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<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN"

"http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">

<head>

<title>8. 錯誤和例外 — Python 3.7.0 說明文件</title>

title="在 Python 3.7.0 說明文件中搜尋"

href="../_static/opensearch.xml"/>

</head><body>

<ul>

<a href="../genindex.html" title="General Index"

accesskey="I">索引</a></li>

<a href="../py-modindex.html" title="Python 模組索引"

>模組</a> |</li>

<a href="classes.html" title="9. Classes"

accesskey="N">下一頁</a> |</li>

<a href="inputoutput.html" title="7. 輸入和輸出"

accesskey="P">上一頁</a> |</li>

<li><img src="../_static/py.png" alt=""

style="vertical-align: middle; margin-top: -1px"/></li>

<li><a href="https://www.python.org/">Python</a> »</li>

<li>

zh_TW

3.7.0

<a href="../index.html">Documentation </a> »

</li>

<li class="nav-item nav-item-1"><a href="index.html" accesskey="U">Python 教學</a> »</li>

</form>

</div>

</li>

</ul>

</div>

<h1>8. 錯誤和例外<a class="headerlink" href="#errors-and-exceptions" title="本標題的永久連結">¶</a></h1>

Until now error messages haven’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.

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-python3 notranslate"><div class="highlight"><pre>>>> while True print('Hello world')

File "<stdin>", line 1

while True print('Hello world')

^

SyntaxError: invalid syntax

</pre></div>

</div>

The parser repeats the offending line and displays a little 『arrow』 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 function <a class="reference internal" href="../library/functions.html#print" title="print"><code class="xref py py-func docutils literal notranslate">print()</code></a>, since a colon

(<code class="docutils literal notranslate">':'</code>) 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>

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-python3 notranslate"><div class="highlight"><pre>>>> 10 * (1/0)

Traceback (most recent call last):

File "<stdin>", line 1, in <module>

ZeroDivisionError: division by zero

>>> 4 + spam*3

Traceback (most recent call last):

File "<stdin>", line 1, in <module>

NameError: name 'spam' is not defined

>>> '2' + 2

Traceback (most recent call last):

File "<stdin>", line 1, in <module>

TypeError: Can't convert 'int' object to str implicitly

</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#ZeroDivisionError" title="ZeroDivisionError"><code class="xref py py-exc docutils literal notranslate">ZeroDivisionError</code></a>, <a class="reference internal" href="../library/exceptions.html#NameError" title="NameError"><code class="xref py py-exc docutils literal notranslate">NameError</code></a> and <a class="reference internal" href="../library/exceptions.html#TypeError" title="TypeError"><code class="xref py py-exc docutils literal notranslate">TypeError</code></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">內建的例外</a> lists the built-in exceptions and their meanings.

</div>

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 <kbd class="kbd docutils literal notranslate">Control-C</kbd> 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#KeyboardInterrupt" title="KeyboardInterrupt"><code class="xref py py-exc docutils literal notranslate">KeyboardInterrupt</code></a> exception.

<div class="highlight-python3 notranslate"><div class="highlight"><pre>>>> while True:

... try:

... x = int(input("Please enter a number: "))

... break

... except ValueError:

... print("Oops! That was no valid number. Try again...")

...

</pre></div>

</div>

<li>First, the try clause (the statement(s) between the <a class="reference internal" href="../reference/compound_stmts.html#try"><code class="xref std std-keyword docutils literal notranslate">try</code></a> and

<li>If no exception occurs, the except clause is skipped and execution of the

<a class="reference internal" href="../reference/compound_stmts.html#try"><code class="xref std std-keyword docutils literal notranslate">try</code></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

continues after the <a class="reference internal" href="../reference/compound_stmts.html#try"><code class="xref std std-keyword docutils literal notranslate">try</code></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"><code class="xref std std-keyword docutils literal notranslate">try</code></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"><code class="xref std std-keyword docutils literal notranslate">try</code></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"><code class="xref std std-keyword docutils literal notranslate">try</code></a> statement. An except clause may

name multiple exceptions as a parenthesized tuple, for example:

<div class="highlight-python3 notranslate"><div class="highlight"><pre>... except (RuntimeError, TypeError, NameError):

... pass

</pre></div>

</div>

A class in an <a class="reference internal" href="../reference/compound_stmts.html#except"><code class="xref std std-keyword docutils literal notranslate">except</code></a> clause is compatible with an exception if it is

the same class or a base class thereof (but not the other way around — an

except clause listing a derived class is not compatible with a base class). For

example, the following code will print B, C, D in that order:

<div class="highlight-python3 notranslate"><div class="highlight"><pre>class B(Exception):

pass

class C(B):

pass

class D(C):

pass

for cls in [B, C, D]:

try:

raise cls()

except D:

print("D")

except C:

print("C")

except B:

print("B")

</pre></div>

</div>

Note that if the except clauses were reversed (with <code class="docutils literal notranslate">except B</code> first), it

would have printed B, B, B — the first matching except clause is triggered.

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-python3 notranslate"><div class="highlight"><pre>import sys

try:

f = open('myfile.txt')

s = f.readline()

i = int(s.strip())

except OSError as err:

print("OS error: {0}".format(err))

except ValueError:

print("Could not convert data to an integer.")

except:

print("Unexpected error:", sys.exc_info()[0])

raise

</pre></div>

</div>

The <a class="reference internal" href="../reference/compound_stmts.html#try"><code class="xref std std-keyword docutils literal notranslate">try</code></a> … <a class="reference internal" href="../reference/compound_stmts.html#except"><code class="xref std std-keyword docutils literal notranslate">except</code></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-python3 notranslate"><div class="highlight"><pre>for arg in sys.argv[1:]:

try:

f = open(arg, 'r')

except OSError:

print('cannot open', arg)

else:

print(arg, 'has', len(f.readlines()), 'lines')

f.close()

</pre></div>

</div>

The use of the <a class="reference internal" href="../reference/compound_stmts.html#else"><code class="xref std std-keyword docutils literal notranslate">else</code></a> clause is better than adding additional code to

the <a class="reference internal" href="../reference/compound_stmts.html#try"><code class="xref std std-keyword docutils literal notranslate">try</code></a> clause because it avoids accidentally catching an exception

that wasn’t raised by the code being protected by the <a class="reference internal" href="../reference/compound_stmts.html#try"><code class="xref std std-keyword docutils literal notranslate">try</code></a> …

<a class="reference internal" href="../reference/compound_stmts.html#except"><code class="xref std std-keyword docutils literal notranslate">except</code></a> statement.

When an exception occurs, it may have an associated value, also known as the

exception’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. The

variable is bound to an exception instance with the arguments stored in

<code class="docutils literal notranslate">instance.args</code>. For convenience, the exception instance defines

<a class="reference internal" href="../reference/datamodel.html#object.__str__" title="object.__str__"><code class="xref py py-meth docutils literal notranslate">__str__()</code></a> so the arguments can be printed directly without having to

reference <code class="docutils literal notranslate">.args</code>. One may also instantiate an exception first before

raising it and add any attributes to it as desired.

<div class="highlight-python3 notranslate"><div class="highlight"><pre>>>> try:

... raise Exception('spam', 'eggs')

... except Exception as inst:

... print(type(inst)) # the exception instance

... print(inst.args) # arguments stored in .args

... print(inst) # __str__ allows args to be printed directly,

... # but may be overridden in exception subclasses

... x, y = inst.args # unpack args

... print('x =', x)

... print('y =', y)

...

('spam', 'eggs')

x = spam

y = eggs

</pre></div>

</div>

If an exception has arguments, they are printed as the last part (『detail』) of

the message for unhandled exceptions.

