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Built-in Types" accesskey="P">previous</a> |</li> <li><img src="../_static/py.png" alt="" style="vertical-align: middle; margin-top: -1px"/></li> <li><a href="http://www.python.org/">Python</a> &raquo;</li> <li> <a href="../index.html">Python 2.7.5 documentation</a> &raquo; </li> <li><a href="index.html" accesskey="U">The Python Standard Library</a> &raquo;</li> </ul> </div> <div class="document"> <div class="documentwrapper"> <div class="bodywrapper"> <div class="body"> <div class="section" id="module-exceptions"> <span id="built-in-exceptions"></span><span id="bltin-exceptions"></span><h1>6. Built-in Exceptions<a class="headerlink" href="#module-exceptions" title="Permalink to this headline">¶</a></h1> <p>Exceptions should be class objects. The exceptions are defined in the module <a class="reference internal" href="#module-exceptions" title="exceptions: Standard exception classes."><tt class="xref py py-mod docutils literal"><span class="pre">exceptions</span></tt></a>. This module never needs to be imported explicitly: the exceptions are provided in the built-in namespace as well as the <a class="reference internal" href="#module-exceptions" title="exceptions: Standard exception classes."><tt class="xref py py-mod docutils literal"><span class="pre">exceptions</span></tt></a> module.</p> <p id="index-0">For class exceptions, in a <a class="reference internal" href="../reference/compound_stmts.html#try"><tt class="xref std std-keyword docutils literal"><span class="pre">try</span></tt></a> statement with an <a class="reference internal" href="../reference/compound_stmts.html#except"><tt class="xref std std-keyword docutils literal"><span class="pre">except</span></tt></a> clause that mentions a particular class, that clause also handles any exception classes derived from that class (but not exception classes from which <em>it</em> is derived). Two exception classes that are not related via subclassing are never equivalent, even if they have the same name.</p> <p id="index-1">The built-in exceptions listed below can be generated by the interpreter or built-in functions. Except where mentioned, they have an &#8220;associated value&#8221; indicating the detailed cause of the error. This may be a string or a tuple containing several items of information (e.g., an error code and a string explaining the code). The associated value is the second argument to the <a class="reference internal" href="../reference/simple_stmts.html#raise"><tt class="xref std std-keyword docutils literal"><span class="pre">raise</span></tt></a> statement. If the exception class is derived from the standard root class <a class="reference internal" href="#exceptions.BaseException" title="exceptions.BaseException"><tt class="xref py py-exc docutils literal"><span class="pre">BaseException</span></tt></a>, the associated value is present as the exception instance&#8217;s <tt class="xref py py-attr docutils literal"><span class="pre">args</span></tt> attribute.</p> <p>User code can raise built-in exceptions. This can be used to test an exception handler or to report an error condition &#8220;just like&#8221; the situation in which the interpreter raises the same exception; but beware that there is nothing to prevent user code from raising an inappropriate error.</p> <p>The built-in exception classes can be sub-classed to define new exceptions; programmers are encouraged to at least derive new exceptions from the <a class="reference internal" href="#exceptions.Exception" title="exceptions.Exception"><tt class="xref py py-exc docutils literal"><span class="pre">Exception</span></tt></a> class and not <a class="reference internal" href="#exceptions.BaseException" title="exceptions.BaseException"><tt class="xref py py-exc docutils literal"><span class="pre">BaseException</span></tt></a>. More information on defining exceptions is available in the Python Tutorial under <a class="reference internal" href="../tutorial/errors.html#tut-userexceptions"><em>User-defined Exceptions</em></a>.</p> <p>The following exceptions are only used as base classes for other exceptions.</p> <dl class="exception"> <dt id="exceptions.BaseException"> <em class="property">exception </em><tt class="descname">BaseException</tt><a class="headerlink" href="#exceptions.BaseException" title="Permalink to this definition">¶</a></dt> <dd><p>The base class for all built-in exceptions. It is not meant to be directly inherited by user-defined classes (for that, use <a class="reference internal" href="#exceptions.Exception" title="exceptions.Exception"><tt class="xref py py-exc docutils literal"><span class="pre">Exception</span></tt></a>). If <a class="reference internal" href="functions.html#str" title="str"><tt class="xref py py-func docutils literal"><span class="pre">str()</span></tt></a> or <a class="reference internal" href="functions.html#unicode" title="unicode"><tt class="xref py py-func docutils literal"><span class="pre">unicode()</span></tt></a> is called on an instance of this class, the representation of the argument(s) to the instance are returned, or the empty string when there were no arguments.</p> <p class="versionadded"> <span class="versionmodified">New in version 2.5.</span></p> <dl class="attribute"> <dt id="exceptions.BaseException.args"> <tt class="descname">args</tt><a class="headerlink" href="#exceptions.BaseException.args" title="Permalink to this definition">¶</a></dt> <dd><p>The tuple of arguments given to the exception constructor. Some built-in exceptions (like <a class="reference internal" href="#exceptions.IOError" title="exceptions.IOError"><tt class="xref py py-exc docutils literal"><span class="pre">IOError</span></tt></a>) expect a certain number of arguments and assign a special meaning to the elements of this tuple, while others are usually called only with a single string giving an error message.</p> </dd></dl> </dd></dl> <dl class="exception"> <dt id="exceptions.Exception"> <em class="property">exception </em><tt class="descname">Exception</tt><a class="headerlink" href="#exceptions.Exception" title="Permalink to this definition">¶</a></dt> <dd><p>All built-in, non-system-exiting exceptions are derived from this class. All user-defined exceptions should also be derived from this class.</p> <p class="versionchanged"> <span class="versionmodified">Changed in version 2.5: </span>Changed to inherit from <a class="reference internal" href="#exceptions.BaseException" title="exceptions.BaseException"><tt class="xref py py-exc docutils literal"><span class="pre">BaseException</span></tt></a>.