All config options used within Mercurial should be registered.
Config options used by Mercurial core are registered in the "mercurial.configitems" module.
A registration entry typically looks like:
coreconfigitem('section', 'option',
default=MyDefaultValue,
)
Once registered, Mercurial will know that "section.option" is a legitimate config option and that "MyDefaultValue" should be used if no other values are defined in configuration files.
If the default provided is a callable, it is called to retrieve the default value when accessing the config option. This is useful for default values that are mutable like the empty list:
coreconfigitem('pager', 'ignore',
default=list,
)
In addition, there are cases where the default is not fixed, but computed from other properties. In this case, use the "dynamicdefault" object as the value for the "default" parameter. A default value is then explicitly required when reading the option:
# registration
coreconfigitem('web', 'name',
default=dynamicdefault,
)
# usage
ui.config('web', 'name', dirname)
Some config sections use free form options (e.g. "paths"). You can register them using the "generic" parameters:
coreconfigitem('paths', '.*',
default=None,
generic=True,
)
When "generic=True" is set, the option name is matched as a regular expression (rooted to string start). It can be used to select specific sub parameters:
coreconfigitem('merge-tools', br'.*\.args$',
default="$local $base $other",
generic=True,
priority=-1,
)
The "priority" parameter controls the order used to match the generic pattern (lower first).
Extensions should register config items through the "registrar" API (also used for commands and others):
configtable = {}
configitem = registrar.configitem(configtable)
configitem('blackbox', 'dirty',
default=False,
)
The "dynamicdefault" object is then available as "configitem.dynamicdefault".
The registrar was introduced in Mercurial 4.3, and the "generic" parameter was introduced in 4.4. Starting with Mercurial 4.4, all core options were registered and developer warnings are emitted when accessing unregistered option.
Extensions supporting versions older than Mercurial 4.3 cannot rely on the default value being registered. The simplest way to register an option while still supporting an older version is to use "dynamicdefault" for options requiring a default value. The existing code passing an explicit default can then stay in use until compatibility with Mercurial 4.2 is dropped.
As reminder, here are the default values for each config type: