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67 lines
6.7 KiB
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<html><body><div class="content"><div class="section" id="the-raise-statement">
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<span id="raise"></span><h2>The <a class="reference internal" href="#raise"><tt class="xref std std-keyword docutils literal"><span class="pre">raise</span></tt></a> statement</h2>
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<pre id="index-24">
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<strong id="grammar-token-raise_stmt">raise_stmt</strong> ::= "raise" [<a class="reference internal" href="expressions.html#grammar-token-expression"><tt class="xref docutils literal"><span class="pre">expression</span></tt></a> ["from" <a class="reference internal" href="expressions.html#grammar-token-expression"><tt class="xref docutils literal"><span class="pre">expression</span></tt></a>]]
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</pre>
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<p>If no expressions are present, <a class="reference internal" href="#raise"><tt class="xref std std-keyword docutils literal"><span class="pre">raise</span></tt></a> re-raises the last exception
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that was active in the current scope. If no exception is active in the current
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scope, a <a class="reference internal" href="../library/exceptions.html#RuntimeError" title="RuntimeError"><tt class="xref py py-exc docutils literal"><span class="pre">RuntimeError</span></tt></a> exception is raised indicating that this is an
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error.</p>
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<p>Otherwise, <a class="reference internal" href="#raise"><tt class="xref std std-keyword docutils literal"><span class="pre">raise</span></tt></a> evaluates the first expression as the exception
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object. It must be either a subclass or an instance of <a class="reference internal" href="../library/exceptions.html#BaseException" title="BaseException"><tt class="xref py py-class docutils literal"><span class="pre">BaseException</span></tt></a>.
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If it is a class, the exception instance will be obtained when needed by
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instantiating the class with no arguments.</p>
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<p>The <em class="dfn">type</em> of the exception is the exception instance’s class, the
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<em class="dfn">value</em> is the instance itself.</p>
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<p id="index-25">A traceback object is normally created automatically when an exception is raised
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and attached to it as the <tt class="xref py py-attr docutils literal"><span class="pre">__traceback__</span></tt> attribute, which is writable.
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You can create an exception and set your own traceback in one step using the
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<tt class="xref py py-meth docutils literal"><span class="pre">with_traceback()</span></tt> exception method (which returns the same exception
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instance, with its traceback set to its argument), like so:</p>
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<div class="highlight-python3"><div class="highlight"><pre><span class="k">raise</span> <span class="ne">Exception</span><span class="p">(</span><span class="s">"foo occurred"</span><span class="p">)</span><span class="o">.</span><span class="n">with_traceback</span><span class="p">(</span><span class="n">tracebackobj</span><span class="p">)</span>
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</pre></div>
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</div>
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<p id="index-26">The <tt class="docutils literal"><span class="pre">from</span></tt> clause is used for exception chaining: if given, the second
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<em>expression</em> must be another exception class or instance, which will then be
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attached to the raised exception as the <tt class="xref py py-attr docutils literal"><span class="pre">__cause__</span></tt> attribute (which is
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writable). If the raised exception is not handled, both exceptions will be
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printed:</p>
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<div class="highlight-python3"><div class="highlight"><pre><span class="gp">>>> </span><span class="k">try</span><span class="p">:</span>
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<span class="gp">... </span> <span class="nb">print</span><span class="p">(</span><span class="mi">1</span> <span class="o">/</span> <span class="mi">0</span><span class="p">)</span>
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<span class="gp">... </span><span class="k">except</span> <span class="ne">Exception</span> <span class="k">as</span> <span class="n">exc</span><span class="p">:</span>
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<span class="gp">... </span> <span class="k">raise</span> <span class="ne">RuntimeError</span><span class="p">(</span><span class="s">"Something bad happened"</span><span class="p">)</span> <span class="kn">from</span> <span class="nn">exc</span>
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<span class="gp">...</span>
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<span class="gt">Traceback (most recent call last):</span>
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File <span class="nb">"<stdin>"</span>, line <span class="m">2</span>, in <span class="n"><module></span>
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<span class="gr">ZeroDivisionError</span>: <span class="n">int division or modulo by zero</span>
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<span class="go">The above exception was the direct cause of the following exception:</span>
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<span class="gt">Traceback (most recent call last):</span>
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File <span class="nb">"<stdin>"</span>, line <span class="m">4</span>, in <span class="n"><module></span>
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<span class="gr">RuntimeError</span>: <span class="n">Something bad happened</span>
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</pre></div>
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</div>
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<p>A similar mechanism works implicitly if an exception is raised inside an
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exception handler or a <a class="reference internal" href="compound_stmts.html#finally"><tt class="xref std std-keyword docutils literal"><span class="pre">finally</span></tt></a> clause: the previous exception is then
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attached as the new exception’s <tt class="xref py py-attr docutils literal"><span class="pre">__context__</span></tt> attribute:</p>
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<div class="highlight-python3"><div class="highlight"><pre><span class="gp">>>> </span><span class="k">try</span><span class="p">:</span>
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<span class="gp">... </span> <span class="nb">print</span><span class="p">(</span><span class="mi">1</span> <span class="o">/</span> <span class="mi">0</span><span class="p">)</span>
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<span class="gp">... </span><span class="k">except</span><span class="p">:</span>
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<span class="gp">... </span> <span class="k">raise</span> <span class="ne">RuntimeError</span><span class="p">(</span><span class="s">"Something bad happened"</span><span class="p">)</span>
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<span class="gp">...</span>
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<span class="gt">Traceback (most recent call last):</span>
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File <span class="nb">"<stdin>"</span>, line <span class="m">2</span>, in <span class="n"><module></span>
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<span class="gr">ZeroDivisionError</span>: <span class="n">int division or modulo by zero</span>
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<span class="go">During handling of the above exception, another exception occurred:</span>
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<span class="gt">Traceback (most recent call last):</span>
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File <span class="nb">"<stdin>"</span>, line <span class="m">4</span>, in <span class="n"><module></span>
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<span class="gr">RuntimeError</span>: <span class="n">Something bad happened</span>
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</pre></div>
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</div>
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<p>Additional information on exceptions can be found in section <a class="reference internal" href="executionmodel.html#exceptions"><em>Exceptions</em></a>,
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and information about handling exceptions is in section <a class="reference internal" href="compound_stmts.html#try"><em>The try statement</em></a>.</p>
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</div>
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</div></body></html> |