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 <div class="section" id="errors-and-exceptions">
<h1>8. Errors and Exceptions<a class="headerlink" href="#errors-and-exceptions" title="Permalink to this headline">¶</a></h1>
Until now error messages haven&#8217;t been more than mentioned, but if you have tried
out the examples you have probably seen some. There are (at least) two
distinguishable kinds of errors: syntax errors and exceptions.
<div class="section" id="syntax-errors">
<h2>8.1. Syntax Errors<a class="headerlink" href="#syntax-errors" title="Permalink to this headline">¶</a></h2>
Syntax errors, also known as parsing errors, are perhaps the most common kind of
complaint you get while you are still learning Python:
<div class="highlight-python"><div class="highlight"><pre>&gt;&gt;&gt; while True print &#39;Hello world&#39;
 File &quot;&lt;stdin&gt;&quot;, line 1, in ?
 while True print &#39;Hello world&#39;
 ^
SyntaxError: invalid syntax
</pre></div>
</div>
The parser repeats the offending line and displays a little &#8216;arrow&#8217; pointing at
the earliest point in the line where the error was detected. The error is
caused by (or at least detected at) the token preceding the arrow: in the
example, the error is detected at the keyword <a class="reference internal" href="../reference/simple_stmts.html#print"><tt class="xref std std-keyword docutils literal">print</tt></a>, since a colon
(<tt class="docutils literal">':'</tt>) is missing before it. File name and line number are printed so you
know where to look in case the input came from a script.
</div>
<div class="section" id="exceptions">
<h2>8.2. Exceptions<a class="headerlink" href="#exceptions" title="Permalink to this headline">¶</a></h2>
Even if a statement or expression is syntactically correct, it may cause an
error when an attempt is made to execute it. Errors detected during execution
are called exceptions and are not unconditionally fatal: you will soon learn
how to handle them in Python programs. Most exceptions are not handled by
programs, however, and result in error messages as shown here:
<div class="highlight-python"><div class="highlight"><pre>&gt;&gt;&gt; 10 * (1/0)
Traceback (most recent call last):
 File &quot;&lt;stdin&gt;&quot;, line 1, in ?
ZeroDivisionError: integer division or modulo by zero
&gt;&gt;&gt; 4 + spam*3
Traceback (most recent call last):
 File &quot;&lt;stdin&gt;&quot;, line 1, in ?
NameError: name &#39;spam&#39; is not defined
&gt;&gt;&gt; &#39;2&#39; + 2
Traceback (most recent call last):
 File &quot;&lt;stdin&gt;&quot;, line 1, in ?
TypeError: cannot concatenate &#39;str&#39; and &#39;int&#39; objects
</pre></div>
</div>
The last line of the error message indicates what happened. Exceptions come in
different types, and the type is printed as part of the message: the types in
the example are <a class="reference internal" href="../library/exceptions.html#exceptions.ZeroDivisionError" title="exceptions.ZeroDivisionError"><tt class="xref py py-exc docutils literal">ZeroDivisionError</tt></a>, <a class="reference internal" href="../library/exceptions.html#exceptions.NameError" title="exceptions.NameError"><tt class="xref py py-exc docutils literal">NameError</tt></a> and <a class="reference internal" href="../library/exceptions.html#exceptions.TypeError" title="exceptions.TypeError"><tt class="xref py py-exc docutils literal">TypeError</tt></a>.
The string printed as the exception type is the name of the built-in exception
that occurred. This is true for all built-in exceptions, but need not be true
for user-defined exceptions (although it is a useful convention). Standard
exception names are built-in identifiers (not reserved keywords).
The rest of the line provides detail based on the type of exception and what
caused it.
The preceding part of the error message shows the context where the exception
happened, in the form of a stack traceback. In general it contains a stack
traceback listing source lines; however, it will not display lines read from
standard input.
<a class="reference internal" href="../library/exceptions.html#bltin-exceptions">Built-in Exceptions</a> lists the built-in exceptions and their meanings.
