. Swapped the order of the arguments for timer.end() from the debug.timing
   module. Now the msg argument comes first, to make it more convenient as
   it is the most used argument.


New in version 1.3.0
--------------------

Removed the obsolete application.python.util module. It's contents was moved
to application.python and application.python.types in version 1.2.8


New in version 1.2.9
--------------------

Added execute_once decorator, that allows a function or method that is
decorated with it to be executed only once for each entity to which it
belongs. For example a decorated method will be executed only once for
each instance of that class.
The decorator will add the 'called' and 'lock' attributes to the decorated
function/method and these attributes can be inspected/used from the outside.


New in version 1.2.8
--------------------

1. Reorganized the application.python package contents

   - the types module was added to hold python types.
   - the util module was made obsolete and is scheduled to be removed in 1.3.0
   - the NullType was moved to application.python.types
   - Null was moved to application.python

2. Added functions/descriptors:

   - added limit function to restrict a numeric value to a given range
   - added function to recursively create directory and ignore error if exists
   - added classproperty descriptor for defining properties at the class level

3. Removed obsolete default_host_ip attribute from application.system


New in version 1.2.5
--------------------

1. An outgoing_ip_for method was added to the host object from
   application.system which determines the IP address which would be used as a
   source for sending packets to the IP address specified as an argument. This
   always works correctly as long as no port or protocol based routing is
   performed. The method needs to be called with an IP address as an argument:

   host.outgoing_ip_for('192.0.2.1')

2. The application.log module can now be used on systems that do not have
   syslog. The start_syslog function will raise a RuntimeError if it is called
   on such a system.


New in version 1.2.4
--------------------

The Null class in in application.python.util is no longer overwritten with an
instance. Instead the Null class was renamed to NullType and is now available
from the module, while Null remains to be an instance that can be used as a
singleton as before. This was done mainly because overwriting the Null class
with an instance, prevented it to be pickled as the class could not be found
anymore during pickling.

NullType was also transformed into a singleton in order to prevent multiple
Null instances when unpickling.


New in version 1.2.3
--------------------

The memory_dump function from application.debug.memory was enhanced to detect
and report the cycles of objects whose reference count reached zero. The
function now receives two arguments, show_cycles and show_objects, defaulting to
True and False respectively. The former controls whether the object cycles are
printed, while the latter whether the garbage objects are printed. In order to
obtain the behaviour found in the previous versions, the function needs to be
called as:

memory_dump(show_cycles=False, show_objects=True)

In addition, the representation of garbage objects is truncated according to the
width of the console, rather than at the fixed size of 80 characters.


New in version 1.2.2
--------------------

Null from application.python.util was transformed into an instance. This
means that one is no longer required to instantiate it after importing it.
Exiting code that did that should be unaffected as calling on a Null
instance returns Null itself. This change will make Null easier to use and
similar in semantics with None. If the type is still needed (for subclassing
for example) it can be obtained as type(Null) or Null.__class__


New in version 1.2.0
--------------------

1. python-application now depends on python2.5 or later.

2. The functionality which was deprecated up to this version was removed. This
   includes:
    - Within the configuration system:
      * The ConfigFile.read_settings and ConfigFile.get_option methods
      * The ConfigSection.__configfile__ and ConfigSection._datatypes special
        attributes
      * The dump_settings global function
      * The NetworkAddress._defaultPort, EndpointAddress._defaultPort and
        EndpointAddress._name special attributes
    - Within the logging system:
      * The startSyslog global function
    - Within the application.process module:
      * The Process.config_directory special attribute

   The corresponding entries within this file point to the equivalent
   functionality provided.

3. The notification system has received a number of enhancements which improve
   its usability.

   Two new methods were added to the NotificationCenter:
    - discard_observer: which is equivalent to remove_observer but does not
      raise an error if the observer is not subscribed for the specified
      entry in the NotificationCenter
    - purge_observer: which will remove all of the observer's subscriptions
      within the NotificationCenter

   A new class, ObserverWeakrefProxy, has been added which allows the
   NotificationCenter to keep a weak reference to the observer and automatically
   removes all of its subscriptions once the observer object's reference count
   reaches zero. Please see examples/notification.py for an example of using the
   class.

