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<h1 id="use-case-3-wiring-runtime-injection">Use Case 3: Wiring &amp; Runtime Injection<a class="headerlink" href="#use-case-3-wiring-runtime-injection" title="Permanent link">&para;</a></h1>
<p><strong>Scenario:</strong> you have concrete classes with annotated slots and you want a fully wired instance — including runtime values (repos, clients) and config values (retries, labels, thresholds) — without writing any assembly code.</p>
<hr />
<h2 id="whats-new">📦 What's New?<a class="headerlink" href="#whats-new" title="Permanent link">&para;</a></h2>
<table>
<thead>
<tr>
<th>Component</th>
<th>Description</th>
</tr>
</thead>
<tbody>
<tr>
<td><code>build</code></td>
<td>Recursively instantiates the port tree from annotations.</td>
</tr>
<tr>
<td><code>instances=</code></td>
<td>Inject objects verbatim (never reconstructed) by name.</td>
</tr>
<tr>
<td><code>config=</code></td>
<td>Propagate plain config values by name across the whole tree.</td>
</tr>
</tbody>
</table>
<hr />
<h2 id="example">🚀 Example<a class="headerlink" href="#example" title="Permanent link">&para;</a></h2>
<p>This mirrors <code>tests/test_container.py</code> exactly. Define leaf, mid-level, and root classes purely with annotations:</p>
<div class="language-python highlight"><span class="filename">Python</span><pre><span></span><code><a id="__codelineno-0-1" name="__codelineno-0-1" href="#__codelineno-0-1"></a><span class="kn">from</span><span class="w"> </span><span class="nn">hexa</span><span class="w"> </span><span class="kn">import</span> <span class="n">build</span>
<a id="__codelineno-0-2" name="__codelineno-0-2" href="#__codelineno-0-2"></a>
<a id="__codelineno-0-3" name="__codelineno-0-3" href="#__codelineno-0-3"></a>
<a id="__codelineno-0-4" name="__codelineno-0-4" href="#__codelineno-0-4"></a><span class="k">class</span><span class="w"> </span><span class="nc">_Repo</span><span class="p">:</span>
<a id="__codelineno-0-5" name="__codelineno-0-5" href="#__codelineno-0-5"></a> <span class="k">def</span><span class="w"> </span><span class="fm">__init__</span><span class="p">(</span><span class="bp">self</span><span class="p">):</span>
<a id="__codelineno-0-6" name="__codelineno-0-6" href="#__codelineno-0-6"></a> <span class="bp">self</span><span class="o">.</span><span class="n">name</span> <span class="o">=</span> <span class="s2">"injected-repo"</span>
<a id="__codelineno-0-7" name="__codelineno-0-7" href="#__codelineno-0-7"></a>
<a id="__codelineno-0-8" name="__codelineno-0-8" href="#__codelineno-0-8"></a>
<a id="__codelineno-0-9" name="__codelineno-0-9" href="#__codelineno-0-9"></a><span class="k">class</span><span class="w"> </span><span class="nc">_Leaf</span><span class="p">:</span>
<a id="__codelineno-0-10" name="__codelineno-0-10" href="#__codelineno-0-10"></a> <span class="n">field</span><span class="p">:</span> <span class="nb">int</span> <span class="o">=</span> <span class="mi">0</span>
<a id="__codelineno-0-11" name="__codelineno-0-11" href="#__codelineno-0-11"></a>
<a id="__codelineno-0-12" name="__codelineno-0-12" href="#__codelineno-0-12"></a>
<a id="__codelineno-0-13" name="__codelineno-0-13" href="#__codelineno-0-13"></a><span class="k">class</span><span class="w"> </span><span class="nc">_Mid</span><span class="p">:</span>
<a id="__codelineno-0-14" name="__codelineno-0-14" href="#__codelineno-0-14"></a> <span class="n">leaf</span><span class="p">:</span> <span class="n">_Leaf</span>
<a id="__codelineno-0-15" name="__codelineno-0-15" href="#__codelineno-0-15"></a> <span class="n">threshold</span><span class="p">:</span> <span class="nb">float</span> <span class="o">=</span> <span class="mf">0.0</span>
<a id="__codelineno-0-16" name="__codelineno-0-16" href="#__codelineno-0-16"></a>
<a id="__codelineno-0-17" name="__codelineno-0-17" href="#__codelineno-0-17"></a>
<a id="__codelineno-0-18" name="__codelineno-0-18" href="#__codelineno-0-18"></a><span class="k">class</span><span class="w"> </span><span class="nc">_Root</span><span class="p">:</span>
<a id="__codelineno-0-19" name="__codelineno-0-19" href="#__codelineno-0-19"></a> <span class="n">repo</span><span class="p">:</span> <span class="n">_Repo</span>
<a id="__codelineno-0-20" name="__codelineno-0-20" href="#__codelineno-0-20"></a> <span class="n">mid</span><span class="p">:</span> <span class="n">_Mid</span>
<a id="__codelineno-0-21" name="__codelineno-0-21" href="#__codelineno-0-21"></a> <span class="n">retries</span><span class="p">:</span> <span class="nb">int</span> <span class="o">=</span> <span class="mi">0</span>
<a id="__codelineno-0-22" name="__codelineno-0-22" href="#__codelineno-0-22"></a> <span class="n">label</span><span class="p">:</span> <span class="nb">str</span> <span class="o">=</span> <span class="s2">""</span>
