代码收藏家 Python 异常处理 详解
Python 异常处理 详解
1、错误和异常
1.1 错误 Error
1.2 异常 Exception
1.3 总结
2、产生异常
1 raise 语句显式的抛出异常
2 Python 解释器自己检测到异常并引发它
3 程序会在异常抛出的地方中断执行，如果不捕获，就会提前结束程序（其实是终止当前线程的执行）
def foo():
print('before')
print(1/0) # 捕获异常
print('after')
foo()
# ZeroDivisionError: division by zero
def foo():
print('before')
raise Exception('My Exception') # raise 主动抛出异常
print('after')
foo()
# Exception: My Exception
3、捕获异常
3.1 语法
try:
待捕获异常的代码块
except [异常类型]:
异常的处理代码块
3.2 示例 1
此例执行到c = 1/0时产生异常并抛出，由于使用了try...except语句块则捕捉到了这个异常
异常生成位置之后语句块将不再执行，转而执行对应的except部分的语句
最后执行try...except语句块之外的语句
def foo():
try:
print('before')
c = 1/0
print('after')
except:
print('error')
print('catch the exception')
foo()
print('====== end ======')
before
error
catch the exception
====== end ======
3.3 示例 2
def foo():
try:
print('before')
print(1/0)
print('after')
except ArithmeticError: # 指定捕获的类型
print('error')
print('catch the exception')
foo()
print('====== end ======')
before
error
catch the exception
====== end ======
4、 异常类
4.1 Exception hierarchy
The class hierarchy for built-in exceptions is:
BaseException
+-- SystemExit
+-- KeyboardInterrupt
+-- GeneratorExit
+-- Exception
+-- StopIteration
+-- StopAsyncIteration
+-- ArithmeticError
| +-- FloatingPointError
| +-- OverflowError
| +-- ZeroDivisionError
+-- AssertionError
+-- AttributeError
+-- BufferError
+-- EOFError
+-- ImportError
| +-- ModuleNotFoundError
+-- LookupError
| +-- IndexError
| +-- KeyError
+-- MemoryError
+-- NameError
| +-- UnboundLocalError
+-- OSError
| +-- BlockingIOError
| +-- ChildProcessError
| +-- ConnectionError
| | +-- BrokenPipeError
| | +-- ConnectionAbortedError
| | +-- ConnectionRefusedError
| | +-- ConnectionResetError
| +-- FileExistsError
| +-- FileNotFoundError
| +-- InterruptedError
| +-- IsADirectoryError
| +-- NotADirectoryError
| +-- PermissionError
| +-- ProcessLookupError
| +-- TimeoutError
+-- ReferenceError
+-- RuntimeError
| +-- NotImplementedError
| +-- RecursionError
+-- SyntaxError
| +-- IndentationError
| +-- TabError
+-- SystemError
+-- TypeError
+-- ValueError
| +-- UnicodeError
| +-- UnicodeDecodeError
| +-- UnicodeEncodeError
| +-- UnicodeTranslateError
+-- Warning
+-- DeprecationWarning
+-- PendingDeprecationWarning
+-- RuntimeWarning
+-- SyntaxWarning
+-- UserWarning
+-- FutureWarning
+-- ImportWarning
+-- UnicodeWarning
+-- BytesWarning
+-- EncodingWarning
+-- ResourceWarning
4.2 BaseException及子类
BaseException：所有内建异常类的基类是 BaseExceptionSystemExit：sys.exit()函数引发的异常，异常不捕获处理，就直接交给Python解释器，解释器退出KeyboardInterrupt：对应的捕获用户中断行为 Ctrl + CException及子类：1、
Exception是所有内建的、非系统退出的异常的基类，自定义异常类应该继承自它2、
SyntaxError：Python将这种错误也归到异常类下面的Exception下的子类，但是这种错误是不可捕获的3、
ArithmeticError：所有算术计算引发的异常，其子类有除零异常等4、
LookupError：使用映射的键或序列的索引无效时引发的异常的基类：IndexError, KeyError5、 自定义异常类：
从Exception继承的类4.3 sys.exit()示例 1
import sys
KeyboardInterrupt
print('Before')
sys.exit(1)
print('SysExit')
print('outer')
D:\PycharmProjects\pythonProject\202108089\venv\Scripts\python.exe D:/PycharmProjects/pythonProject/202108089/test6.py
Before
Process finished with exit code 1
4.4 sys.exit()示例 2
import sys
try:
sys.exit(1)
except SystemExit:
print('SysExit')
print('outer')
D:\PycharmProjects\pythonProject\202108089\venv\Scripts\python.exe D:/PycharmProjects/pythonProject/202108089/test6.py
SysExit
outer
Process finished with exit code 0
4.5 KeyboardInterrupt示例
try:
import time
while True:
time.sleep(1)
print('!!!')
