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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 //package:bar (and its dependencies) will be //package:foo’s configuration plus the changes specified by the transition function.

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Members

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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.

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Parameters

ParameterDescription
implementationcallable; 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 dicts or a dict of dicts in the case of a split transition.
inputssequence 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.
outputssequence 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.