</p> </dd></dl> <dl class="exception"> <dt id="exceptions.StandardError"> <em class="property">exception </em><tt class="descname">StandardError</tt><a class="headerlink" href="#exceptions.StandardError" title="Permalink to this definition">¶</a></dt> <dd><p>The base class for all built-in exceptions except <a class="reference internal" href="#exceptions.StopIteration" title="exceptions.StopIteration"><tt class="xref py py-exc docutils literal"><span class="pre">StopIteration</span></tt></a>, <a class="reference internal" href="#exceptions.GeneratorExit" title="exceptions.GeneratorExit"><tt class="xref py py-exc docutils literal"><span class="pre">GeneratorExit</span></tt></a>, <a class="reference internal" href="#exceptions.KeyboardInterrupt" title="exceptions.KeyboardInterrupt"><tt class="xref py py-exc docutils literal"><span class="pre">KeyboardInterrupt</span></tt></a> and <a class="reference internal" href="#exceptions.SystemExit" title="exceptions.SystemExit"><tt class="xref py py-exc docutils literal"><span class="pre">SystemExit</span></tt></a>. <a class="reference internal" href="#exceptions.StandardError" title="exceptions.StandardError"><tt class="xref py py-exc docutils literal"><span class="pre">StandardError</span></tt></a> itself is derived from <a class="reference internal" href="#exceptions.Exception" title="exceptions.Exception"><tt class="xref py py-exc docutils literal"><span class="pre">Exception</span></tt></a>.</p> </dd></dl> <dl class="exception"> <dt id="exceptions.ArithmeticError"> <em class="property">exception </em><tt class="descname">ArithmeticError</tt><a class="headerlink" href="#exceptions.ArithmeticError" title="Permalink to this definition">¶</a></dt> <dd><p>The base class for those built-in exceptions that are raised for various arithmetic errors: <a class="reference internal" href="#exceptions.OverflowError" title="exceptions.OverflowError"><tt class="xref py py-exc docutils literal"><span class="pre">OverflowError</span></tt></a>, <a class="reference internal" href="#exceptions.ZeroDivisionError" title="exceptions.ZeroDivisionError"><tt class="xref py py-exc docutils literal"><span class="pre">ZeroDivisionError</span></tt></a>, <a class="reference internal" href="#exceptions.FloatingPointError" title="exceptions.FloatingPointError"><tt class="xref py py-exc docutils literal"><span class="pre">FloatingPointError</span></tt></a>.</p> </dd></dl> <dl class="exception"> <dt id="exceptions.BufferError"> <em class="property">exception </em><tt class="descname">BufferError</tt><a class="headerlink" href="#exceptions.BufferError" title="Permalink to this definition">¶</a></dt> <dd><p>Raised when a <a class="reference internal" href="../c-api/buffer.html#bufferobjects"><em>buffer</em></a> related operation cannot be performed.</p> </dd></dl> <dl class="exception"> <dt id="exceptions.LookupError"> <em class="property">exception </em><tt class="descname">LookupError</tt><a class="headerlink" href="#exceptions.LookupError" title="Permalink to this definition">¶</a></dt> <dd><p>The base class for the exceptions that are raised when a key or index used on a mapping or sequence is invalid: <a class="reference internal" href="#exceptions.IndexError" title="exceptions.IndexError"><tt class="xref py py-exc docutils literal"><span class="pre">IndexError</span></tt></a>, <a class="reference internal" href="#exceptions.KeyError" title="exceptions.KeyError"><tt class="xref py py-exc docutils literal"><span class="pre">KeyError</span></tt></a>. This can be raised directly by <a class="reference internal" href="codecs.html#codecs.lookup" title="codecs.lookup"><tt class="xref py py-func docutils literal"><span class="pre">codecs.lookup()</span></tt></a>.</p> </dd></dl> <dl class="exception"> <dt id="exceptions.EnvironmentError"> <em class="property">exception </em><tt class="descname">EnvironmentError</tt><a class="headerlink" href="#exceptions.EnvironmentError" title="Permalink to this definition">¶</a></dt> <dd><p>The base class for exceptions that can occur outside the Python system: <a class="reference internal" href="#exceptions.IOError" title="exceptions.IOError"><tt class="xref py py-exc docutils literal"><span class="pre">IOError</span></tt></a>, <a class="reference internal" href="#exceptions.OSError" title="exceptions.OSError"><tt class="xref py py-exc docutils literal"><span class="pre">OSError</span></tt></a>. When exceptions of this type are created with a 2-tuple, the first item is available on the instance&#8217;s <a class="reference internal" href="errno.html#module-errno" title="errno: Standard errno system symbols."><tt class="xref py py-attr docutils literal"><span class="pre">errno</span></tt></a> attribute (it is assumed to be an error number), and the second item is available on the <tt class="xref py py-attr docutils literal"><span class="pre">strerror</span></tt> attribute (it is usually the associated error message). The tuple itself is also available on the <tt class="xref py py-attr docutils literal"><span class="pre">args</span></tt> attribute.</p> <p class="versionadded"> <span class="versionmodified">New in version 1.5.2.</span></p> <p>When an <a class="reference internal" href="#exceptions.EnvironmentError" title="exceptions.EnvironmentError"><tt class="xref py py-exc docutils literal"><span class="pre">EnvironmentError</span></tt></a> exception is instantiated with a 3-tuple, the first two items are available as above, while the third item is available on the <tt class="xref py py-attr docutils literal"><span class="pre">filename</span></tt> attribute. However, for backwards compatibility, the <tt class="xref py py-attr docutils literal"><span class="pre">args</span></tt> attribute contains only a 2-tuple of the first two constructor arguments.</p> <p>The <tt class="xref py py-attr docutils literal"><span class="pre">filename</span></tt> attribute is <tt class="docutils literal"><span class="pre">None</span></tt> when this exception is created with other than 3 arguments. The <a class="reference internal" href="errno.html#module-errno" title="errno: Standard errno system symbols."><tt class="xref py py-attr docutils literal"><span class="pre">errno</span></tt></a> and <tt class="xref py py-attr docutils literal"><span class="pre">strerror</span></tt> attributes are also <tt class="docutils literal"><span class="pre">None</span></tt> when the instance was created with other than 2 or 3 arguments. In this last case, <tt class="xref py py-attr docutils literal"><span class="pre">args</span></tt> contains the verbatim constructor arguments as a tuple.