</div>
<div class="section" id="handling-exceptions">
<h2>8.3. Handling Exceptions<a class="headerlink" href="#handling-exceptions" title="Permalink to this headline">¶</a></h2>
It is possible to write programs that handle selected exceptions. Look at the
following example, which asks the user for input until a valid integer has been
entered, but allows the user to interrupt the program (using <tt class="kbd docutils literal">Control-C</tt> or
whatever the operating system supports); note that a user-generated interruption
is signalled by raising the <a class="reference internal" href="../library/exceptions.html#exceptions.KeyboardInterrupt" title="exceptions.KeyboardInterrupt"><tt class="xref py py-exc docutils literal">KeyboardInterrupt</tt></a> exception.
<div class="highlight-python"><div class="highlight"><pre>&gt;&gt;&gt; while True:
... try:
... x = int(raw_input(&quot;Please enter a number: &quot;))
... break
... except ValueError:
... print &quot;Oops! That was no valid number. Try again...&quot;
...
</pre></div>
</div>
The <a class="reference internal" href="../reference/compound_stmts.html#try"><tt class="xref std std-keyword docutils literal">try</tt></a> statement works as follows.
<ul class="simple">
<li>First, the try clause (the statement(s) between the <a class="reference internal" href="../reference/compound_stmts.html#try"><tt class="xref std std-keyword docutils literal">try</tt></a> and
<a class="reference internal" href="../reference/compound_stmts.html#except"><tt class="xref std std-keyword docutils literal">except</tt></a> keywords) is executed.</li>
<li>If no exception occurs, the except clause is skipped and execution of the
<a class="reference internal" href="../reference/compound_stmts.html#try"><tt class="xref std std-keyword docutils literal">try</tt></a> statement is finished.</li>
<li>If an exception occurs during execution of the try clause, the rest of the
clause is skipped. Then if its type matches the exception named after the
<a class="reference internal" href="../reference/compound_stmts.html#except"><tt class="xref std std-keyword docutils literal">except</tt></a> keyword, the except clause is executed, and then execution
continues after the <a class="reference internal" href="../reference/compound_stmts.html#try"><tt class="xref std std-keyword docutils literal">try</tt></a> statement.</li>
<li>If an exception occurs which does not match the exception named in the except
clause, it is passed on to outer <a class="reference internal" href="../reference/compound_stmts.html#try"><tt class="xref std std-keyword docutils literal">try</tt></a> statements; if no handler is
found, it is an unhandled exception and execution stops with a message as
shown above.</li>
</ul>
A <a class="reference internal" href="../reference/compound_stmts.html#try"><tt class="xref std std-keyword docutils literal">try</tt></a> statement may have more than one except clause, to specify
handlers for different exceptions. At most one handler will be executed.
Handlers only handle exceptions that occur in the corresponding try clause, not
in other handlers of the same <a class="reference internal" href="../reference/compound_stmts.html#try"><tt class="xref std std-keyword docutils literal">try</tt></a> statement. An except clause may
name multiple exceptions as a parenthesized tuple, for example:
<div class="highlight-python"><div class="highlight"><pre>... except (RuntimeError, TypeError, NameError):
... pass
</pre></div>
</div>
Note that the parentheses around this tuple are required, because
<tt class="docutils literal">except ValueError, e:</tt> was the syntax used for what is normally
written as <tt class="docutils literal">except ValueError as e:</tt> in modern Python (described
below). The old syntax is still supported for backwards compatibility.
This means <tt class="docutils literal">except RuntimeError, TypeError</tt> is not equivalent to
<tt class="docutils literal">except (RuntimeError, TypeError):</tt> but to <tt class="docutils literal">except RuntimeError as
TypeError:</tt> which is not what you want.
The last except clause may omit the exception name(s), to serve as a wildcard.