4. The application.system module now contains a host object with dynamic
   properties that reflect the default IP address, hostname, fully qualified
   domain name, domain name and host aliases of the running system.

   In addition, the default_host_ip attribute has been deprecated and will be
   removed in a future release. This attribute is not dynamic and its value is
   frozen to that determined at module load time. The default_host_ip attribute
   should be replaced by host.default_ip.

5. The logging system was improved to offer better consistency. The twisted
   backend was removed and the application.log module is built exclusively upon
   the standard logging module. This allows better integration with applications
   which don't use twisted but run on a system where it is available. For the
   applications which do use it, twisted's log module is hooked automatically to
   use the logging system in python-application the first time a twisted module
   is imported. This improvement also simplifies the log module, allowing it to
   be more easily extended, and in the same time makes it faster.

   The log functions now pass their keyword arguments as a dictionary to the
   `extra' argument of the functions in the logging module for use by
   applications which define their own log formatter:

   application.log.info('message', foo=1, bar=2)

   translates to

   logging.info('message', extra={'foo': 1, 'bar': 2})

   The functions log.warn, log.fatal and log.err have been replaced with
   log.warning, log.critical and log.exception respectively. The previous names
   have been kept as aliases and can continue to be used as they are not
   deprecated.

   The log.exception/log.err function now accepts an optional message which, if
   present, is logged at ERROR level in addition to the exception traceback,
   which otherwise is logged as before.

6. Miscellaneous improvements:
    - The Hostname configuration data type no longer tries to resolve the
      provided value in DNS; this is due to the fact that a glitch in DNS could
      have resulted in a configuration setting being ignored.
    - The Null object has been improved with several new methods for descriptor
      and sequence protocol support.


New in version 1.1.5
--------------------

The configuration framework has received a number of enhancements to make it
easier to use.

1. Made ConfigSection iterable as (name, value) pairs. With this it became
   very easy to go through all the settings in a section with a simple
   for loop or list comprehension: for name, value in MyConfig: ...
   Saving the current state of a ConfigSection has also become as easy as
   writing saved_state = dict(MyConfig)

2. Allow any callable to used as a data type converter (or data validator).
   This was a small regression from version 1.1.1, fixed in this version.

3. The initial state of a ConfigSection, given by the factory default values
   is saved as a dict in ConfigSection.__defaults__, right after the class is
   created, before any of the settings is modified or read from a section.
   With this it's possible to reset to the factory defaults at a later time,
   or to inspect the factory defaults even after the settings were modified
   when read from a configuration file.

4. Added ConfigSection.reset() to allow a ConfigSection to revert itself to
   the factory default values. The method is mirrored in __reset__, in case
   the reset keyword is used by the section as a setting name and overwrites
   the method.

5. Added ConfigSection.set() which is used to modify multiple settings at a
   time. It takes name=value keyword arguments and modifies the settings
   which are referred by name. Using a keyword argument that doesn't map to
   an existing setting name will result in an exception. The method is
   mirrored in __set__, in case the set keyword is used as a setting name
   in the section and overwrites the method.

   Another use of this function is to restore a previously saved state, which
   together with the ability to make a snapshot of the current state provide
   the foundation for implementing a versioning system on top of the existing
   configuration framework.

     saved_state = dict(MyConfig) # save the current state
     MyConfig.foo = 'bar'         # change it
     MyConfig.bar = 'baz'
     ...
     MyConfig.reset()             # reset it to factory defaults
     ...
     MyConfig.set(saved_state)    # revert to the saved state

6. Added ConfigFile.read() which is used to read the settings from a given
   configuration file and section (or sections) into the class attributes
   with the corresponding names. This replaces ConfigFile.read_settings
   (which was obsoleted), providing a better interface which decouples
   ConfigSection from ConfigFile.
   