</p> </dd></dl> <p>The following exceptions are the exceptions that are actually raised.</p> <dl class="exception"> <dt id="exceptions.AssertionError"> <em class="property">exception </em><tt class="descname">AssertionError</tt><a class="headerlink" href="#exceptions.AssertionError" title="Permalink to this definition">¶</a></dt> <dd><p id="index-2">Raised when an <a class="reference internal" href="../reference/simple_stmts.html#assert"><tt class="xref std std-keyword docutils literal"><span class="pre">assert</span></tt></a> statement fails.</p> </dd></dl> <dl class="exception"> <dt id="exceptions.AttributeError"> <em class="property">exception </em><tt class="descname">AttributeError</tt><a class="headerlink" href="#exceptions.AttributeError" title="Permalink to this definition">¶</a></dt> <dd><p>Raised when an attribute reference (see <a class="reference internal" href="../reference/expressions.html#attribute-references"><em>Attribute references</em></a>) or assignment fails. (When an object does not support attribute references or attribute assignments at all, <a class="reference internal" href="#exceptions.TypeError" title="exceptions.TypeError"><tt class="xref py py-exc docutils literal"><span class="pre">TypeError</span></tt></a> is raised.)</p> </dd></dl> <dl class="exception"> <dt id="exceptions.EOFError"> <em class="property">exception </em><tt class="descname">EOFError</tt><a class="headerlink" href="#exceptions.EOFError" title="Permalink to this definition">¶</a></dt> <dd><p>Raised when one of the built-in functions (<a class="reference internal" href="functions.html#input" title="input"><tt class="xref py py-func docutils literal"><span class="pre">input()</span></tt></a> or <a class="reference internal" href="functions.html#raw_input" title="raw_input"><tt class="xref py py-func docutils literal"><span class="pre">raw_input()</span></tt></a>) hits an end-of-file condition (EOF) without reading any data. (N.B.: the <a class="reference internal" href="stdtypes.html#file.read" title="file.read"><tt class="xref py py-meth docutils literal"><span class="pre">file.read()</span></tt></a> and <a class="reference internal" href="stdtypes.html#file.readline" title="file.readline"><tt class="xref py py-meth docutils literal"><span class="pre">file.readline()</span></tt></a> methods return an empty string when they hit EOF.)</p> </dd></dl> <dl class="exception"> <dt id="exceptions.FloatingPointError"> <em class="property">exception </em><tt class="descname">FloatingPointError</tt><a class="headerlink" href="#exceptions.FloatingPointError" title="Permalink to this definition">¶</a></dt> <dd><p>Raised when a floating point operation fails. This exception is always defined, but can only be raised when Python is configured with the <tt class="docutils literal"><span class="pre">--with-fpectl</span></tt> option, or the <tt class="xref py py-const docutils literal"><span class="pre">WANT_SIGFPE_HANDLER</span></tt> symbol is defined in the <tt class="file docutils literal"><span class="pre">pyconfig.h</span></tt> file.</p> </dd></dl> <dl class="exception"> <dt id="exceptions.GeneratorExit"> <em class="property">exception </em><tt class="descname">GeneratorExit</tt><a class="headerlink" href="#exceptions.GeneratorExit" title="Permalink to this definition">¶</a></dt> <dd><p>Raise when a <a class="reference internal" href="../glossary.html#term-generator"><em class="xref std std-term">generator</em></a>&#8216;s <tt class="xref py py-meth docutils literal"><span class="pre">close()</span></tt> method is called. It directly inherits from <a class="reference internal" href="#exceptions.BaseException" title="exceptions.BaseException"><tt class="xref py py-exc docutils literal"><span class="pre">BaseException</span></tt></a> instead of <a class="reference internal" href="#exceptions.StandardError" title="exceptions.StandardError"><tt class="xref py py-exc docutils literal"><span class="pre">StandardError</span></tt></a> since it is technically not an error.</p> <p class="versionadded"> <span class="versionmodified">New in version 2.5.</span></p> <p class="versionchanged"> <span class="versionmodified">Changed in version 2.6: </span>Changed to inherit from <a class="reference internal" href="#exceptions.BaseException" title="exceptions.BaseException"><tt class="xref py py-exc docutils literal"><span class="pre">BaseException</span></tt></a>.</p> </dd></dl> <dl class="exception"> <dt id="exceptions.IOError"> <em class="property">exception </em><tt class="descname">IOError</tt><a class="headerlink" href="#exceptions.IOError" title="Permalink to this definition">¶</a></dt> <dd><p>Raised when an I/O operation (such as a <a class="reference internal" href="../reference/simple_stmts.html#print"><tt class="xref std std-keyword docutils literal"><span class="pre">print</span></tt></a> statement, the built-in <a class="reference internal" href="functions.html#open" title="open"><tt class="xref py py-func docutils literal"><span class="pre">open()</span></tt></a> function or a method of a file object) fails for an I/O-related reason, e.g., &#8220;file not found&#8221; or &#8220;disk full&#8221;.</p> <p>This class is derived from <a class="reference internal" href="#exceptions.EnvironmentError" title="exceptions.EnvironmentError"><tt class="xref py py-exc docutils literal"><span class="pre">EnvironmentError</span></tt></a>. See the discussion above for more information on exception instance attributes.</p> <p class="versionchanged"> <span class="versionmodified">Changed in version 2.6: </span>Changed <a class="reference internal" href="socket.html#socket.error" title="socket.error"><tt class="xref py py-exc docutils literal"><span class="pre">socket.error</span></tt></a> to use this as a base class.</p> </dd></dl> <dl class="exception"> <dt id="exceptions.ImportError"> <em class="property">exception </em><tt class="descname">ImportError</tt><a class="headerlink" href="#exceptions.ImportError" title="Permalink to this definition">¶</a></dt> <dd><p>Raised when an <a class="reference internal" href="../reference/simple_stmts.html#import"><tt class="xref std std-keyword docutils literal"><span class="pre">import</span></tt></a> statement fails to find the module definition or when a <tt class="docutils literal"><span class="pre">from</span> <span class="pre">...