docs/versions/9.0.0/rules/lib/toplevel/config.mdx

---
title: 'config'
---

This is a top-level module for creating configuration transitions and build setting descriptors which describe what kind of build setting (if any) a rule is.

ex: the following rule is marked as a build setting by setting the `build_setting` parameter of the `rule()` function. Specifically it is a build setting of type `int` and is a `flag` which means this build setting is callable on the command line.

```
  my_rule = rule(
    implementation = _impl,
    build_setting = config.int(flag = True),
    ...
  )
```

## Members

* [bool](#bool)
* [exec](#exec)
* [int](#int)
* [none](#none)
* [string](#string)
* [string\_list](#string_list)
* [string\_set](#string_set)
* [target](#target)

## bool

```
BuildSetting config.bool(*, flag=False)
```

A bool-typed build setting

### Parameters

| Parameter | Description |
| --- | --- |
| `flag` | [bool](/versions/9.0.0/rules/lib/core/bool); default is `False` |

## exec

```
ExecTransitionFactory config.exec(exec_group=None)
```

Creates an execution transition.

### Parameters

| Parameter | Description |
| --- | --- |
| `exec_group` | [string](/versions/9.0.0/rules/lib/core/string); or `None`; default is `None`  The name of the exec group whose execution platform this transition will use. If not provided, this exec transition will use the target's default execution platform. |

## int

```
BuildSetting config.int(*, flag=False)
```

An integer-typed build setting

### Parameters

| Parameter | Description |
| --- | --- |
| `flag` | [bool](/versions/9.0.0/rules/lib/core/bool); default is `False`  Whether or not this build setting is callable on the command line. |

## none

```
transition config.none()
```

Creates a transition which removes all configuration, unsetting all flags. Intended for the case where a dependency is data-only and contains no code that needs to be built, but should only be analyzed once.

## string

```
BuildSetting config.string(*, flag=False, allow_multiple=False)
```

A string-typed build setting

### Parameters

| Parameter | Description |
| --- | --- |
| `flag` | [bool](/versions/9.0.0/rules/lib/core/bool); default is `False`  Whether or not this build setting is callable on the command line. |
| `allow_multiple` | [bool](/versions/9.0.0/rules/lib/core/bool); default is `False`  Deprecated, use a `string_list` setting with `repeatable = True` instead. If set, this flag is allowed to be set multiple times on the command line. The Value of the flag as accessed in transitions and build setting implementation function will be a list of strings. Insertion order and repeated values are both maintained. This list can be post-processed in the build setting implementation function if different behavior is desired. |

## string\_list

```
BuildSetting config.string_list(*, flag=False, repeatable=False)
```

A string list-typed build setting. On the command line pass a list using comma-separated value like `--//my/setting=foo,bar`.

### Parameters

| Parameter | Description |
| --- | --- |
| `flag` | [bool](/versions/9.0.0/rules/lib/core/bool); default is `False`  Whether or not this build setting is callable on the command line. |
| `repeatable` | [bool](/versions/9.0.0/rules/lib/core/bool); default is `False`  If set, instead of expecting a comma-separated value, this flag is allowed to be set multiple times on the command line with each individual value treated as a single string to add to the list value. Insertion order and repeated values are both maintained. This list can be post-processed in the build setting implementation function if different behavior is desired. |

## string\_set

```
BuildSetting config.string_set(*, flag=False, repeatable=False)
```

A string set-typed build setting. The value of this setting will be a [set](https://bazel.build/versions/9.0.0/rules/lib/core/set) of strings in Starlark. On the command line, pass a set using a comma-separated value like `--//my/setting=foo,bar`.

Unlike with a `string_list`, the order of the elements doesn't matter and only a single instance of each element is maintained. This is recommended over `string_list` for flags where these properties are not needed as it can improve build performance by avoiding unnecessary configurations forking.

### Parameters

| Parameter | Description |
| --- | --- |
| `flag` | [bool](/versions/9.0.0/rules/lib/core/bool); default is `False`  Whether or not this build setting is callable on the command line. |
| `repeatable` | [bool](/versions/9.0.0/rules/lib/core/bool); default is `False`  If set, instead of expecting a comma-separated value, this flag is allowed to be set multiple times on the command line with each individual value treated as a single string to add to the set value. Only a single instance of repeated values is maintained and the insertion order does not matter. |

## target

```
transition config.target()
```

Creates a target transition. This is a no-op transition intended for the case where a transition object is needed, but doesn't want to actually change anything. Equivalent to `cfg = "target"` in `attr.label()`.