   Together with the reset method, read allows a ConfigSection to reload
   its settings when the configuration file changes. ConfigFile provides
   an internal caching mechanism that returns the same instance as long
   as it has not changed on disk, but it will automatically reload it if
   it detects that it has changed since the last time it was requested.
   Because of this caching mechanism, a ConfigSection only needs to call
   its read method to have the content reloaded if it has changed, with
   no need to perform any other action. By using the reset method it can
   assure that it doesn't retain old values for settings that were removed
   from the configuration file since it was last red. If the removed values
   need to be preserved instead of being reset to factory defaults, then
   the reset call can be skipped.
   
     MyConfig.reset() # revert to factory defaults
     MyConfig.read()  # reload settings by reading them from the file
   
   The read method accepts optional cfgfile and section arguments that
   override the __cfgfile__ and __section__ attributes. If the arguments
   are not provided, the class attributes will used instead if defined.
   If not the arguments nor the class attributes are specified it will
   raise an exception as it cannot perform the requested action.

7. Improved the ConfigFile caching mechanism to take into account the files
   that were red and their timestamp, in order to reload the configuration
   files when they change, instead of retaining them forever once read.
   This allows ConfigSection.read() to work transparently when reloading a
   configuration file that changed.

8. The __configfile__ property on ConfigSection was deprecated in favor of
   __cfgfile__

9. Added ConfigSection.__cfgtype__ to specify the type to use to create
   instances that handle a configuration file. It defaults to ConfigFile,
   but it can be overwritten in case one wants to implement a different
   backend than the one based on ConfigParser.

10. A ConfigSection can now be printed and it will show all its settings
    in a manner similar to the deprecated dump_settings. This is done by
    implementing the __str__ method, which makes it easy to include the
    string representation of a ConfigSection into a message using %s

11. Added ConfigSection.__tracing__ that can hold one of log.level.INFO,
    log.level.DEBUG or None, indicating where to send messages about the
    inner working of the ConfigSection. When set to None, no information
    will be logged, while the other 2 values indicate the log level to
    which the messages should be logged. When enabled, it will output
    detailed descriptions of what it does and the state it reaches after
    operations like read(), set() or reset() as well as the initial class
    creation or individual changes to settings.

12. Modified the data converters/validators in datatypes to also accept on
    input what they generate on output. Since they are not real types and
    do not return instances of themselves, they need to accept what they
    generate, not only strings in order for ConfigSection.set() to work
    when restoring a previously saved state. Care must be taken with custom
    defined data converters/validators to make sure they are able to process
    not only the input strings that come from the configuration, but also
    the values they generate out of those strings. This is because when
    ConfigSection.set() is later called on a saved state, it will no longer
    receive strings as input values anymore, but it will receive the values
    that were previously returned by the data converters/validators.
    If real data types are used instead to define settings, this is not an
    issue as then the values can be identified by their type and if already
    the right type the type will no longer be applied on them before being
    assigned as settings.

Deprecated in this version and scheduled to be removed in 1.2.0:

- ConfigFile.read_settings() in favor of ConfigSection.read()
- dump_settings(MyConfig) in favor of print MyConfig
- ConfigSection.__configfile__ in favor of ConfigSection.__cfgfile__

All the deprecated features issue a deprecation warning when used/accessed.


The Process class in the process module has received a new property to control
the local config directory. Until now, the local config directory could not be
modified and always pointed to the directory containing the script being run.
Starting with this version, the local config directory only defaults to the
script's directory but can be later changed using the local_config_directory
property, similar to the system_config_directory which controls the system
config directory.


New in version 1.1.4
--------------------

1. Version enhancements

Version can now handle version numbers which may not contain the micro or
even the minor number. It also added the concept of an undefined version,
that compares lower than any other version. The undefined version can be
obtained either by parsing the 'undefined' string, or by creating an
instance of Version with all arguments None.