</span> <span class="pre">import</span></tt> fails to find a name that is to be imported.</p> </dd></dl> <dl class="exception"> <dt id="exceptions.IndexError"> <em class="property">exception </em><tt class="descname">IndexError</tt><a class="headerlink" href="#exceptions.IndexError" title="Permalink to this definition">¶</a></dt> <dd><p>Raised when a sequence subscript is out of range. (Slice indices are silently truncated to fall in the allowed range; if an index is not a plain integer, <a class="reference internal" href="#exceptions.TypeError" title="exceptions.TypeError"><tt class="xref py py-exc docutils literal"><span class="pre">TypeError</span></tt></a> is raised.)</p> </dd></dl> <dl class="exception"> <dt id="exceptions.KeyError"> <em class="property">exception </em><tt class="descname">KeyError</tt><a class="headerlink" href="#exceptions.KeyError" title="Permalink to this definition">¶</a></dt> <dd><p>Raised when a mapping (dictionary) key is not found in the set of existing keys.</p> </dd></dl> <dl class="exception"> <dt id="exceptions.KeyboardInterrupt"> <em class="property">exception </em><tt class="descname">KeyboardInterrupt</tt><a class="headerlink" href="#exceptions.KeyboardInterrupt" title="Permalink to this definition">¶</a></dt> <dd><p>Raised when the user hits the interrupt key (normally <tt class="kbd docutils literal"><span class="pre">Control-C</span></tt> or <tt class="kbd docutils literal"><span class="pre">Delete</span></tt>). During execution, a check for interrupts is made regularly. Interrupts typed when a built-in function <a class="reference internal" href="functions.html#input" title="input"><tt class="xref py py-func docutils literal"><span class="pre">input()</span></tt></a> or <a class="reference internal" href="functions.html#raw_input" title="raw_input"><tt class="xref py py-func docutils literal"><span class="pre">raw_input()</span></tt></a> is waiting for input also raise this exception. The exception inherits from <a class="reference internal" href="#exceptions.BaseException" title="exceptions.BaseException"><tt class="xref py py-exc docutils literal"><span class="pre">BaseException</span></tt></a> so as to not be accidentally caught by code that catches <a class="reference internal" href="#exceptions.Exception" title="exceptions.Exception"><tt class="xref py py-exc docutils literal"><span class="pre">Exception</span></tt></a> and thus prevent the interpreter from exiting.</p> <p class="versionchanged"> <span class="versionmodified">Changed in version 2.5: </span>Changed to inherit from <a class="reference internal" href="#exceptions.BaseException" title="exceptions.BaseException"><tt class="xref py py-exc docutils literal"><span class="pre">BaseException</span></tt></a>.</p> </dd></dl> <dl class="exception"> <dt id="exceptions.MemoryError"> <em class="property">exception </em><tt class="descname">MemoryError</tt><a class="headerlink" href="#exceptions.MemoryError" title="Permalink to this definition">¶</a></dt> <dd><p>Raised when an operation runs out of memory but the situation may still be rescued (by deleting some objects). The associated value is a string indicating what kind of (internal) operation ran out of memory. Note that because of the underlying memory management architecture (C&#8217;s <tt class="xref c c-func docutils literal"><span class="pre">malloc()</span></tt> function), the interpreter may not always be able to completely recover from this situation; it nevertheless raises an exception so that a stack traceback can be printed, in case a run-away program was the cause.</p> </dd></dl> <dl class="exception"> <dt id="exceptions.NameError"> <em class="property">exception </em><tt class="descname">NameError</tt><a class="headerlink" href="#exceptions.NameError" title="Permalink to this definition">¶</a></dt> <dd><p>Raised when a local or global name is not found. This applies only to unqualified names. The associated value is an error message that includes the name that could not be found.</p> </dd></dl> <dl class="exception"> <dt id="exceptions.NotImplementedError"> <em class="property">exception </em><tt class="descname">NotImplementedError</tt><a class="headerlink" href="#exceptions.NotImplementedError" title="Permalink to this definition">¶</a></dt> <dd><p>This exception is derived from <a class="reference internal" href="#exceptions.RuntimeError" title="exceptions.RuntimeError"><tt class="xref py py-exc docutils literal"><span class="pre">RuntimeError</span></tt></a>. In user defined base classes, abstract methods should raise this exception when they require derived classes to override the method.</p> <p class="versionadded"> <span class="versionmodified">New in version 1.5.2.</span></p> </dd></dl> <dl class="exception"> <dt id="exceptions.OSError"> <em class="property">exception </em><tt class="descname">OSError</tt><a class="headerlink" href="#exceptions.OSError" title="Permalink to this definition">¶</a></dt> <dd><p id="index-3">This exception is derived from <a class="reference internal" href="#exceptions.EnvironmentError" title="exceptions.EnvironmentError"><tt class="xref py py-exc docutils literal"><span class="pre">EnvironmentError</span></tt></a>. It is raised when a function returns a system-related error (not for illegal argument types or other incidental errors). The <a class="reference internal" href="errno.html#module-errno" title="errno: Standard errno system symbols."><tt class="xref py py-attr docutils literal"><span class="pre">errno</span></tt></a> attribute is a numeric error code from <tt class="xref c c-data docutils literal"><span class="pre">errno</span></tt>, and the <tt class="xref py py-attr docutils literal"><span class="pre">strerror</span></tt> attribute is the corresponding string, as would be printed by the C function <tt class="xref c c-func docutils literal"><span class="pre">perror()</span></tt>. See the module <a class="reference internal" href="errno.html#module-errno" title="errno: Standard errno system symbols."><tt class="xref py py-mod docutils literal"><span class="pre">errno</span></tt></a>, which contains names for the error codes defined by the underlying operating system.</p> <p>For exceptions that involve a file system path (such as <tt class="xref py py-func docutils literal"><span class="pre">chdir()</span></tt> or <tt class="xref py py-func docutils literal"><span class="pre">unlink()</span></tt>), the exception instance will contain a third attribute, <tt class="xref py py-attr docutils literal"><span class="pre">filename</span></tt>, which is the file name passed to the function.