PyMOTW: exceptions

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2023-12-01

模块： exceptions
目的： excpetions模块定义了在整个标准库和解释器中使用的内置错误.
python版本： 1.5+

描述

Python之前就支持将简单的字符串信息和类一样作为异常. 从Python 1.5开始, 所有的标准库模块使用类作为异常. 而从Python 2.5开始, 字符串的异常会导致一个DeprecationWarning, 而对于字符串异常的支持会在未来的版本中去掉.

基类

在标准库文档的描述中, 异常类以分层结构定义. 这样做除了结构清晰之外, 异常的继承关系也很有用, 可以通过捕获基类的异常来捕获相关的异常. 大部分情况下, 这些基类异常不会直接被引发.

BaseException

所有异常的基类. 实现逻辑为使用 str() 将传给构造函数的参数创建为一个异常的字符串表达式.

Exception

不会导致运行的应用程序退出的异常的基类. 所有用户自定义的异常应该使用Exception作为基类.

StandardError

在标准库中使用的内置异常的基类.

ArithmeticError

数学相关错误的基类.

LookupError

因无法找到某些东西产生的错误的基类.

EnvironmentError

因Python之外的原因(操作系统, 文件系统, 等等)产生的错误的基类.

异常的引发

AssertionError

一个AssertionError由一个失败的 assert 语句引发.

assert False, 'The assertion failed'

$ python exceptions_AssertionError_assert.py
Traceback (most recent call last):
File "/Users/dhellmann/Documents/PyMOTW/in_progress/exceptions/PyMOTW/exceptions/exceptions_AssertionError_assert.py", line 12, in <module>
assert False, 'The assertion failed'
AssertionError: The assertion failed

也可以在 unittest 模块中像 failIf() 一样使用.

import unittest

class AssertionExample(unittest.TestCase):
    def test(self):
        self.failUnless(False)

unittest.main()

$ python exceptions_AssertionError_unittest.py
F
======================================================================
FAIL: test (__main__.AssertionExample)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/Users/dhellmann/Documents/PyMOTW/in_progress/exceptions/PyMOTW/exceptions/exceptions_AssertionError_unittest.py", line 17, in test
  self.failUnless(False)
AssertionError

----------------------------------------------------------------------
Ran 1 test in 0.000s

FAILED (failures=1)

AttributeError

若引用一个属性或对一个属性赋值时发生错误, 则会引发AttributeError.

例如, 当引用一个不存在的属性时:

class NoAttributes(object):
    pass

o = NoAttributes()
print o.attribute

$ python exceptions_AttributeError.py
Traceback (most recent call last):
  File "/Users/dhellmann/Documents/PyMOTW/in_progress/exceptions/PyMOTW/exceptions/exceptions_AttributeError.py", line 16, in <module>
  print o.attribute
AttributeError: 'NoAttributes' object has no attribute 'attribute'

或者尝试修改一个只读属性:

class MyClass(object):

    @property
    def attribute(self):
        return 'This is the attribute value'

o = MyClass()
print o.attribute
o.attribute = 'New value'

$ python exceptions_AttributeError_assignment.py
This is the attribute value
Traceback (most recent call last):
  File "/Users/dhellmann/Documents/PyMOTW/in_progress/exceptions/PyMOTW/exceptions/exceptions_AttributeError_assignment.py", line 20, in <module>
  o.attribute = 'New value'
AttributeError: can't set attribute

EOFError

当一个内置函数, 如 input() 或 raw_input(), 直到输入流的结尾也没有读到任何数据时, 会引发EOFError异常. 而对于文件的方法, 如 read(), 如果文件为空, 则会在文件末尾处返回一个空字符串.

while True:
    data = raw_input('prompt:')
    print 'READ:', data

$ echo hello | python PyMOTW/exceptions/exceptions_EOFError.py
prompt:READ: hello
prompt:Traceback (most recent call last):
  File "PyMOTW/exceptions/exceptions_EOFError.py", line 13, in <module>
  data = raw_input('prompt:')
EOFError: EOF when reading a line

FloatingPointError

若浮点异常控制(fpectl)启用, 则会在浮点运算出错时引发FloatingPointError. 启用:mod:fpectl 需要有 --with-fpectl 选项编译的解释器. fpectl 在stdlib文档 <http://docs.python.org/lib/module-fpectl.html>中不推荐使用。

import math
import fpectl

print 'Control off:', math.exp(1000)
fpectl.turnon_sigfpe()
print 'Control on:', math.exp(1000)

GeneratorExit

若调用generator的 close() 方法, 则在generator内引发GeneratorExit异常.