except KeyboardInterrupt:
print('Ctrl + C')
print('=== end ===')
PS D:\PycharmProjects\pythonProject\202108089> python .\test8.py
!!!
!!!
Ctrl + C
=== end ===
PS D:\PycharmProjects\pythonProject\202108089>
4.6 SyntaxError示例
try:
0a = 5
except SyntaxError: # 无法捕获
print('after')
SyntaxError: invalid syntax
print('=' * 10)
def 0a():
print('begin')
try:
0a()
except Exception as e:
print(e, type(e))
D:\PycharmProjects\pythonProject\202108089\venv\Scripts\python.exe D:/PycharmProjects/pythonProject/202108089/test8.py
File "D:/PycharmProjects/pythonProject/202108089/test8.py", line 1
def 0a():
^
SyntaxError: invalid syntax
Process finished with exit code 1
4.7 自定义类示例
class MyException(Exception):
pass
try:
raise MyException
except MyException: # 捕捉自定义异常
print('Catch the exception')
Out:
Catch the exception
5、多种捕获
多种捕获：except可以指定捕获的类型，捕获多种异常。
5.1 捕获规则
5.2 示例 1
import sys
class MyException(Exception):
pass
try:
print('before')
a = 1/0
# raise MyException()
# open('t')
# sys.exit(1)
except MyException:
print('catch my exception')
except ArithmeticError: # 可以捕获除零异常
print('ari')
except ZeroDivisionError: # 可以捕获除零异常
print('zero')
except MyException:
print('catch my exception')
except BaseException: # 可以捕获除零异常
print('Base')
except Exception: # 可以捕获除零异常
print('exception')
except:
print('sysexit')
5.3 示例 2
class A:pass
try:
raise A()
except:
print('catch the exception')
Out：
catch the exception
6、其它子句介绍
as子句：被抛出的异常，应该时异常类的实例，可以使用as子句获得这个对象raise语句：raise后要求应该是BaseException类的子类或实例，如果是类，将被无参实例化；raise后什么都没有，表示抛出最近一个被激活的异常，如果没有被激活的异常，则抛出类型异常；raise是用raise语句来引发一个异常。异常/错误对象必须有一个名字，且它们应是Error或Exception类的子类Python用异常对象(exception object)表示异常情况，遇到错误后，会引发异常Traceback,一种错误信息)终止执行finally子句：最终，即最后一定要执行的，try...finally语句块中，不管是否发生了异常，都要执行finally的部分else子句：没有任何异常发生，则执行6.1 示例 1
class MyException(Exception):
def __init__(self, code, message):
self.code = code
self.message = message
try:
raise MyException()
raise 1/0
# raise
except MyException as e:
print('catch my exception')
except Exception as e:
print(e)
print('=== end ===')
Out:
__init__() missing 2 required positional arguments: 'code' and 'message'
=== end ===
6.2 示例 2
class MyException(Exception):
def __init__(self, code, message):
self.code = code
self.message = message
try:
# raise MyException()
raise 1/0
# raise
except MyException as e:
print('catch my exception')
except Exception as e:
print(e)
print('=== end ===')
Out：
division by zero
=== end ===
6.3 示例 3
f = None
try:
f = open('111.txt')
except Exception as e:
print(e.__class__, e.errno, e.strerror)
finally:
print('clear working')
try:
f.close()
except Exception as e:
print(e)
Out：
<class 'FileNotFoundError'> 2 No such file or directory
clear working
'NoneType' object has no attribute 'close'
6.4 示例 4
# 函数的返回值取决于最后一个执行的return语句，而finally则是try...finally中最后执行的语句块
def foo():
# return 1
try:
return 3
finally:
return 5
print('Finally')
print('===')
print(foo())
Out：
5
6.5 示例 5
try:
ret = 1/0
# ret = 1 * 0
except ArithmeticError as e:
print(e)
else:
print('OK')
finally:
print('fin')
division by zero
fin
6.6 示例 6
try:
# ret = 1/0
ret = 1 * 0
except ArithmeticError as e:
print(e)
else:
print('OK')
finally:
print('fin')
OK
fin
7、异常的传递
7.1 示例 1
foo2调用了foo1，foo1产生的异常，传递到了foo2中
异常总是向外层抛出，如果外层没有处理这个异常，就会继续向外抛出
如果内层捕获并处理了异常，外部就不能捕获到了
如果到了最外层还是没有被处理，就会中断异常所在的线程的执行
注意整个程序结束的状态返回值
def foo1():
return 1/0
def foo2():
print('foo2 start')
foo1()
print('foo2 stop')
foo2()
D:\PycharmProjects\pythonProject\202108089\venv\Scripts\python.exe D:/PycharmProjects/pythonProject/202108089/test6.py
Traceback (most recent call last):
... ...