</p> <p class="versionadded"> <span class="versionmodified">New in version 1.5.2.</span></p> </dd></dl> <dl class="exception"> <dt id="exceptions.OverflowError"> <em class="property">exception </em><tt class="descname">OverflowError</tt><a class="headerlink" href="#exceptions.OverflowError" title="Permalink to this definition">¶</a></dt> <dd><p>Raised when the result of an arithmetic operation is too large to be represented. This cannot occur for long integers (which would rather raise <a class="reference internal" href="#exceptions.MemoryError" title="exceptions.MemoryError"><tt class="xref py py-exc docutils literal"><span class="pre">MemoryError</span></tt></a> than give up) and for most operations with plain integers, which return a long integer instead. Because of the lack of standardization of floating point exception handling in C, most floating point operations also aren&#8217;t checked.</p> </dd></dl> <dl class="exception"> <dt id="exceptions.ReferenceError"> <em class="property">exception </em><tt class="descname">ReferenceError</tt><a class="headerlink" href="#exceptions.ReferenceError" title="Permalink to this definition">¶</a></dt> <dd><p>This exception is raised when a weak reference proxy, created by the <a class="reference internal" href="weakref.html#weakref.proxy" title="weakref.proxy"><tt class="xref py py-func docutils literal"><span class="pre">weakref.proxy()</span></tt></a> function, is used to access an attribute of the referent after it has been garbage collected. For more information on weak references, see the <a class="reference internal" href="weakref.html#module-weakref" title="weakref: Support for weak references and weak dictionaries."><tt class="xref py py-mod docutils literal"><span class="pre">weakref</span></tt></a> module.</p> <p class="versionadded"> <span class="versionmodified">New in version 2.2: </span>Previously known as the <a class="reference internal" href="weakref.html#weakref.ReferenceError" title="weakref.ReferenceError"><tt class="xref py py-exc docutils literal"><span class="pre">weakref.ReferenceError</span></tt></a> exception.</p> </dd></dl> <dl class="exception"> <dt id="exceptions.RuntimeError"> <em class="property">exception </em><tt class="descname">RuntimeError</tt><a class="headerlink" href="#exceptions.RuntimeError" title="Permalink to this definition">¶</a></dt> <dd><p>Raised when an error is detected that doesn&#8217;t fall in any of the other categories. The associated value is a string indicating what precisely went wrong. (This exception is mostly a relic from a previous version of the interpreter; it is not used very much any more.)</p> </dd></dl> <dl class="exception"> <dt id="exceptions.StopIteration"> <em class="property">exception </em><tt class="descname">StopIteration</tt><a class="headerlink" href="#exceptions.StopIteration" title="Permalink to this definition">¶</a></dt> <dd><p>Raised by an <a class="reference internal" href="../glossary.html#term-iterator"><em class="xref std std-term">iterator</em></a>&#8216;s <a class="reference internal" href="stdtypes.html#iterator.next" title="iterator.next"><tt class="xref py py-meth docutils literal"><span class="pre">next()</span></tt></a> method to signal that there are no further values. This is derived from <a class="reference internal" href="#exceptions.Exception" title="exceptions.Exception"><tt class="xref py py-exc docutils literal"><span class="pre">Exception</span></tt></a> rather than <a class="reference internal" href="#exceptions.StandardError" title="exceptions.StandardError"><tt class="xref py py-exc docutils literal"><span class="pre">StandardError</span></tt></a>, since this is not considered an error in its normal application.</p> <p class="versionadded"> <span class="versionmodified">New in version 2.2.</span></p> </dd></dl> <dl class="exception"> <dt id="exceptions.SyntaxError"> <em class="property">exception </em><tt class="descname">SyntaxError</tt><a class="headerlink" href="#exceptions.SyntaxError" title="Permalink to this definition">¶</a></dt> <dd><p>Raised when the parser encounters a syntax error. This may occur in an <a class="reference internal" href="../reference/simple_stmts.html#import"><tt class="xref std std-keyword docutils literal"><span class="pre">import</span></tt></a> statement, in an <a class="reference internal" href="../reference/simple_stmts.html#exec"><tt class="xref std std-keyword docutils literal"><span class="pre">exec</span></tt></a> statement, in a call to the built-in function <a class="reference internal" href="functions.html#eval" title="eval"><tt class="xref py py-func docutils literal"><span class="pre">eval()</span></tt></a> or <a class="reference internal" href="functions.html#input" title="input"><tt class="xref py py-func docutils literal"><span class="pre">input()</span></tt></a>, or when reading the initial script or standard input (also interactively).</p> <p>Instances of this class have attributes <tt class="xref py py-attr docutils literal"><span class="pre">filename</span></tt>, <tt class="xref py py-attr docutils literal"><span class="pre">lineno</span></tt>, <tt class="xref py py-attr docutils literal"><span class="pre">offset</span></tt> and <tt class="xref py py-attr docutils literal"><span class="pre">text</span></tt> for easier access to the details. <a class="reference internal" href="functions.html#str" title="str"><tt class="xref py py-func docutils literal"><span class="pre">str()</span></tt></a> of the exception instance returns only the message.</p> </dd></dl> <dl class="exception"> <dt id="exceptions.IndentationError"> <em class="property">exception </em><tt class="descname">IndentationError</tt><a class="headerlink" href="#exceptions.IndentationError" title="Permalink to this definition">¶</a></dt> <dd><p>Base class for syntax errors related to incorrect indentation. This is a subclass of <a class="reference internal" href="#exceptions.SyntaxError" title="exceptions.SyntaxError"><tt class="xref py py-exc docutils literal"><span class="pre">SyntaxError</span></tt></a>.