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<h1 class="page-title" id="modules.transition">transition</h1>

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

<h2>Members</h2>
<ul>
    <li>
    <a href="#transition">transition</a>
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      <h2 id="transition">transition</h2>
        <p><pre class="rule-signature"><a class="anchor" href="transition.html">transition</a> transition(implementation, inputs, outputs)</pre></p>

    A transition that reads a set of input build settings and writes a set of output build settings.<p>Example:</p><p><pre class="language-python">
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"],
)</pre></p><p>For more details see <a href="https://bazel.build/versions/6.1.0/rules/config#user-defined-transitions">here</a>.</p>

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      <h3 class="hide-from-toc">Parameters</h3>
      <table class="table table-bordered table-condensed table-params">
        <colgroup>
          <col class="col-param">
          <col class="param-description">
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        <thead>
          <tr>
            <th>Parameter</th>
            <th>Description</th>
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        <tbody>
                  <tr>
            <td id="transition.implementation">
              <code>implementation</code>
            </td>
            <td>
                                     required<br/>
                          The function implementing this transition. This function always has two parameters: <code>settings</code> and <code>attr</code>. The <code>settings</code> param is a dictionary whose set of keys is defined by the inputs parameter. So, for each build setting <code>--//foo=bar</code>, if <code>inputs</code> contains <code>//foo</code>, <code>settings</code> will have an entry <code>settings['//foo']='bar'</code>.<p>The <code>attr</code> param is a reference to <code>ctx.attr</code>. This gives the implementation function access to the rule's attributes to make attribute-parameterized transitions possible.<p>This function must return a <code>dict</code> from build setting identifier to build setting value; this represents the configuration transition: for each entry in the returned <code>dict</code>, the transition updates that setting to the new value. All other settings are unchanged. This function can also return a <code>list</code> of <code>dict</code>s or a <code>dict</code> of <code>dict</code>s in the case of a split transition.
            </td>
          </tr>
                  <tr>
            <td id="transition.inputs">
              <code>inputs</code>
            </td>
            <td>
                          <code><a class="anchor" href="list.html">sequence</a> of <a class="anchor" href="string.html">string</a>s</code>;
                                     required<br/>
                          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.
            </td>
          </tr>
                  <tr>
            <td id="transition.outputs">
              <code>outputs</code>
            </td>
            <td>
                          <code><a class="anchor" href="list.html">sequence</a> of <a class="anchor" href="string.html">string</a>s</code>;
                                     required<br/>
                          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.
            </td>
          </tr>
                </tbody>
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