return 1/0
ZeroDivisionError: division by zero
foo2 start
Process finished with exit code 1
7.2 示例 2
# 线程中测试异常
import threading
import time
def foo1():
return 1/0
def foo2():
time.sleep(3)
print('foo2 start')
foo1()
print('foo2 stop')
t = threading.Thread(target=foo2)
t.start()
while True:
time.sleep(1)
print('Everything is OK.')
print(threading.enumerate())
8、try语句
8.1 try 语法
try:
<语句> # 运行别的代码
except <异常类>:
<语句> # 捕获某种类型的异常
except <异常类> as <变量名>:
<语句> # 捕获某种类型的异常并获得对象
else:
<语句> # 如果没有异常发生
finally:
<语句> # 退出try时总会执行
8.2 try 工作原理
8.3 示例 1
# try嵌套
# 内部捕获不到异常，会向外层传递异常
# 但是如果函数内层有finally且其中有return break语句，则异常就不会继续向外抛出
try:
try:
ret = 1/0
except KeyError as e:
# except ZeroDivisionError as e:
print(e)
finally:
print('inner fin')
except:
print('outer catch')
finally:
print('outer fin')
inner fin
outer catch
outer fin
8.4 示例 2
# try嵌套
# 内部捕获不到异常，会向外层传递异常
# 但是如果函数内层有finally且其中有return break语句，则异常就不会继续向外抛出
def foo():
try:
ret = 1/0
except KeyError as e:
print(e)
finally:
print('inner fin')
return
try:
foo()
except:
print('outer catch')
finally:
print('outer fin')
inner fin
outer fin
9、异常的捕获时机
1 立即捕获：需要立即返回一个明确的结果
2 边界捕获：封装产生了边界
def parse_int(s):
try:
return int(s)
except:
return 0
print(parse_int('s'))
Out:
0
10、自定义异常类注意事项
10.1 异常类写的有问题
# 自己写的异常类没写对，抛出异常，被exception捕获
class MyException(Exception):
def __init__(self, code, msg):
self.code = code
self.msg = msg
try:
raise MyException # 抛出的异常都是实例
except NotImplementedError as e:
print('not imp', e, type(e))
except MyException as e:
print('my exception', e, type(e))
except Exception as e:
print('all exception', e, type(e))
print('end')
all exception __init__() missing 2 required positional arguments: 'code' and 'msg' <class 'TypeError'>
end
10.2 异常类传入参数
class MyException(Exception):
def __init__(self, code, msg):
self.code = code
self.msg = msg
try:
raise MyException(999, '123')
except NotImplementedError as e:
print('not imp', e, type(e))
except MyException as e:
print('my exception', e, type(e))
except Exception as e:
print('all exception', e, type(e))
print('end')
my exception (999, '123') <class '__main__.MyException'>
end
10.3 异常类传入默认参数
class MyException(Exception):
def __init__(self, code="200", msg="Not OK"):
self.code = code
self.msg = msg
try:
raise MyException # 等效于raise MyException()
except NotImplementedError as e:
print('not imp', e, type(e))
except MyException as e:
print('my exception', e, type(e), e.code)
except Exception as e:
print('all exception', e, type(e))
print('end')
my exception <class '__main__.MyException'> 200
end
来源：Lee木木
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