Use this with extreme caution, since it is easy to mask a real programming error
in this way! It can also be used to print an error message and then re-raise
the exception (allowing a caller to handle the exception as well):
<div class="highlight-python"><div class="highlight"><pre>import sys

try:
 f = open(&#39;myfile.txt&#39;)
 s = f.readline()
 i = int(s.strip())
except IOError as e:
 print &quot;I/O error({0}): {1}&quot;.format(e.errno, e.strerror)
except ValueError:
 print &quot;Could not convert data to an integer.&quot;
except:
 print &quot;Unexpected error:&quot;, sys.exc_info()[0]
 raise
</pre></div>
</div>
The <a class="reference internal" href="../reference/compound_stmts.html#try"><tt class="xref std std-keyword docutils literal">try</tt></a> ... <a class="reference internal" href="../reference/compound_stmts.html#except"><tt class="xref std std-keyword docutils literal">except</tt></a> statement has an optional else
clause, which, when present, must follow all except clauses. It is useful for
code that must be executed if the try clause does not raise an exception. For
example:
<div class="highlight-python"><div class="highlight"><pre>for arg in sys.argv[1:]:
 try:
 f = open(arg, &#39;r&#39;)
 except IOError:
 print &#39;cannot open&#39;, arg
 else:
 print arg, &#39;has&#39;, len(f.readlines()), &#39;lines&#39;
 f.close()
</pre></div>
</div>
The use of the <a class="reference internal" href="../reference/compound_stmts.html#else"><tt class="xref std std-keyword docutils literal">else</tt></a> clause is better than adding additional code to
the <a class="reference internal" href="../reference/compound_stmts.html#try"><tt class="xref std std-keyword docutils literal">try</tt></a> clause because it avoids accidentally catching an exception
that wasn&#8217;t raised by the code being protected by the <a class="reference internal" href="../reference/compound_stmts.html#try"><tt class="xref std std-keyword docutils literal">try</tt></a> ...
<a class="reference internal" href="../reference/compound_stmts.html#except"><tt class="xref std std-keyword docutils literal">except</tt></a> statement.
When an exception occurs, it may have an associated value, also known as the
exception&#8217;s argument. The presence and type of the argument depend on the
exception type.
The except clause may specify a variable after the exception name (or tuple).
The variable is bound to an exception instance with the arguments stored in
<tt class="docutils literal">instance.args</tt>. For convenience, the exception instance defines
<a class="reference internal" href="../reference/datamodel.html#object.__str__" title="object.__str__"><tt class="xref py py-meth docutils literal">__str__()</tt></a> so the arguments can be printed directly without having to
reference <tt class="docutils literal">.args</tt>.
One may also instantiate an exception first before raising it and add any
attributes to it as desired.
<div class="highlight-python"><div class="highlight"><pre>&gt;&gt;&gt; try:
... raise Exception(&#39;spam&#39;, &#39;eggs&#39;)
... except Exception as inst:
... print type(inst) # the exception instance
... print inst.args # arguments stored in .args
... print inst # __str__ allows args to printed directly
... x, y = inst.args
... print &#39;x =&#39;, x
... print &#39;y =&#39;, y
...
&lt;type &#39;exceptions.Exception&#39;&gt;
(&#39;spam&#39;, &#39;eggs&#39;)
(&#39;spam&#39;, &#39;eggs&#39;)
x = spam
y = eggs
</pre></div>
</div>
If an exception has an argument, it is printed as the last part (&#8216;detail&#8217;) of
the message for unhandled exceptions.
Exception handlers don&#8217;t just handle exceptions if they occur immediately in the
try clause, but also if they occur inside functions that are called (even
indirectly) in the try clause. For example:
<div class="highlight-python"><div class="highlight"><pre>&gt;&gt;&gt; def this_fails():
... x = 1/0
...
&gt;&gt;&gt; try:
... this_fails()
... except ZeroDivisionError as detail:
... print &#39;Handling run-time error:&#39;, detail
...
