错误 vs 异常

语法错误，无法被识别与执行

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name = 'x'
if name is not None  # SyntaxError: invalid syntax
    print(name)

异常 - 语法正确，可以被执行，但在执行过程中遇到错误，抛出异常，并终止程序

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# 10 / 0  # ZeroDivisionError: division by zero
# order * 2  # NameError: name 'order' is not defined
# 1 + [1, 2]  # TypeError: unsupported operand type(s) for +: 'int' and 'list'

处理异常

try-except
except block 只接受与它相匹配的异常类型并执行

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try:
    s = input('please enter two numbers separated by comma: ')
    num1 = int(s.split(',')[0].strip())
    num2 = int(s.split(',')[1].strip())
except ValueError as err:
    print('Value Error: {}'.format(err))

在 except block 中加入多种异常的类型

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try:
    s = input('please enter two numbers separated by comma: ')
    num1 = int(s.split(',')[0].strip())
    num2 = int(s.split(',')[1].strip())
except (ValueError, IndexError) as err:
    print('Error: {}'.format(err))
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try:
    s = input('please enter two numbers separated by comma: ')
    num1 = int(s.split(',')[0].strip())
    num2 = int(s.split(',')[1].strip())
except ValueError as err:
    print('Value Error: {}'.format(err))
except IndexError as err:
    print('Index Error: {}'.format(err))

很难覆盖所有的异常类型，在最后的一个 except block，声明其处理的异常类型为 Exception
Exception - 是其它所有非系统异常的基类

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try:
    s = input('please enter two numbers separated by comma: ')
    num1 = int(s.split(',')[0].strip())
    num2 = int(s.split(',')[1].strip())
except ValueError as err:
    print('Value Error: {}'.format(err))
except IndexError as err:
    print('Index Error: {}'.format(err))
except Exception as err:
    print('Other Error: {}'.format(err))

在 except 后省略异常类型，表示与任意异常（包括系统异常和非系统异常等）匹配 - 不推荐（过于宽泛）

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try:
    s = input('please enter two numbers separated by comma: ')
    num1 = int(s.split(',')[0].strip())
    num2 = int(s.split(',')[1].strip())
except ValueError as err:
    print('Value Error: {}'.format(err))
except IndexError as err:
    print('Index Error: {}'.format(err))
except:  # too broad exception
    print('Other Error')

多个 except 声明的异常类型与实际相匹配，只有最前面的 except block 会被执行

finally - 无论发生什么情况，finally block 中的语句都会被执行，即便在 try 和 except 中使用了 return 语句

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import sys

f = None
try:
    f = open('params.json', 'r')
    print(f.read())
except OSError as err:
    print(f"Error: {err}")
except:
    print("Unexpected error: ", sys.exc_info()[0])
finally:  # always executed
    f.close()

with open 会在最后自动关闭文件，更加简洁

自定义异常

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class MyInputError(Exception):
    """
    Exception raised when there are errors in input
    """

    def __init__(self, value):  # Constructor
        self.value = value

    def __str__(self):  # __str__ is to print() the value
        return "{} is invalid input".format(repr(self.value))


try:
    raise MyInputError(1)
except MyInputError as err:
    print('error: {}'.format(err))  # error: 1 is invalid input