docs/versions/8.2.1/rules/lib/builtins/transition.mdx

---
title: 'transition'
---

Represents a configuration transition across a dependency edge. For example, if `//package:foo` depends on `//package:bar` with a configuration transition, then the configuration of these two targets will differ: `//package:bar`'s transition will be determined by that of `//package:foo`, as subject to the function defined by a transition object.

## Members

* [transition](#transition)

## transition

```
transition transition(implementation, inputs, outputs)
```

A transition that reads a set of input build settings and writes a set of output build settings.

Example:

```
def _transition_impl(settings, attr):
    # This transition just reads the current CPU value as a demonstration.
    # A real transition could incorporate this into its followup logic.
    current_cpu = settings["//command_line_option:cpu"]
    return {"//command_line_option:compilation_mode": "dbg"}

build_in_debug_mode = transition(
    implementation = _transition_impl,
    inputs = ["//command_line_option:cpu"],
    outputs = ["//command_line_option:compilation_mode"],
)
```

For more details see [here](https://bazel.build/versions/8.2.1/rules/config#user-defined-transitions).

### Parameters

| Parameter | Description |
| --- | --- |
| `implementation` | callable; required  The function implementing this transition. This function always has two parameters: `settings` and `attr`. The `settings` param is a dictionary whose set of keys is defined by the inputs parameter. So, for each build setting `--//foo=bar`, if `inputs` contains `//foo`, `settings` will have an entry `settings['//foo']='bar'`. The `attr` param is a reference to `ctx.attr`. This gives the implementation function access to the rule's attributes to make attribute-parameterized transitions possible. This function must return a `dict` from build setting identifier to build setting value; this represents the configuration transition: for each entry in the returned `dict`, the transition updates that setting to the new value. All other settings are unchanged. This function can also return a `list` of `dict`s or a `dict` of `dict`s in the case of a split transition. |
| `inputs` | [sequence](../core/list) of [string](../core/string)s; required  List of build settings that can be read by this transition. This becomes the key set of the settings parameter of the implementation function parameter. |
| `outputs` | [sequence](../core/list) of [string](../core/string)s; required  List of build settings that can be written by this transition. This must be a superset of the key set of the dictionary returned by this transition. |