Handling run-time error: integer division or modulo by zero
</pre></div>
</div>
</div>
<div class="section" id="raising-exceptions">
<h2>8.4. Raising Exceptions<a class="headerlink" href="#raising-exceptions" title="Permalink to this headline">¶</a></h2>
The <a class="reference internal" href="../reference/simple_stmts.html#raise"><tt class="xref std std-keyword docutils literal">raise</tt></a> statement allows the programmer to force a specified
exception to occur. For example:
<div class="highlight-python"><div class="highlight"><pre>&gt;&gt;&gt; raise NameError(&#39;HiThere&#39;)
Traceback (most recent call last):
 File &quot;&lt;stdin&gt;&quot;, line 1, in ?
NameError: HiThere
</pre></div>
</div>
The sole argument to <a class="reference internal" href="../reference/simple_stmts.html#raise"><tt class="xref std std-keyword docutils literal">raise</tt></a> indicates the exception to be raised.
This must be either an exception instance or an exception class (a class that
derives from <tt class="xref py py-class docutils literal">Exception</tt>).
If you need to determine whether an exception was raised but don&#8217;t intend to
handle it, a simpler form of the <a class="reference internal" href="../reference/simple_stmts.html#raise"><tt class="xref std std-keyword docutils literal">raise</tt></a> statement allows you to
re-raise the exception:
<div class="highlight-python"><div class="highlight"><pre>&gt;&gt;&gt; try:
... raise NameError(&#39;HiThere&#39;)
... except NameError:
... print &#39;An exception flew by!&#39;
... raise
...
An exception flew by!
Traceback (most recent call last):
 File &quot;&lt;stdin&gt;&quot;, line 2, in ?
NameError: HiThere
</pre></div>
</div>
</div>
<div class="section" id="user-defined-exceptions">
<h2>8.5. User-defined Exceptions<a class="headerlink" href="#user-defined-exceptions" title="Permalink to this headline">¶</a></h2>
Programs may name their own exceptions by creating a new exception class (see
<a class="reference internal" href="classes.html#tut-classes">Classes</a> for more about Python classes). Exceptions should typically
be derived from the <a class="reference internal" href="../library/exceptions.html#exceptions.Exception" title="exceptions.Exception"><tt class="xref py py-exc docutils literal">Exception</tt></a> class, either directly or indirectly. For
example:
<div class="highlight-python"><div class="highlight"><pre>&gt;&gt;&gt; class MyError(Exception):
... def __init__(self, value):
... self.value = value
... def __str__(self):
... return repr(self.value)
...
&gt;&gt;&gt; try:
... raise MyError(2*2)
... except MyError as e:
... print &#39;My exception occurred, value:&#39;, e.value
...
My exception occurred, value: 4
&gt;&gt;&gt; raise MyError(&#39;oops!&#39;)
Traceback (most recent call last):
 File &quot;&lt;stdin&gt;&quot;, line 1, in ?
__main__.MyError: &#39;oops!&#39;
</pre></div>
</div>
In this example, the default <a class="reference internal" href="../reference/datamodel.html#object.__init__" title="object.__init__"><tt class="xref py py-meth docutils literal">__init__()</tt></a> of <tt class="xref py py-class docutils literal">Exception</tt> has been
overridden. The new behavior simply creates the value attribute. This
replaces the default behavior of creating the args attribute.
Exception classes can be defined which do anything any other class can do, but
are usually kept simple, often only offering a number of attributes that allow
information about the error to be extracted by handlers for the exception. When
creating a module that can raise several distinct errors, a common practice is
to create a base class for exceptions defined by that module, and subclass that
to create specific exception classes for different error conditions:
<div class="highlight-python"><div class="highlight"><pre>class Error(Exception):
 &quot;&quot;&quot;Base class for exceptions in this module.&quot;&quot;&quot;
 pass

class InputError(Error):
 &quot;&quot;&quot;Exception raised for errors in the input.

