Python dataclass field defaults and post-init processing

<p>I am using Python dataclasses for internal data objects. I need mutable default values (lists, dicts), computed fields that derive from other fields, and validation in <strong>post_init</strong>.</p>

<p>Dataclass patterns for complex defaults:</p> <div class="highlight"><pre><span></span><code><span class="kn">from</span><span class="w"> </span><span class="nn">dataclasses</span><span class="w"> </span><span class="kn">import</span> <span class="n">dataclass</span><span class="p">,</span> <span class="n">field</span> <span class="kn">from</span><span class="w"> </span><span class="nn">datetime</span><span class="w"> </span><span class="kn">import</span> <span class="n">datetime</span><span class="p">,</span> <span class="n">timezone</span> <span class="kn">from</span><span class="w"> </span><span class="nn">typing</span><span class="w"> </span><span class="kn">import</span> <span class="n">Optional</span> <span class="kn">import</span><span class="w"> </span><span class="nn">uuid</span> <span class="nd">@dataclass</span> <span class="k">class</span><span class="w"> </span><span class="nc">TraceRecord</span><span class="p">:</span> <span class="n">title</span><span class="p">:</span> <span class="nb">str</span> <span class="n">context_text</span><span class="p">:</span> <span class="nb">str</span> <span class="n">solution_text</span><span class="p">:</span> <span class="nb">str</span> <span class="c1"># Mutable defaults -- use field(default_factory=) not []</span> <span class="n">tags</span><span class="p">:</span> <span class="nb">list</span><span class="p">[</span><span class="nb">str</span><span class="p">]</span> <span class="o">=</span> <span class="n">field</span><span class="p">(</span><span class="n">default_factory</span><span class="o">=</span><span class="nb">list</span><span class="p">)</span> <span class="n">metadata</span><span class="p">:</span> <span class="nb">dict</span><span class="p">[</span><span class="nb">str</span><span class="p">,</span> <span class="nb">str</span><span class="p">]</span> <span class="o">=</span> <span class="n">field</span><span class="p">(</span><span class="n">default_factory</span><span class="o">=</span><span class="nb">dict</span><span class="p">)</span> <span class="c1"># Computed at creation time:</span> <span class="nb">id</span><span class="p">:</span> <span class="nb">str</span> <span class="o">=</span> <span class="n">field</span><span class="p">(</span><span class="n">default_factory</span><span class="o">=</span><span class="k">lambda</span><span class="p">:</span> <span class="nb">str</span><span class="p">(</span><span class="n">uuid</span><span class="o">.</span><span class="n">uuid4</span><span class="p">()))</span> <span class="n">created_at</span><span class="p">:</span> <span class="n">datetime</span> <span class="o">=</span> <span class="n">field</span><span class="p">(</span><span class="n">default_factory</span><span class="o">=</span><span class="k">lambda</span><span class="p">:</span> <span class="n">datetime</span><span class="o">.</span><span class="n">now</span><span class="p">(</span><span class="n">timezone</span><span class="o">.</span><span class="n">utc</span><span class="p">))</span> <span class="c1"># Not included in __init__:</span> <span class="n">_cache</span><span class="p">:</span> <span class="nb">dict</span> <span class="o">=</span> <span class="n">field</span><span class="p">(</span><span class="n">default_factory</span><span class="o">=</span><span class="nb">dict</span><span class="p">,</span> <span class="n">init</span><span class="o">=</span><span class="kc">False</span><span class="p">,</span> <span class="nb">repr</span><span class="o">=</span><span class="kc">False</span><span class="p">)</span> <span class="k">def</span><span class="w"> </span><span class="nf">__post_init__</span><span class="p">(</span><span class="bp">self</span><span class="p">):</span> <span class="c1"># Validation:</span> <span class="k">if</span> <span class="ow">not</span> <span class="bp">self</span><span class="o">.</span><span class="n">title</span><span class="o">.</span><span class="n">strip</span><span class="p">():</span> <span class="k">raise</span> <span class="ne">ValueError</span><span class="p">(</span><span class="s1">'title cannot be empty'</span><span class="p">)</span> <span class="k">if</span> <span class="nb">len</span><span class="p">(</span><span class="bp">self</span><span class="o">.</span><span class="n">tags</span><span class="p">)</span> <span class="o">&gt;</span> <span class="mi">10</span><span class="p">:</span> <span class="k">raise</span> <span class="ne">ValueError</span><span class="p">(</span><span class="s1">'maximum 10 tags'</span><span class="p">)</span> <span class="c1"># Normalization:</span> <span class="bp">self</span><span class="o">.</span><span class="n">title</span> <span class="o">=</span> <span class="bp">self</span><span class="o">.</span><span class="n">title</span><span class="o">.</span><span class="n">strip</span><span class="p">()</span> <span class="bp">self</span><span class="o">.</span><span class="n">tags</span> <span class="o">=</span> <span class="p">[</span><span class="n">t</span><span class="o">.</span><span class="n">lower</span><span class="p">()</span><span class="o">.</span><span class="n">strip</span><span class="p">()</span> <span class="k">for</span> <span class="n">t</span> <span class="ow">in</span> <span class="bp">self</span><span class="o">.</span><span class="n">tags</span><span class="p">]</span> <span class="c1"># Usage:</span> <span class="n">trace</span> <span class="o">=</span> <span class="n">TraceRecord</span><span class="p">(</span> <span class="n">title</span><span class="o">=</span><span class="s1">'FastAPI setup'</span><span class="p">,</span> <span class="n">context_text</span><span class="o">=</span><span class="s1">'Setting up FastAPI...'</span><span class="p">,</span> <span class="n">solution_text</span><span class="o">=</span><span class="s1">'Here is how...'</span><span class="p">,</span> <span class="n">tags</span><span class="o">=</span><span class="p">[</span><span class="s1">'python'</span><span class="p">,</span> <span class="s1">'FastAPI'</span><span class="p">],</span> <span class="c1"># Normalized to ['python', 'fastapi']</span> <span class="p">)</span> <span class="c1"># Immutable dataclass:</span> <span class="nd">@dataclass</span><span class="p">(</span><span class="n">frozen</span><span class="o">=</span><span class="kc">True</span><span class="p">)</span> <span class="k">class</span><span class="w"> </span><span class="nc">TagKey</span><span class="p">:</span> <span class="n">name</span><span class="p">:</span> <span class="nb">str</span> <span class="n">domain</span><span class="p">:</span> <span class="nb">str</span> <span class="k">def</span><span class="w"> </span><span class="fm">__hash__</span><span class="p">(</span><span class="bp">self</span><span class="p">):</span> <span class="k">return</span> <span class="nb">hash</span><span class="p">((</span><span class="bp">self</span><span class="o">.</span><span class="n">name</span><span class="p">,</span> <span class="bp">self</span><span class="o">.</span><span class="n">domain</span><span class="p">))</span> </code></pre></div> <p>Key points: - field(default_factory=list) not tags: list = [] -- mutable defaults shared across instances - field(init=False) excludes from <strong>init</strong> -- computed internally - <strong>post_init</strong> runs after <strong>init</strong> -- use for validation and normalization - frozen=True makes instances hashable and immutable (usable as dict keys) - @dataclass(eq=True) (default) generates <strong>eq</strong> based on all fields</p>