</p> </dd></dl> <dl class="exception"> <dt id="exceptions.TabError"> <em class="property">exception </em><tt class="descname">TabError</tt><a class="headerlink" href="#exceptions.TabError" title="Permalink to this definition">¶</a></dt> <dd><p>Raised when indentation contains an inconsistent use of tabs and spaces. This is a subclass of <a class="reference internal" href="#exceptions.IndentationError" title="exceptions.IndentationError"><tt class="xref py py-exc docutils literal"><span class="pre">IndentationError</span></tt></a>.</p> </dd></dl> <dl class="exception"> <dt id="exceptions.SystemError"> <em class="property">exception </em><tt class="descname">SystemError</tt><a class="headerlink" href="#exceptions.SystemError" title="Permalink to this definition">¶</a></dt> <dd><p>Raised when the interpreter finds an internal error, but the situation does not look so serious to cause it to abandon all hope. The associated value is a string indicating what went wrong (in low-level terms).</p> <p>You should report this to the author or maintainer of your Python interpreter. Be sure to report the version of the Python interpreter (<tt class="docutils literal"><span class="pre">sys.version</span></tt>; it is also printed at the start of an interactive Python session), the exact error message (the exception&#8217;s associated value) and if possible the source of the program that triggered the error.</p> </dd></dl> <dl class="exception"> <dt id="exceptions.SystemExit"> <em class="property">exception </em><tt class="descname">SystemExit</tt><a class="headerlink" href="#exceptions.SystemExit" title="Permalink to this definition">¶</a></dt> <dd><p>This exception is raised by the <a class="reference internal" href="sys.html#sys.exit" title="sys.exit"><tt class="xref py py-func docutils literal"><span class="pre">sys.exit()</span></tt></a> function. When it is not handled, the Python interpreter exits; no stack traceback is printed. If the associated value is a plain integer, it specifies the system exit status (passed to C&#8217;s <tt class="xref c c-func docutils literal"><span class="pre">exit()</span></tt> function); if it is <tt class="docutils literal"><span class="pre">None</span></tt>, the exit status is zero; if it has another type (such as a string), the object&#8217;s value is printed and the exit status is one.</p> <p>Instances have an attribute <a class="reference internal" href="code.html#module-code" title="code: Facilities to implement read-eval-print loops."><tt class="xref py py-attr docutils literal"><span class="pre">code</span></tt></a> which is set to the proposed exit status or error message (defaulting to <tt class="docutils literal"><span class="pre">None</span></tt>). Also, this exception derives directly from <a class="reference internal" href="#exceptions.BaseException" title="exceptions.BaseException"><tt class="xref py py-exc docutils literal"><span class="pre">BaseException</span></tt></a> and not <a class="reference internal" href="#exceptions.StandardError" title="exceptions.StandardError"><tt class="xref py py-exc docutils literal"><span class="pre">StandardError</span></tt></a>, since it is not technically an error.</p> <p>A call to <a class="reference internal" href="sys.html#sys.exit" title="sys.exit"><tt class="xref py py-func docutils literal"><span class="pre">sys.exit()</span></tt></a> is translated into an exception so that clean-up handlers (<a class="reference internal" href="../reference/compound_stmts.html#finally"><tt class="xref std std-keyword docutils literal"><span class="pre">finally</span></tt></a> clauses of <a class="reference internal" href="../reference/compound_stmts.html#try"><tt class="xref std std-keyword docutils literal"><span class="pre">try</span></tt></a> statements) can be executed, and so that a debugger can execute a script without running the risk of losing control. The <a class="reference internal" href="os.html#os._exit" title="os._exit"><tt class="xref py py-func docutils literal"><span class="pre">os._exit()</span></tt></a> function can be used if it is absolutely positively necessary to exit immediately (for example, in the child process after a call to <tt class="xref py py-func docutils literal"><span class="pre">fork()</span></tt>).</p> <p>The exception inherits from <a class="reference internal" href="#exceptions.BaseException" title="exceptions.BaseException"><tt class="xref py py-exc docutils literal"><span class="pre">BaseException</span></tt></a> instead of <a class="reference internal" href="#exceptions.StandardError" title="exceptions.StandardError"><tt class="xref py py-exc docutils literal"><span class="pre">StandardError</span></tt></a> or <a class="reference internal" href="#exceptions.Exception" title="exceptions.Exception"><tt class="xref py py-exc docutils literal"><span class="pre">Exception</span></tt></a> so that it is not accidentally caught by code that catches <a class="reference internal" href="#exceptions.Exception" title="exceptions.Exception"><tt class="xref py py-exc docutils literal"><span class="pre">Exception</span></tt></a>. This allows the exception to properly propagate up and cause the interpreter to exit.</p> <p class="versionchanged"> <span class="versionmodified">Changed in version 2.5: </span>Changed to inherit from <a class="reference internal" href="#exceptions.BaseException" title="exceptions.BaseException"><tt class="xref py py-exc docutils literal"><span class="pre">BaseException</span></tt></a>.</p> </dd></dl> <dl class="exception"> <dt id="exceptions.TypeError"> <em class="property">exception </em><tt class="descname">TypeError</tt><a class="headerlink" href="#exceptions.TypeError" title="Permalink to this definition">¶</a></dt> <dd><p>Raised when an operation or function is applied to an object of inappropriate type. The associated value is a string giving details about the type mismatch.</p> </dd></dl> <dl class="exception"> <dt id="exceptions.UnboundLocalError"> <em class="property">exception </em><tt class="descname">UnboundLocalError</tt><a class="headerlink" href="#exceptions.UnboundLocalError" title="Permalink to this definition">¶</a></dt> <dd><p>Raised when a reference is made to a local variable in a function or method, but no value has been bound to that variable. This is a subclass of <a class="reference internal" href="#exceptions.NameError" title="exceptions.NameError"><tt class="xref py py-exc docutils literal"><span class="pre">NameError</span></tt></a>.</p> <p class="versionadded"> <span class="versionmodified">New in version 2.0.