2. New dependency module

Added a new module application.dependency that can define application
dependencies on certain packages having minimum required versions and
allow the application to check if they are satisfied at runtime.
For usage examples please check examples/dependency.py


New in version 1.1.3
--------------------

Added the isdescriptor function to the application.python.descriptor module.
As name suggests, it can be used to determine if an object is a descriptor.


New in version 1.1.2
--------------------

1. Configuration framework improvements

The use of the _datatypes attribute of ConfigSection has been deprecated in
favor of a descriptor based syntax. The newly added ConfigSetting descriptor
is now used to declare the type and default value of a setting. The
descriptor is only necessary if validators are used as types, otherwise the
initial value can be directly assigned to the setting and a descriptor is
automatically generated using its type and value. The deprecated _datatypes
attribute can still be used until version 1.2.0 and descriptors will be
automatically generated from its contents.

The ConfigSection class now has two attributes, __configfile__ and
__section__, which if set to non-None values are used to automatically load
the settings from the specified filename and section. Settings in
ConfigSection classes can still be loaded explicitly using the read_settings
method of ConfigFile.

The following example describes the new use of the ConfigSection class:

class MyConfigSection(ConfigSection):
    __configfile__ = 'config.ini'
    __section__ = 'MySettings'
    address = ConfigSetting(type=datatypes.IPAddress, value='0.0.0.0')
    interval = 100

The Boolean configuration datatype has become a validator and instantiating
it will return a bool instance. With this change, all the configuration
datatypes are now validators.

The get_option method of ConfigFile has been deprecated in favor of the
get_setting method whose name is consistent with the rest of the terminology
used in the configuration package. The signatures of the two methods are
similar, but not identical:

def get_option(self, section, option, default='', type=str)
def get_setting(self, section, setting, type=str, default='')


2. Descriptors

Added a new descriptor that can be used to implement write once attributes,
named WriteOnceAttribute. It should be noted that the descriptor can only
enforce this while the object's attributes are accessed directly. It is
still possible to modify/delete such an attribute by messing around with
the descriptor's internal data. Thus the descriptor should not be used to
guarantee such a behavior, because given Python's introspective abilities,
it can be circumvented. Instead, the descriptor is meant to be used as an
indication that the attribute should be writable only once and the code
using it should play along and honor the limitation.


Given a recent bug fix for a memory leak in the ThreadLocal descriptor, now
both the ThreadLocal and the WriteOnceAttribute descriptors can only be used
inside classes that allow weak referencing to their instances. Currently, in
order to clean up internal data from objects that are no longer available,
both descriptors use weak references with a callback to the objects for which
they are an attribute, in order to be notified of the objects being released
when no longer referenced. Due to this change, the following classes cannot
use these descriptors anymore: subclasses of str, tuple and long as well as
classes that define __slots__ and do not include __weakref__ in it.


3. New version module

Added a new module to handle version numbers for applications, modules and
packages. The module offers a Version class that is a subclass of str and
can represent a major.minor.micro[extraversion] version string that offers
version wise comparison, rather than string wise, when comparing against
another version instance. When compared against a string instance it will
compare as a string.

The class understands a number of standard conventions for the extraversion
part, like alphaN, betaN, preN and rcN notations and is able to compare them
correctly (alpha < beta < pre < rc). If the extraversion is missing or is a
number it ranks higher than rcN. If the extraversion is not a number and it
doesn't fit into these conventions is compared as a string and ranks higher
than any of them. If the extraversion is a number or follows the alpha, beta,
pre, rc convention and is prefixed by a dash or a dot, the dash/dot doesn't
take part in the comparison.

The Version class has a parse classmethod, that is able to take a version
number expressed as a string and turn it into a Version instance that can be
compared. This is useful when one needs to compare version numbers, but the
package provides them as simple strings, not as comparable Version instances.