<a id="__codelineno-0-23" name="__codelineno-0-23" href="#__codelineno-0-23"></a>
<a id="__codelineno-0-24" name="__codelineno-0-24" href="#__codelineno-0-24"></a>
<a id="__codelineno-0-25" name="__codelineno-0-25" href="#__codelineno-0-25"></a><span class="c1"># Provide a runtime instance for a slot, and config values by name.</span>
<a id="__codelineno-0-26" name="__codelineno-0-26" href="#__codelineno-0-26"></a><span class="n">repo</span> <span class="o">=</span> <span class="n">_Repo</span><span class="p">()</span>
<a id="__codelineno-0-27" name="__codelineno-0-27" href="#__codelineno-0-27"></a><span class="n">root</span> <span class="o">=</span> <span class="n">build</span><span class="p">(</span>
<a id="__codelineno-0-28" name="__codelineno-0-28" href="#__codelineno-0-28"></a> <span class="n">_Root</span><span class="p">,</span>
<a id="__codelineno-0-29" name="__codelineno-0-29" href="#__codelineno-0-29"></a> <span class="n">instances</span><span class="o">=</span><span class="p">{</span><span class="s2">"repo"</span><span class="p">:</span> <span class="n">repo</span><span class="p">},</span>
<a id="__codelineno-0-30" name="__codelineno-0-30" href="#__codelineno-0-30"></a> <span class="n">config</span><span class="o">=</span><span class="p">{</span><span class="s2">"retries"</span><span class="p">:</span> <span class="mi">3</span><span class="p">,</span> <span class="s2">"label"</span><span class="p">:</span> <span class="s2">"fast"</span><span class="p">,</span> <span class="s2">"threshold"</span><span class="p">:</span> <span class="mf">2.5</span><span class="p">},</span>
<a id="__codelineno-0-31" name="__codelineno-0-31" href="#__codelineno-0-31"></a><span class="p">)</span>
<a id="__codelineno-0-32" name="__codelineno-0-32" href="#__codelineno-0-32"></a>
<a id="__codelineno-0-33" name="__codelineno-0-33" href="#__codelineno-0-33"></a><span class="k">assert</span> <span class="n">root</span><span class="o">.</span><span class="n">repo</span> <span class="ow">is</span> <span class="n">repo</span> <span class="c1"># injected verbatim, not reconstructed</span>
<a id="__codelineno-0-34" name="__codelineno-0-34" href="#__codelineno-0-34"></a><span class="k">assert</span> <span class="n">root</span><span class="o">.</span><span class="n">retries</span> <span class="o">==</span> <span class="mi">3</span> <span class="c1"># config applied</span>
<a id="__codelineno-0-35" name="__codelineno-0-35" href="#__codelineno-0-35"></a><span class="k">assert</span> <span class="n">root</span><span class="o">.</span><span class="n">label</span> <span class="o">==</span> <span class="s2">"fast"</span>
<a id="__codelineno-0-36" name="__codelineno-0-36" href="#__codelineno-0-36"></a><span class="k">assert</span> <span class="n">root</span><span class="o">.</span><span class="n">mid</span><span class="o">.</span><span class="n">leaf</span> <span class="ow">is</span> <span class="ow">not</span> <span class="kc">None</span> <span class="c1"># nested ports built recursively</span>
<a id="__codelineno-0-37" name="__codelineno-0-37" href="#__codelineno-0-37"></a><span class="k">assert</span> <span class="n">root</span><span class="o">.</span><span class="n">mid</span><span class="o">.</span><span class="n">threshold</span> <span class="o">==</span> <span class="mf">2.5</span> <span class="c1"># config propagates by name into nested nodes</span>
</code></pre></div>
<p>Without any overrides, the same annotations still produce a working tree — every slot is instantiated from its annotated type and config fields keep their class defaults:</p>
<div class="language-python highlight"><span class="filename">Python</span><pre><span></span><code><a id="__codelineno-1-1" name="__codelineno-1-1" href="#__codelineno-1-1"></a><span class="n">root</span> <span class="o">=</span> <span class="n">build</span><span class="p">(</span><span class="n">_Root</span><span class="p">)</span>
<a id="__codelineno-1-2" name="__codelineno-1-2" href="#__codelineno-1-2"></a><span class="k">assert</span> <span class="nb">isinstance</span><span class="p">(</span><span class="n">root</span><span class="o">.</span><span class="n">repo</span><span class="p">,</span> <span class="n">_Repo</span><span class="p">)</span>
<a id="__codelineno-1-3" name="__codelineno-1-3" href="#__codelineno-1-3"></a><span class="k">assert</span> <span class="nb">isinstance</span><span class="p">(</span><span class="n">root</span><span class="o">.</span><span class="n">mid</span><span class="p">,</span> <span class="n">_Mid</span><span class="p">)</span>
