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| <h1 class="post-title">Error Handling Guide</h1> |
| <h3>Utilize structured error types in MXNet for modern cross-language error handling.</h3><a style="float:left; margin-top:20px" href="/versions/master/community/index" class="btn btn-action">Contribute |
| <span class="span-accented">›</span></a></header> |
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| <!--- Licensed to the Apache Software Foundation (ASF) under one --> |
| <!--- or more contributor license agreements. See the NOTICE file --> |
| <!--- distributed with this work for additional information --> |
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| <!--- to you under the Apache License, Version 2.0 (the --> |
| <!--- "License"); you may not use this file except in compliance --> |
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| <!--- under the License. --> |
| |
| <h1 id="error-handling-guide">Error Handling Guide</h1> |
| |
| <p>MXNet contains structured error classes to indicate specific types of |
| error. Please raise a specific error type when possible, so that users |
| can write code to handle a specific error category if necessary. You can |
| directly raise the specific error object in python. In other languages |
| like c++, you simply add <code class="highlighter-rouge"><ErrorType>:</code> prefix to the error message(see |
| below).</p> |
| |
| <div class="alert alert-info" role="alert"><i class="fa fa-info-circle"></i> <b>Note:</b> Please refer to <a href="https://github.com/apache/incubator-mxnet/blob/master/python/mxnet/error.py">/python/mxnet/error.py</a> for the list of errors.</div> |
| |
| <h2 id="raise-a-specific-error-in-c">Raise a Specific Error in C++</h2> |
| |
| <p>You can add <code class="highlighter-rouge"><ErrorType>:</code> prefix to your error message to raise an |
| error of the corresponding type. Note that you do not have to add a new |
| type <code class="highlighter-rouge">mxnet.base.MXNetError</code> will be |
| raised by default when there is no error type prefix in the message. |
| This mechanism works for both <code class="highlighter-rouge">LOG(FATAL)</code> and <code class="highlighter-rouge">CHECK</code> macros. The |
| following code gives an example on how to do so.</p> |
| |
| <div class="language-cpp highlighter-rouge"><div class="highlight"><pre class="highlight"><code><span class="c1">// Python frontend receives the following error type:</span> |
| <span class="c1">// ValueError: Check failed: x == y (0 vs. 1) : expect x and y to be equal.</span> |
| <span class="n">CHECK_EQ</span><span class="p">(</span><span class="mi">0</span><span class="p">,</span> <span class="mi">1</span><span class="p">)</span> <span class="o"><<</span> <span class="s">"ValueError: expect x and y to be equal."</span> |
| |
| |
| <span class="c1">// Python frontend receives the following error type:</span> |
| <span class="c1">// InternalError: cannot reach here</span> |
| <span class="n">LOG</span><span class="p">(</span><span class="n">FATAL</span><span class="p">)</span> <span class="o"><<</span> <span class="s">"InternalError: cannot reach here"</span><span class="p">;</span> |
| </code></pre></div></div> |
| |
| <p>As you can see in the above example, MXNet’s ffi system combines both the |
| python and C++’s stacktrace into a single message, and generate the |
| corresponding error class automatically.</p> |
| |
| <h2 id="how-to-choose-an-error-type">How to choose an Error Type</h2> |
| |
| <p>You can go through the error types are listed below, try to use common |
| sense and also refer to the choices in the existing code. We try to keep |
| a reasonable amount of error types. If you feel there is a need to add a |
| new error type, do the following steps:</p> |
| |
| <ul> |
| <li>Send a RFC proposal with a description and usage examples in the |
| current codebase.</li> |
| <li>Add the new error type to <code class="highlighter-rouge">mxnet.error</code> with clear documents.</li> |
| <li>Update the list in this file to include the new error type.</li> |
| <li>Change the code to use the new error type.</li> |
| </ul> |
| |
| <p>We also recommend to use less abstraction when creating the short error |
| messages. The code is more readable in this way, and also opens path to |
| craft specific error messages when necessary.</p> |
| |
| <div class="language-python highlighter-rouge"><div class="highlight"><pre class="highlight"><code><span class="k">def</span> <span class="nf">preferred</span><span class="p">():</span> |
| <span class="c1"># Very clear about what is being raised and what is the error message. |
| </span> <span class="k">raise</span> <span class="n">OpNotImplemented</span><span class="p">(</span><span class="s">"Operator relu is not implemented in the MXNet frontend"</span><span class="p">)</span> |
| |
| <span class="k">def</span> <span class="nf">_op_not_implemented</span><span class="p">(</span><span class="n">op_name</span><span class="p">):</span> |
| <span class="k">return</span> <span class="n">OpNotImplemented</span><span class="p">(</span><span class="s">"Operator {} is not implemented."</span><span class="p">)</span><span class="o">.</span><span class="nb">format</span><span class="p">(</span><span class="n">op_name</span><span class="p">)</span> |
| |
| <span class="k">def</span> <span class="nf">not_preferred</span><span class="p">():</span> |
| <span class="c1"># Introduces another level of indirection. |
| </span> <span class="k">raise</span> <span class="n">_op_not_implemented</span><span class="p">(</span><span class="s">"relu"</span><span class="p">)</span> |
| </code></pre></div></div> |
| |
| <p>If we need to introduce a wrapper function that constructs multi-line |
| error messages, please put wrapper in the same file so other developers |
| can look up the implementation easily.</p> |
| |
| <h2 id="signal-handling">Signal Handling</h2> |
| |
| <p>When not careful, some errors can occur in the form of a <a href="https://en.wikipedia.org/wiki/Signal_(IPC)">signal</a>, |
| which is handled by the OS kernel. In MXNet, you can choose to handle certain signals in the form of |
| a catchable exception. This can be combined with the error type selection above so that it can be |
| caught in the Python frontend. Currently, the following signals are handled this way:</p> |
| |
| <ul> |
| <li><code class="highlighter-rouge">SIGFPE</code>: throws <code class="highlighter-rouge">FloatingPointError</code></li> |
| <li><code class="highlighter-rouge">SIGBUS</code>: throws <code class="highlighter-rouge">IOError</code></li> |
| </ul> |
| |
| <p>To extend this to other signals, you can modify the signal handler registration in |
| <a href="https://github.com/apache/incubator-mxnet/blob/72eff9b66ecc683c3e7f9ad2c0ba69efa8dd423b/src/initialize.cc#L347-L376">/src/initialize.cc</a>.</p> |
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