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# PyPy’s Configuration Handling

Due to more and more available configuration options it became quite annoying to hand the necessary options to where they are actually used and even more annoying to add new options. To circumvent these problems configuration management was introduced. There all the necessary options are stored in a configuration object, which is available nearly everywhere in the RPython toolchain and in the standard interpreter so that adding new options becomes trivial. Options are organized into a tree. Configuration objects can be created in different ways, there is support for creating an optparse command line parser automatically.

RPython toolchain: translation.html

## Main Assumption

Configuration objects are produced at the entry points and handed down to where they are actually used. This keeps configuration local but available everywhere and consistent. The configuration values are created using the command line.

## API Details

The handling of options is split into two parts: the description of which options are available, what their possible values and defaults are and how they are organized into a tree. A specific choice of options is bundled into a configuration object which has a reference to its option description (and therefore makes sure that the configuration values adhere to the option description). This splitting is remotely similar to the distinction between types and instances in the type systems of the rtyper: the types describe what sort of fields the instances have.

The Options are organized in a tree. Every option has a name, as does every option group. The parts of the full name of the option are separated by dots: e.g. `config.translation.thread`.

### Description of Options

All the constructors take a `name` and a `doc` argument as first arguments to give the option or option group a name and to document it. Most constructors take a `default` argument that specifies the default value of the option. If this argument is not supplied the default value is assumed to be `None`. Most constructors also take a `cmdline` argument where you can specify what the command line option should look like (for example cmdline=”-v –version”). If `cmdline` is not specified a default cmdline option is created that uses the name of the option together with its full path. If `None` is passed in as `cmdline` then no command line option is created at all.

Some options types can specify requirements to specify that a particular choice for one option works only if a certain choice for another option is used. A requirement is specified using a list of pairs. The first element of the pair gives the path of the option that is required to be set and the second element gives the required value.

#### `OptionDescription`

This class is used to group suboptions.

> `__init__(self, name, doc, children)`
>   `children` is a list of option descriptions (including `OptionDescription` instances for nested namespaces).

#### `ChoiceOption`

Represents a choice out of several objects. The option can also have the value `None`.

> `__init__(self, name, doc, values, default=None, requires=None, cmdline=DEFAULT)`
>   `values` is a list of values the option can possibly take, `requires` is a dictionary mapping values to lists of of two-element tuples.

#### `BoolOption`

Represents a choice between `True` and `False`.

> `__init__(self, name, doc, default=None, requires=None, suggests=None, cmdline=DEFAULT, negation=True)`
>   `default` specifies the default value of the option. `requires` is a list of two-element tuples describing the requirements when the option is set to true, `suggests` is a list of the same structure but the options in there are only suggested, not absolutely necessary. The difference is small: if the current option is set to True, both the required and the suggested options are set. The required options cannot be changed later, though. `negation` specifies whether the negative commandline option should be generated.

#### `IntOption`

Represents a choice of an integer.

> `__init__(self, name, doc, default=None, cmdline=DEFAULT)`

#### `FloatOption`

Represents a choice of a floating point number.

> `__init__(self, name, doc, default=None, cmdline=DEFAULT)`

#### `StrOption`

Represents the choice of a string.

> `__init__(self, name, doc, default=None, cmdline=DEFAULT)`

### Configuration Objects

`Config` objects hold the chosen values for the options (of the default, if no choice was made). A `Config` object is described by an `OptionDescription` instance. The attributes of the `Config` objects are the names of the children of the `OptionDescription`. Example:

    >>> from rpython.config.config import OptionDescription, Config, BoolOption
    >>> descr = OptionDescription("options", "", [
    ...     BoolOption("bool", "", default=False)])
    >>>
    >>> config = Config(descr)
    >>> config.bool
    False
    >>> config.bool = True
    >>> config.bool
    True

Description of the (useful) methods on `Config`:

> `__init__(self, descr, **overrides)`:
>   `descr` is an instance of `OptionDescription` that describes the configuration object. `overrides` can be used to set different default values (see method `override`).
> `override(self, overrides)`:
>   override default values. This marks the overridden values as defaults, which makes it possible to change them (you can usually change values only once). `overrides` is a dictionary of path strings to values.
> `set(self, **kwargs)`:
>   “do what I mean”-interface to option setting. Searches all paths starting from that config for matches of the optional arguments and sets the found option if the match is not ambiguous.

### Production of optparse Parsers

To produce an optparse parser use the function `to_optparse`. It will create an option parser using callbacks in such a way that the config object used for creating the parser is updated automatically.

> `to_optparse(config, useoptions=None, parser=None)`:
>   Returns an optparse parser. `config` is the configuration object for which to create the parser. `useoptions` is a list of options for which to create command line options. It can contain full paths to options or also paths to an option description plus an additional ”.*” to produce command line options for all sub-options of that description. If `useoptions` is `None`, then all sub-options are turned into cmdline options. `parser` can be an existing parser object, if `None` is passed in, then a new one is created.

## The usage of config objects in PyPy

The two large parts of PyPy, the Python interpreter and the RPython toolchain toolchain, have two separate sets of options. The translation toolchain options can be found on the `config` attribute of all `TranslationContext` instances and are described in rpython/config/translationoption.py. The interpreter options are attached to the object space, also under the name `config` and are described in pypy/config/pypyoption.py.

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