New in version 1.1.1
--------------------

1. Configuration data type changes

The _defaultPort and _name attributes from the NetworkAddress and the
EndpointAddress classes are being deprecated starting with this version,
being replaced by default_port and respectively name. The deprecated
attributes are still available, but give a deprecation warning and will
be completely removed in version 1.2.0

Added the LogLevel data type that represents a log level and which can be
used to map a log level name as read from a configuration file into a log
level that can be assigned to log.level.current to change the application
log level.


New in version 1.1.0
--------------------

1. Singleton improvements

The Singleton metaclass got enhanced to properly handle classes that take
arguments in the __init__ and/or __new__ methods. Previously, Singleton
created one instance per class, thus conflicting with classes that took
arguments in __init__ and/or __new__, because instantiating the class again
with a different set of arguments would have returned the same instance that
was created before with a different set of arguments. This behavior was not
only confusing, but also wrong.

The new implementation will create multiple singletons per class, each one
corresponding to a unique combination of the arguments used to instantiate
the class. As a result, one will get the same object only by calling the
class with the same arguments.

One restriction of this is that the __init__ and/or __new__ arguments of the
class that uses Singleton as a metaclass can only have immutable values, so
that their unique combination can be used to lookup the proper instance in a
dictionary.

To determine the arguments, the Singleton metaclass will always prefer the
__init__ signature over the __new__ signature if both are defined. When
subclassing always keep this in mind, especially for cases where __init__
is defined only in the parent class, while __new__ is defined in the current
class. In this case, __init__ will still be preferred even though is defined
in a parent class.

For classes that had no arguments in __init__ and or __new__, there is no
change in the behavior.


2. Logging improvements

The logging system was enhanced to also show the message type prefix (error,
fatal, warning, debug) when outputting on the console as it does in syslog.

The startSyslog() function was deprecated and replaced with start_syslog()
for consistency with the naming convention in the other modules.

Another improvement of the logging system is the ability to set a log level
that will be used to filter out all log messages that do not have at least
that level. The default log level is INFO, meaning that any message except
debug messages will be logged. The level can be changed at any time during
runtime:

from application import log
log.level.current = log.level.ERROR # only log error and critical messages

The available log levels are: ALL, DEBUG, INFO, WARNING, ERROR, CRITICAL
and NONE. ALL and NONE are not effective log levels, but they can be used
to output all or no log messages by assigning them to log.level.current.

The logging system also received a major improvement by no longer depending
on twisted for its log module. A new log backend using the python standard
logging module was added that offers the same functionality in the absence
of twisted. The change is transparent to the user who doesn't have to take
any steps to update his code. The functionality offered by the two backends
is identical, so the logging system works the same if twisted is present or
not.

While a strong dependency on twisted was removed by this, a weak dependency
on it still remains, in the sense that if twisted is installed the twisted
log backend will always be preferred over the logging backend. However,
twisted is no longer mandatory for python-application to work.


3. Notification system

A new notification.py module was added to python-application, to implement a
notification system that can be used to communicate between different
components of an application.

The notification system offers a notification center, that is used to post
notifications and keep track of the registered notification observers.

Observers can register to receive certain notifications posted by certain
senders. In particular one observer can watch for all notifications or for
all senders if desired.

The notification system implements a producer/consumer pattern and can be
used to better isolate different components of an application and make them
less dependant of each other. This is done by creating an asynchronous
communication channel between them via the notification center. One entity
can post a notification that publishes some state or information, while not
being interested who will receive this information. In the same manner, some
observers may register to receive that information not necessarily being
interested in who published it or being aware of each other.


4. Miscellaneous improvements

A new module was added in application.python.descriptor to hold various
useful descriptor classes. It currently only has one descriptor named
ThreadLocal that can be used to get object attributes that are of a
given type and hold data that is thread specific. For example:

from application.python.descriptor import ThreadLocal
from collections import deque

class MyClass(object):
    queue = ThreadLocal(deque)

Any instance of this class will have a queue attribute that is of type deque
and its contents is different in each thread. Type can be any python type
that is a mutable data container (it can be immutable too but it doesn't
make much sense that way).