Attributes:
 expr -- input expression in which the error occurred
 msg -- explanation of the error
 &quot;&quot;&quot;

def __init__(self, expr, msg):
 self.expr = expr
 self.msg = msg

class TransitionError(Error):
 &quot;&quot;&quot;Raised when an operation attempts a state transition that&#39;s not
 allowed.

Attributes:
 prev -- state at beginning of transition
 next -- attempted new state
 msg -- explanation of why the specific transition is not allowed
 &quot;&quot;&quot;

def __init__(self, prev, next, msg):
 self.prev = prev
 self.next = next
 self.msg = msg
</pre></div>
</div>
Most exceptions are defined with names that end in &#8220;Error,&#8221; similar to the
naming of the standard exceptions.
Many standard modules define their own exceptions to report errors that may
occur in functions they define. More information on classes is presented in
chapter <a class="reference internal" href="classes.html#tut-classes">Classes</a>.
</div>
<div class="section" id="defining-clean-up-actions">
<h2>8.6. Defining Clean-up Actions<a class="headerlink" href="#defining-clean-up-actions" title="Permalink to this headline">¶</a></h2>
The <a class="reference internal" href="../reference/compound_stmts.html#try"><tt class="xref std std-keyword docutils literal">try</tt></a> statement has another optional clause which is intended to
define clean-up actions that must be executed under all circumstances. For
example:
<div class="highlight-python"><div class="highlight"><pre>&gt;&gt;&gt; try:
... raise KeyboardInterrupt
... finally:
... print &#39;Goodbye, world!&#39;
...
Goodbye, world!
KeyboardInterrupt
</pre></div>
</div>
A finally clause is always executed before leaving the <a class="reference internal" href="../reference/compound_stmts.html#try"><tt class="xref std std-keyword docutils literal">try</tt></a>
statement, whether an exception has occurred or not. When an exception has
occurred in the <a class="reference internal" href="../reference/compound_stmts.html#try"><tt class="xref std std-keyword docutils literal">try</tt></a> clause and has not been handled by an
<a class="reference internal" href="../reference/compound_stmts.html#except"><tt class="xref std std-keyword docutils literal">except</tt></a> clause (or it has occurred in a <a class="reference internal" href="../reference/compound_stmts.html#except"><tt class="xref std std-keyword docutils literal">except</tt></a> or
<a class="reference internal" href="../reference/compound_stmts.html#else"><tt class="xref std std-keyword docutils literal">else</tt></a> clause), it is re-raised after the <a class="reference internal" href="../reference/compound_stmts.html#finally"><tt class="xref std std-keyword docutils literal">finally</tt></a> clause has
been executed. The <a class="reference internal" href="../reference/compound_stmts.html#finally"><tt class="xref std std-keyword docutils literal">finally</tt></a> clause is also executed &#8220;on the way out&#8221;
when any other clause of the <a class="reference internal" href="../reference/compound_stmts.html#try"><tt class="xref std std-keyword docutils literal">try</tt></a> statement is left via a
<a class="reference internal" href="../reference/simple_stmts.html#break"><tt class="xref std std-keyword docutils literal">break</tt></a>, <a class="reference internal" href="../reference/simple_stmts.html#continue"><tt class="xref std std-keyword docutils literal">continue</tt></a> or <a class="reference internal" href="../reference/simple_stmts.html#return"><tt class="xref std std-keyword docutils literal">return</tt></a> statement. A more
complicated example (having <a class="reference internal" href="../reference/compound_stmts.html#except"><tt class="xref std std-keyword docutils literal">except</tt></a> and <a class="reference internal" href="../reference/compound_stmts.html#finally"><tt class="xref std std-keyword docutils literal">finally</tt></a> clauses in
the same <a class="reference internal" href="../reference/compound_stmts.html#try"><tt class="xref std std-keyword docutils literal">try</tt></a> statement works as of Python 2.5):
<div class="highlight-python"><div class="highlight"><pre>&gt;&gt;&gt; def divide(x, y):
... try:
... result = x / y
... except ZeroDivisionError:
... print &quot;division by zero!&quot;
... else:
... print &quot;result is&quot;, result
... finally:
... print &quot;executing finally clause&quot;
...