def my_generator():
    try:
        for i in range(5):
            print 'Yielding', i
            yield i
    except GeneratorExit:
        print 'Exiting early'

g = my_generator()
print g.next()
g.close()

$ python exceptions_GeneratorExit.py
Yielding 0
0
Exiting early

IOError

输入或输出失败时引发IOError. 例如, 磁盘已满或输入文件不存在.

f = open('/does/not/exist', 'r')

$ python exceptions_IOError.py
Traceback (most recent call last):
  File "/Users/dhellmann/Documents/PyMOTW/in_progress/exceptions/PyMOTW/exceptions/exceptions_IOError.py", line 12, in <module>
  f = open('/does/not/exist', 'r')
IOError: [Errno 2] No such file or directory: '/does/not/exist'

ImportError

当一个模块, 或者模块的成员, 不能被引入时引发ImportError. 以下是几种可能引发ImportError的情况.

若一个模块不存在.

import module_does_not_exist

$ python exceptions_ImportError_nomodule.py
Traceback (most recent call last):
  File "/Users/dhellmann/Documents/PyMOTW/in_progress/exceptions/PyMOTW/exceptions/exceptions_ImportError_nomodule.py", line 12, in <module>
  import module_does_not_exist
ImportError: No module named module_does_not_exist

若执行了 from X import Y, 而X模块中不存在Y, 则引发ImportError.

from exceptions import MadeUpName

$ python exceptions_ImportError_missingname.py
Traceback (most recent call last):
  File "/Users/dhellmann/Documents/PyMOTW/in_progress/exceptions/PyMOTW/exceptions/exceptions_ImportError_missingname.py", line 12, in <module>
  from exceptions import MadeUpName
ImportError: cannot import name MadeUpName

IndexError

当引用的序列下标越界时引发IndexError.

my_seq = [ 0, 1, 2 ]
print my_seq[3]

$ python exceptions_IndexError.py
Traceback (most recent call last):
  File "/Users/dhellmann/Documents/PyMOTW/in_progress/exceptions/PyMOTW/exceptions/exceptions_IndexError.py", line 13, in <module>
  print my_seq[3]
IndexError: list index out of range

KeyError

类似地, 当一个字典中的键无法找到时引发KeyError.

d = { 'a':1, 'b':2 }
print d['c']

$ python exceptions_KeyError.py
Traceback (most recent call last):
  File "/Users/dhellmann/Documents/PyMOTW/in_progress/exceptions/PyMOTW/exceptions/exceptions_KeyError.py", line 13, in <module>
  print d['c']
KeyError: 'c'

KeyboardInterrupt ~~~~~~~~~~~~~~~~~~`

KeyboardInterrupt发生在用户按下Ctrl-C(或Delete)使一个正在运行的程序停止时. 和大部分别的异常不同, KeyboardInterrupt直接继承了BaseException, 以防止其被用来捕获Exception的全局异常处理器捕获.

try:
    print 'Press Return or Ctrl-C:',
    ignored = raw_input()
except Exception, err:
    print 'Caught exception:', err
except KeyboardInterrupt, err:
    print 'Caught KeyboardInterrupt'
else:
    print 'No exception'

在提示符下按下Ctrl-C导致KeyboardInterrupt异常.

$ python PyMOTW/exceptions/exceptions_KeyboardInterrupt.py
Press Return or Ctrl-C: ^CCaught KeyboardInterrupt

MemoryError

如果程序用完了内存并且有可能恢复(例如, 通过删除某些对象)时, 引发MemoryError.

import itertools

# Try to create a MemoryError by allocating a lot of memory
l = []
for i in range(3):
    try:
        for j in itertools.count(1):
            print i, j
            l.append('*' * (2**30))
    except MemoryError:
        print '(error, discarding existing list)'
        l = []

$ python PyMOTW/exceptions/exceptions_MemoryError.py
0 1
0 2
0 3
python(13482) malloc: *** mmap(size=1073745920) failed (error code=12)
*** error: can't allocate region
*** set a breakpoint in malloc_error_break to debug
(error, discarding existing list)
1 1
1 2
1 3
python(13482) malloc: *** mmap(size=1073745920) failed (error code=12)
*** error: can't allocate region
*** set a breakpoint in malloc_error_break to debug
(error, discarding existing list)
2 1
2 2
2 3
python(13482) malloc: *** mmap(size=1073745920) failed (error code=12)
*** error: can't allocate region
*** set a breakpoint in malloc_error_break to debug
(error, discarding existing list)