</span></p> </dd></dl> <dl class="exception"> <dt id="exceptions.UnicodeError"> <em class="property">exception </em><tt class="descname">UnicodeError</tt><a class="headerlink" href="#exceptions.UnicodeError" title="Permalink to this definition">¶</a></dt> <dd><p>Raised when a Unicode-related encoding or decoding error occurs. It is a subclass of <a class="reference internal" href="#exceptions.ValueError" title="exceptions.ValueError"><tt class="xref py py-exc docutils literal"><span class="pre">ValueError</span></tt></a>.</p> <p><a class="reference internal" href="#exceptions.UnicodeError" title="exceptions.UnicodeError"><tt class="xref py py-exc docutils literal"><span class="pre">UnicodeError</span></tt></a> has attributes that describe the encoding or decoding error. For example, <tt class="docutils literal"><span class="pre">err.object[err.start:err.end]</span></tt> gives the particular invalid input that the codec failed on.</p> <dl class="attribute"> <dt id="exceptions.UnicodeError.encoding"> <tt class="descname">encoding</tt><a class="headerlink" href="#exceptions.UnicodeError.encoding" title="Permalink to this definition">¶</a></dt> <dd><p>The name of the encoding that raised the error.</p> </dd></dl> <dl class="attribute"> <dt id="exceptions.UnicodeError.reason"> <tt class="descname">reason</tt><a class="headerlink" href="#exceptions.UnicodeError.reason" title="Permalink to this definition">¶</a></dt> <dd><p>A string describing the specific codec error.</p> </dd></dl> <dl class="attribute"> <dt id="exceptions.UnicodeError.object"> <tt class="descname">object</tt><a class="headerlink" href="#exceptions.UnicodeError.object" title="Permalink to this definition">¶</a></dt> <dd><p>The object the codec was attempting to encode or decode.</p> </dd></dl> <dl class="attribute"> <dt id="exceptions.UnicodeError.start"> <tt class="descname">start</tt><a class="headerlink" href="#exceptions.UnicodeError.start" title="Permalink to this definition">¶</a></dt> <dd><p>The first index of invalid data in <a class="reference internal" href="functions.html#object" title="object"><tt class="xref py py-attr docutils literal"><span class="pre">object</span></tt></a>.</p> </dd></dl> <dl class="attribute"> <dt id="exceptions.UnicodeError.end"> <tt class="descname">end</tt><a class="headerlink" href="#exceptions.UnicodeError.end" title="Permalink to this definition">¶</a></dt> <dd><p>The index after the last invalid data in <a class="reference internal" href="functions.html#object" title="object"><tt class="xref py py-attr docutils literal"><span class="pre">object</span></tt></a>.</p> </dd></dl> <p class="versionadded"> <span class="versionmodified">New in version 2.0.</span></p> </dd></dl> <dl class="exception"> <dt id="exceptions.UnicodeEncodeError"> <em class="property">exception </em><tt class="descname">UnicodeEncodeError</tt><a class="headerlink" href="#exceptions.UnicodeEncodeError" title="Permalink to this definition">¶</a></dt> <dd><p>Raised when a Unicode-related error occurs during encoding. It is a subclass of <a class="reference internal" href="#exceptions.UnicodeError" title="exceptions.UnicodeError"><tt class="xref py py-exc docutils literal"><span class="pre">UnicodeError</span></tt></a>.</p> <p class="versionadded"> <span class="versionmodified">New in version 2.3.</span></p> </dd></dl> <dl class="exception"> <dt id="exceptions.UnicodeDecodeError"> <em class="property">exception </em><tt class="descname">UnicodeDecodeError</tt><a class="headerlink" href="#exceptions.UnicodeDecodeError" title="Permalink to this definition">¶</a></dt> <dd><p>Raised when a Unicode-related error occurs during decoding. It is a subclass of <a class="reference internal" href="#exceptions.UnicodeError" title="exceptions.UnicodeError"><tt class="xref py py-exc docutils literal"><span class="pre">UnicodeError</span></tt></a>.</p> <p class="versionadded"> <span class="versionmodified">New in version 2.3.</span></p> </dd></dl> <dl class="exception"> <dt id="exceptions.UnicodeTranslateError"> <em class="property">exception </em><tt class="descname">UnicodeTranslateError</tt><a class="headerlink" href="#exceptions.UnicodeTranslateError" title="Permalink to this definition">¶</a></dt> <dd><p>Raised when a Unicode-related error occurs during translating. It is a subclass of <a class="reference internal" href="#exceptions.UnicodeError" title="exceptions.UnicodeError"><tt class="xref py py-exc docutils literal"><span class="pre">UnicodeError</span></tt></a>.</p> <p class="versionadded"> <span class="versionmodified">New in version 2.3.</span></p> </dd></dl> <dl class="exception"> <dt id="exceptions.ValueError"> <em class="property">exception </em><tt class="descname">ValueError</tt><a class="headerlink" href="#exceptions.ValueError" title="Permalink to this definition">¶</a></dt> <dd><p>Raised when a built-in operation or function receives an argument that has the right type but an inappropriate value, and the situation is not described by a more precise exception such as <a class="reference internal" href="#exceptions.IndexError" title="exceptions.IndexError"><tt class="xref py py-exc docutils literal"><span class="pre">IndexError</span></tt></a>.</p> </dd></dl> <dl class="exception"> <dt id="exceptions.VMSError"> <em class="property">exception </em><tt class="descname">VMSError</tt><a class="headerlink" href="#exceptions.VMSError" title="Permalink to this definition">¶</a></dt> <dd><p>Only available on VMS. Raised when a VMS-specific error occurs.</p> </dd></dl> <dl class="exception"> <dt id="exceptions.WindowsError"> <em class="property">exception </em><tt class="descname">WindowsError</tt><a class="headerlink" href="#exceptions.WindowsError" title="Permalink to this definition">¶</a></dt> <dd><p>Raised when a Windows-specific error occurs or when the error number does not correspond to an <tt class="xref c c-data docutils literal"><span class="pre">errno</span></tt> value. The <tt class="xref py py-attr docutils literal"><span class="pre">winerror</span></tt> and <tt class="xref py py-attr docutils literal"><span class="pre">strerror</span></tt> values are created from the return values of the <tt class="xref c c-func docutils literal"><span class="pre">GetLastError()</span></tt> and <tt class="xref c c-func docutils literal"><span class="pre">FormatMessage()</span></tt> functions from the Windows Platform API. The <a class="reference internal" href="errno.html#module-errno" title="errno: Standard errno system symbols."