&gt;&gt;&gt; divide(2, 1)
result is 2
executing finally clause
&gt;&gt;&gt; divide(2, 0)
division by zero!
executing finally clause
&gt;&gt;&gt; divide(&quot;2&quot;, &quot;1&quot;)
executing finally clause
Traceback (most recent call last):
 File &quot;&lt;stdin&gt;&quot;, line 1, in ?
 File &quot;&lt;stdin&gt;&quot;, line 3, in divide
TypeError: unsupported operand type(s) for /: &#39;str&#39; and &#39;str&#39;
</pre></div>
</div>
As you can see, the <a class="reference internal" href="../reference/compound_stmts.html#finally"><tt class="xref std std-keyword docutils literal">finally</tt></a> clause is executed in any event. The
<a class="reference internal" href="../library/exceptions.html#exceptions.TypeError" title="exceptions.TypeError"><tt class="xref py py-exc docutils literal">TypeError</tt></a> raised by dividing two strings is not handled by the
<a class="reference internal" href="../reference/compound_stmts.html#except"><tt class="xref std std-keyword docutils literal">except</tt></a> clause and therefore re-raised after the <a class="reference internal" href="../reference/compound_stmts.html#finally"><tt class="xref std std-keyword docutils literal">finally</tt></a>
clause has been executed.
In real world applications, the <a class="reference internal" href="../reference/compound_stmts.html#finally"><tt class="xref std std-keyword docutils literal">finally</tt></a> clause is useful for
releasing external resources (such as files or network connections), regardless
of whether the use of the resource was successful.
</div>
<div class="section" id="predefined-clean-up-actions">
<h2>8.7. Predefined Clean-up Actions<a class="headerlink" href="#predefined-clean-up-actions" title="Permalink to this headline">¶</a></h2>
Some objects define standard clean-up actions to be undertaken when the object
is no longer needed, regardless of whether or not the operation using the object
succeeded or failed. Look at the following example, which tries to open a file
and print its contents to the screen.
<div class="highlight-python"><div class="highlight"><pre>for line in open(&quot;myfile.txt&quot;):
 print line,
</pre></div>
</div>
The problem with this code is that it leaves the file open for an indeterminate
amount of time after the code has finished executing. This is not an issue in
simple scripts, but can be a problem for larger applications. The
<a class="reference internal" href="../reference/compound_stmts.html#with"><tt class="xref std std-keyword docutils literal">with</tt></a> statement allows objects like files to be used in a way that
ensures they are always cleaned up promptly and correctly.
<div class="highlight-python"><div class="highlight"><pre>with open(&quot;myfile.txt&quot;) as f:
 for line in f:
 print line,
</pre></div>
</div>
After the statement is executed, the file f is always closed, even if a
problem was encountered while processing the lines. Other objects which provide
predefined clean-up actions will indicate this in their documentation.
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<li><a class="reference internal" href="#">8. Errors and Exceptions</a><ul>
<li><a class="reference internal" href="#syntax-errors">8.1. Syntax Errors</a></li>
<li><a class="reference internal" href="#exceptions">8.2. Exceptions</a></li>
<li><a class="reference internal" href="#handling-exceptions">8.3. Handling Exceptions</a></li>
<li><a class="reference internal" href="#raising-exceptions">8.4. Raising Exceptions</a></li>
<li><a class="reference internal" href="#user-defined-exceptions">8.5. User-defined Exceptions</a></li>
<li><a class="reference internal" href="#defining-clean-up-actions">8.6. Defining Clean-up Actions</a></li>
<li><a class="reference internal" href="#predefined-clean-up-actions">8.7. Predefined Clean-up Actions</a></li>
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