NameError

若代码引用当前作用域下不存在的名字, 则引发NameError. 例如, 一个不存在的变量名.

def func():
    print unknown_name

func()

$ python exceptions_NameError.py
Traceback (most recent call last):
  File "/Users/dhellmann/Documents/PyMOTW/in_progress/exceptions/PyMOTW/exceptions/exceptions_NameError.py", line 15, in <module>
    func()
  File "/Users/dhellmann/Documents/PyMOTW/in_progress/exceptions/PyMOTW/exceptions/exceptions_NameError.py", line 13, in func
    print unknown_name
NameError: global name 'unknown_name' is not defined

NotImplementedError

用户自定义基类可以引发NotImplementedError来模拟接口 , 意思是一个方法或行为需要在子类中定义.

class BaseClass(object):
    """Defines the interface"""
    def __init__(self):
        super(BaseClass, self).__init__()
    def do_something(self):
        """The interface, not implemented"""
        raise NotImplementedError(self.__class__.__name__ + '.do_something')

class SubClass(BaseClass):
    """Implementes the interface"""
    def do_something(self):
        """really does something"""
        print self.__class__.__name__ + ' doing something!'


SubClass().do_something()
BaseClass().do_something()

$ python exceptions_NotImplementedError.py
SubClass doing something!
Traceback (most recent call last):
  File "/Users/dhellmann/Documents/PyMOTW/in_progress/exceptions/PyMOTW/exceptions/exceptions_NotImplementedError.py", line 27, in <module>
    BaseClass().do_something()
  File "/Users/dhellmann/Documents/PyMOTW/in_progress/exceptions/PyMOTW/exceptions/exceptions_NotImplementedError.py", line 18, in do_something
    raise NotImplementedError(self.__class__.__name__ + '.do_something')
NotImplementedError: BaseClass.do_something

OSError

OSError是为 os 模块服务的错误类, 当一个系统相关的函数返回错误时引发.

import os

for i in range(10):
    print i, os.ttyname(i)

$ python exceptions_OSError.py
0
Traceback (most recent call last):
  File "/Users/dhellmann/Documents/PyMOTW/in_progress/exceptions/PyMOTW/exceptions/exceptions_OSError.py", line 15, in <module>
    print i, os.ttyname(i)
OSError: [Errno 25] Inappropriate ioctl for device

OverflowError

当一个算术操作超过了变量类型的限制时, 引发OverflowError. 长整型会在其值增加时分配更多的空间, 因此会最终引发MemoryError. 浮点数的异常处理不是标准化的, 所以不会检查浮点数. 普通的整数在需要时会被转化为长整型.

import sys

print 'Regular integer: (maxint=%s)' % sys.maxint
try:
    i = sys.maxint * 3
    print 'No overflow for ', type(i), 'i =', i
except OverflowError, err:
    print 'Overflowed at ', i, err

print
print 'Long integer:'
for i in range(0, 100, 10):
    print '%2d' % i, 2L ** i

print
print 'Floating point values:'
try:
    f = 2.0**i
    for i in range(100):
        print i, f
        f = f ** 2
except OverflowError, err:
    print 'Overflowed after ', f, err

$ python exceptions_OverflowError.py
Regular integer: (maxint=2147483647)
No overflow for  <type 'long'> i = 6442450941

Long integer:
0 1
10 1024
20 1048576
30 1073741824
40 1099511627776
50 1125899906842624
60 1152921504606846976
70 1180591620717411303424
80 1208925819614629174706176
90 1237940039285380274899124224

Floating point values:
0 1.23794003929e+27
1 1.53249554087e+54
2 2.34854258277e+108
3 5.5156522631e+216
Overflowed after  5.5156522631e+216 (34, 'Result too large')

ReferenceError

当使用:mod:weakref 代理访问已经被作为垃圾回收的对象时, 引发ReferenceError.

import gc
import weakref

class ExpensiveObject(object):
    def __init__(self, name):
        self.name = name
    def __del__(self):
        print '(Deleting %s)' % self

obj = ExpensiveObject('obj')
p = weakref.proxy(obj)

print 'BEFORE:', p.name
obj = None
print 'AFTER:', p.name

$ python exceptions_ReferenceError.py
BEFORE: obj
(Deleting <__main__.ExpensiveObject object at 0x844f0>)
AFTER:
Traceback (most recent call last):
  File "/Users/dhellmann/Documents/PyMOTW/in_progress/exceptions/PyMOTW/exceptions/exceptions_ReferenceError.py", line 26, in <module>
    print 'AFTER:', p.name
ReferenceError: weakly-referenced object no longer exists

RuntimeError

RuntimeError在没有其它具体的异常可用时被引发. 解释器本身并不会经常引发这个异常, 但是一些用户代码可能会.

