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| <div class="section" id="module-apache_beam.coders.typecoders"> |
| <span id="apache-beam-coders-typecoders-module"></span><h1>apache_beam.coders.typecoders module<a class="headerlink" href="#module-apache_beam.coders.typecoders" title="Permalink to this headline">¶</a></h1> |
| <p>Type coders registration.</p> |
| <p>This module contains functionality to define and use coders for custom classes. |
| Let’s say we have a class Xyz and we are processing a PCollection with elements |
| of type Xyz. If we do not register a coder for Xyz, a default pickle-based |
| fallback coder will be used. This can be undesirable for two reasons. First, we |
| may want a faster coder or a more space efficient one. Second, the pickle-based |
| coder is not deterministic in the sense that objects like dictionaries or sets |
| are not guaranteed to be encoded in the same way every time (elements are not |
| really ordered).</p> |
| <dl class="docutils"> |
| <dt>Two (sometimes three) steps are needed to define and use a custom coder:</dt> |
| <dd><ul class="first last simple"> |
| <li>define the coder class</li> |
| <li>associate the code with the class (a.k.a. coder registration)</li> |
| <li>typehint DoFns or transforms with the new class or composite types using |
| the class.</li> |
| </ul> |
| </dd> |
| </dl> |
| <p>A coder class is defined by subclassing from CoderBase and defining the |
| encode_to_bytes and decode_from_bytes methods. The framework uses duck-typing |
| for coders so it is not strictly required to subclass from CoderBase as long as |
| the encode/decode methods are defined.</p> |
| <p>Registering a coder class is made with a register_coder() call:</p> |
| <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="kn">from</span> <span class="nn">apache_beam</span> <span class="kn">import</span> <span class="n">coders</span> |
| <span class="o">...</span> |
| <span class="n">coders</span><span class="o">.</span><span class="n">registry</span><span class="o">.</span><span class="n">register_coder</span><span class="p">(</span><span class="n">Xyz</span><span class="p">,</span> <span class="n">XyzCoder</span><span class="p">)</span> |
| </pre></div> |
| </div> |
| <p>Additionally, DoFns and PTransforms may need type hints. This is not always |
| necessary since there is functionality to infer the return types of DoFns by |
| analyzing the code. For instance, for the function below the return type of |
| ‘Xyz’ will be inferred:</p> |
| <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="k">def</span> <span class="nf">MakeXyzs</span><span class="p">(</span><span class="n">v</span><span class="p">):</span> |
| <span class="k">return</span> <span class="n">Xyz</span><span class="p">(</span><span class="n">v</span><span class="p">)</span> |
| </pre></div> |
| </div> |
| <p>If Xyz is inferred then its coder will be used whenever the framework needs to |
| serialize data (e.g., writing to the shuffler subsystem responsible for group by |
| key operations). If a typehint is needed it can be specified by decorating the |
| DoFns or using with_input_types/with_output_types methods on PTransforms. For |
| example, the above function can be decorated:</p> |
| <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="nd">@with_output_types</span><span class="p">(</span><span class="n">Xyz</span><span class="p">)</span> |
| <span class="k">def</span> <span class="nf">MakeXyzs</span><span class="p">(</span><span class="n">v</span><span class="p">):</span> |
| <span class="k">return</span> <span class="n">complex_operation_returning_Xyz</span><span class="p">(</span><span class="n">v</span><span class="p">)</span> |
| </pre></div> |
| </div> |
| <p>See apache_beam.typehints.decorators module for more details.</p> |
| </div> |
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