《The Python Tutorial》——Errors and Exceptions 阅读笔记

Errors and Exceptions

官方文档：https://docs.python.org/3.5/tutorial/errors.html

python中所有的异常都继承自BaseException类。

1.1 Syntax Errors

1.2 Exceptions

https://docs.python.org/3.5/library/exceptions.html

1.3 Handling Exception

使用try语句：

>>> while True:
...     try:
...         x = int(input("Please enter a number: "))
...         break
...     except ValueError:
...         print("Oops!  That was no valid number.  Try again...")
except也可以使用元组：

... except (RuntimeError, TypeError, NameError):
...     pass
最后一个except可以省略异常名exception name，此时就跟通配符一样的作用（包含所有其他的异常）。
基于此，可以在最后一句except中打印异常提示并可重新raise对应的异常：

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
还可以在try...except语句后使用else语句，适用于try后没发生相应异常又需要继续完成某种操作的情况：

for arg in sys.argv[1:]:
    try:
        f = open(arg, 'r')
    except IOError:
        print('cannot open', arg)
    else:
        print(arg, 'has', len(f.readlines()), 'lines')
        f.close()
异常handler不仅能够捕捉try语句中的异常，也可以捕捉try语句内被调用函数的异常（嵌套的非直接调用的函数产生的异常也行）：

>>> 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

1.4 Raising Exceptions

raise语句可以人为的让某个异常"发生"：

>>> raise NameError('HiThere')
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
NameError: HiThere

raise的参数只有一个，即为一个异常的实例名或异常类名（当然，这个异常也是继承自Exception类的），
其实，当一个异常类被raise的时候，会自动调用它的构造函数“悄悄的”进行实例化：

raise ValueError  # shorthand for 'raise ValueError()'

1.5 User-defined Exception

所有异常类都继承自Exception类。
通常的做法是先建立一个继承自Exception的Eroor,再根据不同的异常建立不同的异常类（继承Eroor）：

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

1.6 Defineing Clean-up Actions

使用finally语句：

>>> try:
...     raise KeyboardInterrupt
... finally:
...     print('Goodbye, world!')
...
Goodbye, world!
KeyboardInterrupt
Traceback (most recent call last):
  File "<stdin>", line 2, in <module>

1.7 Predefined Clean-up Actions

with语句的使用：

with open("myfile.txt") as f:
    for line in f:
        print(line, end="")