Python Pydantic v2 field validators and model validators

<p>I am migrating from Pydantic v1 to v2 and need custom validation: field-level validators that transform input (normalize strings, coerce types), cross-field validators comparing multiple fields, and custom error messages.</p>

<p>Pydantic v2 validators:</p> <div class="highlight"><pre><span></span><code><span class="kn">from</span><span class="w"> </span><span class="nn">pydantic</span><span class="w"> </span><span class="kn">import</span> <span class="n">BaseModel</span><span class="p">,</span> <span class="n">field_validator</span><span class="p">,</span> <span class="n">model_validator</span><span class="p">,</span> <span class="n">Field</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">Self</span> <span class="k">class</span><span class="w"> </span><span class="nc">TraceCreate</span><span class="p">(</span><span class="n">BaseModel</span><span class="p">):</span> <span class="n">title</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">min_length</span><span class="o">=</span><span class="mi">1</span><span class="p">,</span> <span class="n">max_length</span><span class="o">=</span><span class="mi">500</span><span class="p">)</span> <span class="n">context_text</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">min_length</span><span class="o">=</span><span class="mi">10</span><span class="p">)</span> <span class="n">solution_text</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">min_length</span><span class="o">=</span><span class="mi">10</span><span class="p">)</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">max_length</span><span class="o">=</span><span class="mi">10</span><span class="p">)</span> <span class="nd">@field_validator</span><span class="p">(</span><span class="s1">'title'</span><span class="p">,</span> <span class="s1">'context_text'</span><span class="p">,</span> <span class="s1">'solution_text'</span><span class="p">,</span> <span class="n">mode</span><span class="o">=</span><span class="s1">'before'</span><span class="p">)</span> <span class="nd">@classmethod</span> <span class="k">def</span><span class="w"> </span><span class="nf">strip_whitespace</span><span class="p">(</span><span class="bp">cls</span><span class="p">,</span> <span class="n">v</span><span class="p">:</span> <span class="nb">str</span><span class="p">)</span> <span class="o">-&gt;</span> <span class="nb">str</span><span class="p">:</span> <span class="k">return</span> <span class="n">v</span><span class="o">.</span><span class="n">strip</span><span class="p">()</span> <span class="k">if</span> <span class="nb">isinstance</span><span class="p">(</span><span class="n">v</span><span class="p">,</span> <span class="nb">str</span><span class="p">)</span> <span class="k">else</span> <span class="n">v</span> <span class="nd">@field_validator</span><span class="p">(</span><span class="s1">'tags'</span><span class="p">,</span> <span class="n">mode</span><span class="o">=</span><span class="s1">'before'</span><span class="p">)</span> <span class="nd">@classmethod</span> <span class="k">def</span><span class="w"> </span><span class="nf">normalize_tags</span><span class="p">(</span><span class="bp">cls</span><span class="p">,</span> <span class="n">v</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">-&gt;</span> <span class="nb">list</span><span class="p">[</span><span class="nb">str</span><span class="p">]:</span> <span class="n">seen</span><span class="p">,</span> <span class="n">result</span> <span class="o">=</span> <span class="nb">set</span><span class="p">(),</span> <span class="p">[]</span> <span class="k">for</span> <span class="n">tag</span> <span class="ow">in</span> <span class="n">v</span><span class="p">:</span> <span class="n">normalized</span> <span class="o">=</span> <span class="n">tag</span><span class="o">.</span><span class="n">strip</span><span class="p">()</span><span class="o">.</span><span class="n">lower</span><span class="p">()[:</span><span class="mi">50</span><span class="p">]</span> <span class="k">if</span> <span class="n">normalized</span> <span class="ow">and</span> <span class="n">normalized</span> <span class="ow">not</span> <span class="ow">in</span> <span class="n">seen</span><span class="p">:</span> <span class="n">seen</span><span class="o">.</span><span class="n">add</span><span class="p">(</span><span class="n">normalized</span><span class="p">)</span> <span class="n">result</span><span class="o">.</span><span class="n">append</span><span class="p">(</span><span class="n">normalized</span><span class="p">)</span> <span class="k">return</span> <span class="n">result</span> <span class="nd">@model_validator</span><span class="p">(</span><span class="n">mode</span><span class="o">=</span><span class="s1">'after'</span><span class="p">)</span> <span class="k">def</span><span class="w"> </span><span class="nf">check_solution_length</span><span class="p">(</span><span class="bp">self</span><span class="p">)</span> <span class="o">-&gt;</span> <span class="n">Self</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">solution_text</span><span class="p">)</span> <span class="o">&lt;</span> <span class="nb">len</span><span class="p">(</span><span class="bp">self</span><span class="o">.</span><span class="n">context_text</span><span class="p">)</span> <span class="o">*</span> <span class="mf">0.5</span><span class="p">:</span> <span class="k">raise</span> <span class="ne">ValueError</span><span class="p">(</span><span class="s1">'Solution seems too brief relative to context'</span><span class="p">)</span> <span class="k">return</span> <span class="bp">self</span> </code></pre></div> <p>Key v1 -&gt; v2 changes: - @validator -&gt; @field_validator with explicit mode='before' or 'after' - @root_validator -&gt; @model_validator(mode='after') returning Self - Validators must be @classmethod - model_dump() replaces .dict(), model_validate() replaces .parse_obj()</p>