<a id="__codelineno-1-4" name="__codelineno-1-4" href="#__codelineno-1-4"></a><span class="k">assert</span> <span class="nb">isinstance</span><span class="p">(</span><span class="n">root</span><span class="o">.</span><span class="n">mid</span><span class="o">.</span><span class="n">leaf</span><span class="p">,</span> <span class="n">_Leaf</span><span class="p">)</span>
<a id="__codelineno-1-5" name="__codelineno-1-5" href="#__codelineno-1-5"></a><span class="k">assert</span> <span class="n">root</span><span class="o">.</span><span class="n">retries</span> <span class="o">==</span> <span class="mi">0</span> <span class="ow">and</span> <span class="n">root</span><span class="o">.</span><span class="n">label</span> <span class="o">==</span> <span class="s2">""</span>
</code></pre></div>
<p>Optional / typing-generic config fields are handled the same way — a <code>str | None</code> annotated attribute takes its <code>config</code> value when provided and defaults to the class default otherwise:</p>
<div class="language-python highlight"><span class="filename">Python</span><pre><span></span><code><a id="__codelineno-2-1" name="__codelineno-2-1" href="#__codelineno-2-1"></a><span class="k">class</span><span class="w"> </span><span class="nc">_OptionalConfig</span><span class="p">:</span>
<a id="__codelineno-2-2" name="__codelineno-2-2" href="#__codelineno-2-2"></a> <span class="n">trust_fallback</span><span class="p">:</span> <span class="nb">str</span> <span class="o">|</span> <span class="kc">None</span> <span class="o">=</span> <span class="kc">None</span>
<a id="__codelineno-2-3" name="__codelineno-2-3" href="#__codelineno-2-3"></a> <span class="n">starters</span><span class="p">:</span> <span class="nb">list</span> <span class="o">|</span> <span class="kc">None</span> <span class="o">=</span> <span class="kc">None</span>
<a id="__codelineno-2-4" name="__codelineno-2-4" href="#__codelineno-2-4"></a> <span class="n">port</span><span class="p">:</span> <span class="n">_Leaf</span>
<a id="__codelineno-2-5" name="__codelineno-2-5" href="#__codelineno-2-5"></a>
<a id="__codelineno-2-6" name="__codelineno-2-6" href="#__codelineno-2-6"></a>
<a id="__codelineno-2-7" name="__codelineno-2-7" href="#__codelineno-2-7"></a><span class="n">built</span> <span class="o">=</span> <span class="n">build</span><span class="p">(</span><span class="n">_OptionalConfig</span><span class="p">,</span> <span class="n">config</span><span class="o">=</span><span class="p">{</span><span class="s2">"trust_fallback"</span><span class="p">:</span> <span class="s2">"amount"</span><span class="p">})</span>
<a id="__codelineno-2-8" name="__codelineno-2-8" href="#__codelineno-2-8"></a><span class="k">assert</span> <span class="n">built</span><span class="o">.</span><span class="n">trust_fallback</span> <span class="o">==</span> <span class="s2">"amount"</span>
<a id="__codelineno-2-9" name="__codelineno-2-9" href="#__codelineno-2-9"></a><span class="k">assert</span> <span class="n">built</span><span class="o">.</span><span class="n">starters</span> <span class="ow">is</span> <span class="kc">None</span> <span class="c1"># unrelated optional field keeps its default</span>
</code></pre></div>
<hr />
<h2 id="tips">💡 Tips<a class="headerlink" href="#tips" title="Permanent link">&para;</a></h2>
<ul>
<li><strong><code>instances=</code> wins over reconstruction.</strong> If a name is in <code>instances</code>, its object is <code>setattr</code>ed verbatim at every node that annotates it. Use it for things that must not be re-created (repos, handlers, clients).</li>
<li><strong><code>config=</code> targets non-port annotated fields.</strong> Builtin, <code>typing</code>, and PEP 604-union (<code>X | None</code>) annotated fields are treated as config, applied by name and propagated to every node.</li>
<li><strong>Abstract classes are never instantiated</strong>; they take a <code>config</code> value if one is provided (useful for defaults on CLI-style options).</li>
<li><strong>Only concrete annotated types become wired ports.</strong> String forward references are skipped (the container assumes <code>Impl</code> files pin actual types).</li>
<li>Port instances are wired by <code>setattr(parent, slot, child)</code> — so methods can reference <code>self.portX</code> safely once the root is built.</li>
<li>If <code>cls()</code> raises <code>TypeError</code>, <code>build</code> retries by passing <code>None</code> for each declared <code>__init__</code> parameter — convenient for classes with optional constructor args.</li>
</ul>
<hr />
<h2 id="related">Related<a class="headerlink" href="#related" title="Permanent link">&para;</a></h2>
<ul>
<li><a href="../01_yaml_first/">01 YAML first</a> · <a href="../../06_best_practices/">Best Practices</a> · <a href="../../04_design/">Design</a></li>
</ul>
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