Extensions > API reference > 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.



transition transition(implementation, inputs, outputs)

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


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.


Parameter Description
implementation ; required
The function implementing this transition. This function always has two parammeters: 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 dicts or a dict of dicts in the case of a split transition.

inputs sequence of strings; 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 of strings; 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.