StopIteration

当一个迭代器完成后, 它的 next() 方法引发StopIteration. 这个异常不被认为是错误.

l=[0,1,2]
i=iter(l)

print i
print i.next()
print i.next()
print i.next()
print i.next()

$ python exceptions_StopIteration.py
<listiterator object at 0x808b0>
0
1
2
Traceback (most recent call last):
  File "/Users/dhellmann/Documents/PyMOTW/in_progress/exceptions/PyMOTW/exceptions/exceptions_StopIteration.py", line 19, in <module>
    print i.next()
StopIteration

SyntaxError

分析器发现无法理解的源码时引发SyntaxError. 可能是在引入一个模块, 调用exec, 或 eval() 时发生. 异常的属性确切找到输入的哪一部分导致了异常.

try:
    print eval('five times three')
except SyntaxError, err:
    print 'Syntax error %s (%s-%s): %s' % \
           (err.filename, err.lineno, err.offset, err.text)
    print err

$ python exceptions_SyntaxError.py
Syntax error <string> (1-10): five times three
invalid syntax (<string>, line 1)

SystemError

解释器本身发生错误, 并且有可能继续正常运行时, 引发SystemError. SystemError可能表示解释器本身的bug, 并且需要报告给维护者.

SystemExit

当调用sys.exit()时, 引发SystemExit, 而不是立即退出. 这样允许 try:finally 代码块的完成清理工作, 以及调用者(比如调试器和测试框架)捕获异常, 从而避免退出.

TypeError

连接类型不正确的对象, 或使用类型不正确的对象调用函数时, 引发TypeError.

result = ('tuple',) + 'string'

$ python exceptions_TypeError.py
Traceback (most recent call last):
  File "/Users/dhellmann/Documents/PyMOTW/in_progress/exceptions/PyMOTW/exceptions/exceptions_TypeError.py", line 12, in <module>
    result = ('tuple',) + 'string'
TypeError: can only concatenate tuple (not "str") to tuple

UnboundLocalError

UnboundLocalError是NameError的一种, 针对本地变量名.

def throws_global_name_error():
    print unknown_global_name

def throws_unbound_local():
    local_val = local_val + 1
    print local_val

try:
    throws_global_name_error()
except NameError, err:
    print 'Global name error:', err

try:
    throws_unbound_local()
except UnboundLocalError, err:
    print 'Local name error:', err

全局NameError和UnboundLocal的区别是使用变量名的方式. 因为变量名”local_val”出现在表达式的左边, 所以被解释为一个本地变量名.

$ python exceptions_UnboundLocalError.py
Global name error: global name 'unknown_global_name' is not defined
Local name error: local variable 'local_val' referenced before assignment

UnicodeError

UnicodeError是ValueError的子类, 当出现Unicode问题时引发. UnicodeError有不同的子类: UnicodeEncodeError, UnicodeDecodeError和UnicodeTranslateError.

ValueError

当一个函数得到一个类型正确但是非有效值时, 引发ValueError.

print chr(1024)

$ python exceptions_ValueError.py
Traceback (most recent call last):
  File "/Users/dhellmann/Documents/PyMOTW/in_progress/exceptions/PyMOTW/exceptions/exceptions_ValueError.py", line 12, in <module>
    print chr(1024)
ValueError: chr() arg not in range(256)

ZeroDivisionError

当0作为一个除法操作的分母时, 引发ZeroDivisionError.

print 1/0

$ python exceptions_ZeroDivisionError.py
Traceback (most recent call last):
  File "/Users/dhellmann/Documents/PyMOTW/in_progress/exceptions/PyMOTW/exceptions/exceptions_ZeroDivisionError.py", line 12, in <module>
    print 1/0
ZeroDivisionError: integer division or modulo by zero

Warning列表

还有一些异常用来在:mod:warnings 模块中使用.

Warning: 所有警告的基类.
UserWarning: 所有来自用户代码的警告的基类.
DeprecationWarning: 用在不再被维护的特性上.
PendingDeprecationWarning: 用在很快就会不推荐使用的特性上.
SyntaxWarning: 用在值得商榷的表达式上.
RuntimeWarning: 运行时可能发生问题的事件的警告.
FutureWarning: 对于语言或库的改变会在之后实现时警告.
ImportWarning: 导入一个模块的问题的警告.
UnicodeWarning: 关于unicode文本的问题的警告.

参考

exceptions