Exception handlers don’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-python3 notranslate"><div class="highlight"><pre>>>> def this_fails():

... x = 1/0

...

>>> try:

... this_fails()

... except ZeroDivisionError as err:

... print('Handling run-time error:', err)

...

Handling run-time error: division by zero

</pre></div>

</div>

<h2>8.4. Raising Exceptions<a class="headerlink" href="#raising-exceptions" title="本標題的永久連結">¶</a></h2>

The <a class="reference internal" href="../reference/simple_stmts.html#raise"><code class="xref std std-keyword docutils literal notranslate">raise</code></a> statement allows the programmer to force a specified

exception to occur. For example:

<div class="highlight-python3 notranslate"><div class="highlight"><pre>>>> raise NameError('HiThere')

Traceback (most recent call last):

File "<stdin>", line 1, in <module>

NameError: HiThere

</pre></div>

</div>

The sole argument to <a class="reference internal" href="../reference/simple_stmts.html#raise"><code class="xref std std-keyword docutils literal notranslate">raise</code></a> indicates the exception to be raised.

This must be either an exception instance or an exception class (a class that

derives from <a class="reference internal" href="../library/exceptions.html#Exception" title="Exception"><code class="xref py py-class docutils literal notranslate">Exception</code></a>). If an exception class is passed, it will

be implicitly instantiated by calling its constructor with no arguments:

<div class="highlight-python3 notranslate"><div class="highlight"><pre>raise ValueError # shorthand for 'raise ValueError()'

</pre></div>

</div>

If you need to determine whether an exception was raised but don’t intend to

handle it, a simpler form of the <a class="reference internal" href="../reference/simple_stmts.html#raise"><code class="xref std std-keyword docutils literal notranslate">raise</code></a> statement allows you to

re-raise the exception:

<div class="highlight-python3 notranslate"><div class="highlight"><pre>>>> try:

... raise NameError('HiThere')

... except NameError:

... print('An exception flew by!')

... raise

...

An exception flew by!

Traceback (most recent call last):

File "<stdin>", line 2, in <module>

NameError: HiThere

</pre></div>

</div>

<h2>8.5. User-defined Exceptions<a class="headerlink" href="#user-defined-exceptions" title="本標題的永久連結">¶</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#Exception" title="Exception"><code class="xref py py-exc docutils literal notranslate">Exception</code></a> class, either directly or indirectly.

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-python3 notranslate"><div class="highlight"><pre>class Error(Exception):

"""Base class for exceptions in this module."""

pass

class InputError(Error):

"""Exception raised for errors in the input.

Attributes:

expression -- input expression in which the error occurred

message -- explanation of the error

"""

def __init__(self, expression, message):

self.expression = expression

self.message = message

class TransitionError(Error):

"""Raised when an operation attempts a state transition that's not

allowed.

Attributes:

previous -- state at beginning of transition

next -- attempted new state

message -- explanation of why the specific transition is not allowed

"""

def __init__(self, previous, next, message):

self.previous = previous

self.next = next

self.message = message

</pre></div>

</div>

Most exceptions are defined with names that end in 「Error,」 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>

<h2>8.6. Defining Clean-up Actions<a class="headerlink" href="#defining-clean-up-actions" title="本標題的永久連結">¶</a></h2>

define clean-up actions that must be executed under all circumstances. For

example:

<div class="highlight-python3 notranslate"><div class="highlight"><pre>>>> try:

... raise KeyboardInterrupt

... finally:

... print('Goodbye, world!')

...

Goodbye, world!

