-----学习《Python基础教程第3版》读书笔记-----
2019实战第二期-异常读书打卡
异常是什么
使用异常对象表示异常状态，并在遇到错误时引发异常。异常对象未被处理，程序将终止并显示一条错误信息（traceback）
让事情沿你指定的轨道出错
出现问题时，将自动引发异常。
raise语句
raise Exception
raise Exception('hyperdrive overload')
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虽然内置异常涉及的范围很广，能够满足很多需求，但有时你可能想自己创建异常类。
那么如何创建异常类呢？就像创建其他类一样，但务必直接或间接地继承 Exception （这意味着从任何内置异常类派生都可以）。因此，自定义异常类的代码类似于下面这样：
class SomeCustomException(Exception): pass
捕获异常
异常比较有趣的地方是可对其进行处理，通常称之为捕获异常。为此，可使用try / except 语句。
try:
x = int(input('Enter the first number: '))
y = int(input('Enter the second number: '))
print(x / y)
except ZeroDivisionError:
print("The second number can't be zero!")
异常和函数
不提供参数
class MuffledCalculator:
muffled = False
def calc(self, expr):
try:
return eval(expr)
except ZeroDivisionError:
if self.muffled:
print('Division by zero is illegal')
else:
raise
多个except
try:
x = int(input('Enter the first number: '))
y = int(input('Enter the second number: '))
print(x / y)
except ZeroDivisionError:
print("The second number can't be zero!")
except TypeError:
print("That wasn't a number, was it?")
捕获多个异常
try:
x = int(input('Enter the first number: '))
y = int(input('Enter the second number: '))
print(x / y)
except (ZeroDivisionError, TypeError, NameError):
print('Your numbers were bogus ...')
捕获对象
try:
x = int(input('Enter the first number: '))
y = int(input('Enter the second number: '))
print(x / y)
except (ZeroDivisionError, TypeError) as e:
print(e)
捕获所有异常
try:
x = int(input('Enter the first number: '))
y = int(input('Enter the second number: '))
print(x / y)
except:
print('Something wrong happened ...')
万事大吉
while True:
try:
x = int(input('Enter the first number: '))
y = int(input('Enter the second number: '))
value = x / y
print('x / y is', value)
except Exception as e:
print('Invalid input:', e)
print('Please try again')
else:
break
加finally
try:
1 / 0
except NameError:
print("Unknown variable")
else:
print("That went well!")
finally:
print("Cleaning up.")
异常和函数有着天然的联系。如果不处理函数中引发的异常，它将向上传播到调用函数的地方。如果在那里也未得到处理，异常将继续传播，直至到达主程序（全局作用域）。如果主程序中也没有异常处理程序，程序将终止并显示栈跟踪消息。
>>> def faulty():
... raise Exception('Something is wrong')
...
>>> def ignore_exception():
... faulty()
...
>>> def handle_exception():
... try:
... faulty()
... except:
... print('Exception handled')
...
>>> ignore_exception()
Traceback (most recent call last):
File '', line 1, in ?
File '', line 2, in ignore_exception
File '', line 2, in faulty
Exception: Something is wrong
>>> handle_exception()
Exception handled
异常之禅
def describe_person(person):
print('Description of', person['name'])
print('Age:', person['age'])
try:
print('Occupation:', person['occupation'])
except KeyError: pass
不那么异常的情况
只是发出警告，指出情况偏离了正轨
>>> from warnings import warn
>>> warn("I've got a bad feeling about this.")
__main__:1: UserWarning: I've got a bad feeling about this.
>>> filterwarnings("error")
>>> warn("This function is really old...", DeprecationWarning)
Traceback (most recent call last):
File "", line 1, in
DeprecationWarning: This function is really old...
>>> filterwarnings("ignore", category=DeprecationWarning)
>>> warn("Another deprecation warning.", DeprecationWarning)
>>> warn("Something else.")
Traceback (most recent call last):
File "", line 1, in
UserWarning: Something else.