><tt class="xref py py-attr docutils literal"><span class="pre">errno</span></tt></a> value maps the <tt class="xref py py-attr docutils literal"><span class="pre">winerror</span></tt> value to corresponding <tt class="docutils literal"><span class="pre">errno.h</span></tt> values. This is a subclass of <a class="reference internal" href="#exceptions.OSError" title="exceptions.OSError"><tt class="xref py py-exc docutils literal"><span class="pre">OSError</span></tt></a>.</p> <p class="versionadded"> <span class="versionmodified">New in version 2.0.</span></p> <p class="versionchanged"> <span class="versionmodified">Changed in version 2.5: </span>Previous versions put the <tt class="xref c c-func docutils literal"><span class="pre">GetLastError()</span></tt> codes into <a class="reference internal" href="errno.html#module-errno" title="errno: Standard errno system symbols."><tt class="xref py py-attr docutils literal"><span class="pre">errno</span></tt></a>.</p> </dd></dl> <dl class="exception"> <dt id="exceptions.ZeroDivisionError"> <em class="property">exception </em><tt class="descname">ZeroDivisionError</tt><a class="headerlink" href="#exceptions.ZeroDivisionError" title="Permalink to this definition">¶</a></dt> <dd><p>Raised when the second argument of a division or modulo operation is zero. The associated value is a string indicating the type of the operands and the operation.</p> </dd></dl> <p>The following exceptions are used as warning categories; see the <a class="reference internal" href="warnings.html#module-warnings" title="warnings: Issue warning messages and control their disposition."><tt class="xref py py-mod docutils literal"><span class="pre">warnings</span></tt></a> module for more information.</p> <dl class="exception"> <dt id="exceptions.Warning"> <em class="property">exception </em><tt class="descname">Warning</tt><a class="headerlink" href="#exceptions.Warning" title="Permalink to this definition">¶</a></dt> <dd><p>Base class for warning categories.</p> </dd></dl> <dl class="exception"> <dt id="exceptions.UserWarning"> <em class="property">exception </em><tt class="descname">UserWarning</tt><a class="headerlink" href="#exceptions.UserWarning" title="Permalink to this definition">¶</a></dt> <dd><p>Base class for warnings generated by user code.</p> </dd></dl> <dl class="exception"> <dt id="exceptions.DeprecationWarning"> <em class="property">exception </em><tt class="descname">DeprecationWarning</tt><a class="headerlink" href="#exceptions.DeprecationWarning" title="Permalink to this definition">¶</a></dt> <dd><p>Base class for warnings about deprecated features.</p> </dd></dl> <dl class="exception"> <dt id="exceptions.PendingDeprecationWarning"> <em class="property">exception </em><tt class="descname">PendingDeprecationWarning</tt><a class="headerlink" href="#exceptions.PendingDeprecationWarning" title="Permalink to this definition">¶</a></dt> <dd><p>Base class for warnings about features which will be deprecated in the future.</p> </dd></dl> <dl class="exception"> <dt id="exceptions.SyntaxWarning"> <em class="property">exception </em><tt class="descname">SyntaxWarning</tt><a class="headerlink" href="#exceptions.SyntaxWarning" title="Permalink to this definition">¶</a></dt> <dd><p>Base class for warnings about dubious syntax</p> </dd></dl> <dl class="exception"> <dt id="exceptions.RuntimeWarning"> <em class="property">exception </em><tt class="descname">RuntimeWarning</tt><a class="headerlink" href="#exceptions.RuntimeWarning" title="Permalink to this definition">¶</a></dt> <dd><p>Base class for warnings about dubious runtime behavior.</p> </dd></dl> <dl class="exception"> <dt id="exceptions.FutureWarning"> <em class="property">exception </em><tt class="descname">FutureWarning</tt><a class="headerlink" href="#exceptions.FutureWarning" title="Permalink to this definition">¶</a></dt> <dd><p>Base class for warnings about constructs that will change semantically in the future.</p> </dd></dl> <dl class="exception"> <dt id="exceptions.ImportWarning"> <em class="property">exception </em><tt class="descname">ImportWarning</tt><a class="headerlink" href="#exceptions.ImportWarning" title="Permalink to this definition">¶</a></dt> <dd><p>Base class for warnings about probable mistakes in module imports.</p> <p class="versionadded"> <span class="versionmodified">New in version 2.5.</span></p> </dd></dl> <dl class="exception"> <dt id="exceptions.UnicodeWarning"> <em class="property">exception </em><tt class="descname">UnicodeWarning</tt><a class="headerlink" href="#exceptions.UnicodeWarning" title="Permalink to this definition">¶</a></dt> <dd><p>Base class for warnings related to Unicode.</p> <p class="versionadded"> <span class="versionmodified">New in version 2.5.</span></p> </dd></dl> <div class="section" id="exception-hierarchy"> <h2>6.1. Exception hierarchy<a class="headerlink" href="#exception-hierarchy" title="Permalink to this headline">¶</a></h2> <p>The class hierarchy for built-in exceptions is:</p> <div class="highlight-python"><pre>BaseException +-- SystemExit +-- KeyboardInterrupt +-- GeneratorExit +-- Exception +-- StopIteration +-- StandardError | +-- BufferError | +-- ArithmeticError | | +-- FloatingPointError | | +-- OverflowError | | +-- ZeroDivisionError | +-- AssertionError | +-- AttributeError | +-- EnvironmentError | | +-- IOError | | +-- OSError | | +-- WindowsError (Windows) | | +-- VMSError (VMS) | +-- EOFError | +-- ImportError | +-- LookupError | | +-- IndexError | | +-- KeyError | +-- MemoryError | +-- NameError | | +-- UnboundLocalError | +-- ReferenceError | +-- RuntimeError | | +-- NotImplementedError | +-- SyntaxError | | +-- IndentationError | | +-- TabError | +-- SystemError | +-- TypeError | +-- ValueError | +-- UnicodeError | +-- UnicodeDecodeError | +-- UnicodeEncodeError | +-- UnicodeTranslateError +-- Warning +-- DeprecationWarning +-- PendingDeprecationWarning +-- RuntimeWarning +-- SyntaxWarning +-- UserWarning +-- FutureWarning +-- ImportWarning +-- UnicodeWarning +-- BytesWarning </pre> </div> </div> </div> </div> </div> </div> <div class="sphinxsidebar"> <div class="sphinxsidebarwrapper"> <h3><a href="../contents.html">Table Of Contents</a></h3> <ul> <li><a class="reference internal" href="#">6. Built-in Exceptions</a><ul> <li><a class="reference internal" href="#exception-hierarchy">6.1. Exception hierarchy</a></li> </ul> </li> </ul> <h4>Previous topic</h4> <p class="topless"><a href="stdtypes.html" title="previous chapter">5. Built-in Types</a></p> <h4>Next topic</h4> <p class="topless"><a href="strings.html" title="next chapter">7. 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