KeyboardInterrupt

Traceback (most recent call last):

File "<stdin>", line 2, in <module>

</pre></div>

</div>

A finally clause is always executed before leaving the <a class="reference internal" href="../reference/compound_stmts.html#try"><code class="xref std std-keyword docutils literal notranslate">try</code></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"><code class="xref std std-keyword docutils literal notranslate">try</code></a> clause and has not been handled by an

<a class="reference internal" href="../reference/compound_stmts.html#except"><code class="xref std std-keyword docutils literal notranslate">except</code></a> clause (or it has occurred in an <a class="reference internal" href="../reference/compound_stmts.html#except"><code class="xref std std-keyword docutils literal notranslate">except</code></a> or

<a class="reference internal" href="../reference/compound_stmts.html#else"><code class="xref std std-keyword docutils literal notranslate">else</code></a> clause), it is re-raised after the <a class="reference internal" href="../reference/compound_stmts.html#finally"><code class="xref std std-keyword docutils literal notranslate">finally</code></a> clause has

been executed. The <a class="reference internal" href="../reference/compound_stmts.html#finally"><code class="xref std std-keyword docutils literal notranslate">finally</code></a> clause is also executed 「on the way out」

when any other clause of the <a class="reference internal" href="../reference/compound_stmts.html#try"><code class="xref std std-keyword docutils literal notranslate">try</code></a> statement is left via a

<a class="reference internal" href="../reference/simple_stmts.html#break"><code class="xref std std-keyword docutils literal notranslate">break</code></a>, <a class="reference internal" href="../reference/simple_stmts.html#continue"><code class="xref std std-keyword docutils literal notranslate">continue</code></a> or <a class="reference internal" href="../reference/simple_stmts.html#return"><code class="xref std std-keyword docutils literal notranslate">return</code></a> statement. A more

complicated example:

<div class="highlight-python3 notranslate"><div class="highlight"><pre>>>> def divide(x, y):

... try:

... result = x / y

... except ZeroDivisionError:

... print("division by zero!")

... else:

... print("result is", result)

... finally:

... print("executing finally clause")

...

>>> divide(2, 1)

result is 2.0

executing finally clause

>>> divide(2, 0)

division by zero!

executing finally clause

>>> divide("2", "1")

executing finally clause

Traceback (most recent call last):

File "<stdin>", line 1, in <module>

File "<stdin>", line 3, in divide

TypeError: unsupported operand type(s) for /: 'str' and 'str'

</pre></div>

</div>

As you can see, the <a class="reference internal" href="../reference/compound_stmts.html#finally"><code class="xref std std-keyword docutils literal notranslate">finally</code></a> clause is executed in any event. The

<a class="reference internal" href="../library/exceptions.html#TypeError" title="TypeError"><code class="xref py py-exc docutils literal notranslate">TypeError</code></a> raised by dividing two strings is not handled by the

<a class="reference internal" href="../reference/compound_stmts.html#except"><code class="xref std std-keyword docutils literal notranslate">except</code></a> clause and therefore re-raised after the <a class="reference internal" href="../reference/compound_stmts.html#finally"><code class="xref std std-keyword docutils literal notranslate">finally</code></a>

clause has been executed.

In real world applications, the <a class="reference internal" href="../reference/compound_stmts.html#finally"><code class="xref std std-keyword docutils literal notranslate">finally</code></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>

<h2>8.7. Predefined Clean-up Actions<a class="headerlink" href="#predefined-clean-up-actions" title="本標題的永久連結">¶</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-python3 notranslate"><div class="highlight"><pre>for line in open("myfile.txt"):

print(line, end="")

</pre></div>

</div>

The problem with this code is that it leaves the file open for an indeterminate

amount of time after this part of 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"><code class="xref std std-keyword docutils literal notranslate">with</code></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-python3 notranslate"><div class="highlight"><pre>with open("myfile.txt") as f:

for line in f:

</pre></div>

</div>

After the statement is executed, the file f is always closed, even if a

problem was encountered while processing the lines. Objects which, like files,

provide predefined clean-up actions will indicate this in their documentation.

</div>

<ul>

<li><a class="reference internal" href="#">8. 錯誤和例外</a><ul>

<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>

</ul>

</li>

</ul>

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