1.认识异常

程序执行时会抛出异常

open('abc','r')
# 结果
Traceback (most recent call last):
  File "F:/Pycharm/pyselenium/ExceptionObj.py", line 1, in <module>
    open('abc','r')
FileNotFoundError: [Errno 2] No such file or directory: 'abc'

如果知道报错类型为FileNotFoundError，我们可捕捉异常

try:
    open('abc', 'r')
except FileNotFoundError:
    print('异常了')
# 结果    
异常了

如果程序报错不是FileNotFoundError类型，仍会抛出异常

try:
    print(a)
except FileNotFoundError:
    print('异常了')
# 结果 
Traceback (most recent call last):
  File "F:/Pycharm/pyselenium/ExceptionObj.py", line 9, in <module>
    print(a)
NameError: name 'a' is not defined

若不知异常类型，可捕捉基类BaseException

try:
    open('abc', 'r')
    print(a)
except BaseException as msg:
    print(msg)
 # 结果 
[Errno 2] No such file or directory: 'abc'

2.raise抛出异常

可使用raise自定义抛出的异常

def say_hello(name=None):
    if name is None:
        raise NameError('"name" cannot be empty')
    else:
        print('hello %s'%name)
say_hello()
# 结果
Traceback (most recent call last):
  File "F:/Pycharm/pyselenium/ExceptionObj.py", line 31, in <module>
    say_hello()
  File "F:/Pycharm/pyselenium/ExceptionObj.py", line 26, in say_hello
    raise NameError('"name" cannot be empty')
NameError: "name" cannot be empty

捕捉NameError

def say_hello(name=None):
    if name is None:
        raise NameError('"name" cannot be empty')
    else:
        print('hello %s'%name)
try:
    say_hello()
except NameError as msg:
    print(msg)
# 结果
"name" cannot be empty

注：raise只能使用python提供的异常类。如果想用raise使用自定义异常类，则自定义异常类需要继承Exception类。