docs/versions/6.4.0/rules/lib/transition.mdx - bazel - Git at Google

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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>
   </li>
   </ul>

       <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/6.4.0/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">
         </colgroup>
         <thead>
           <tr>
             <th>Parameter</th>
             <th>Description</th>
           </tr>
         </thead>
         <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>
       </table>


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

	<!-- {% raw %} -->

	<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>
	</li>
	</ul>

	<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/6.4.0/rules/config#user-defined-transitions">here</a>.</p>

	<!-- hide-from-toc is a class used by DevSite for the public Bazel site
	(https://developers.google.com/devsite/reference/styles/headings#hide_headings_from_the_toc) -->
	<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">
	</colgroup>
	<thead>
	<tr>
	<th>Parameter</th>
	<th>Description</th>
	</tr>
	</thead>
	<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>
	</table>


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