异常

>>> 10 * (1/0)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
    10 * (1/0)
          ~^~
ZeroDivisionError: division by zero
>>> 4 + spam*3
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
    4 + spam*3
        ^^^^
NameError: name 'spam' is not defined
>>> '2' + 2
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
    '2' + 2
    ~~~~^~~
TypeError: can only concatenate str (not "int") to str

异常处理（try）

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

异常实例对象：

except Exception as e:
    print(f'caught {type(e)}: e')

类似于 Java 的 catch(Exception e)

异常抛出（raise）

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

• 类似于 Java 的 throw new NameError("HiThere")
• raise (opens new window) 唯一的参数就是要触发的异常。这个参数必须是异常实例或异常类（派生自 BaseException (opens new window) 类，例如 Exception (opens new window) 或其子类）

raise ValueError  # 'raise ValueError()' 的简化

如果只想判断是否触发了异常，但并不打算处理该异常，则可以使用更简单的 raise (opens new window) 语句重新触发异常：

>>> try:
...     raise NameError('HiThere')
... except NameError:
...     print('An exception flew by!')
...     raise
...
An exception flew by!
Traceback (most recent call last):
  File "<stdin>", line 2, in <module>
    raise NameError('HiThere')
NameError: HiThere

异常自定义

直接或间接继承 Exception 类的类，即为异常类，约定异常类命名以 Error 结尾。具体如何定义类参考下章。

finally

与其他语言的 try 语句类似，python 的 try 也有 finally 可选子句，作用也相似：任何情况下都会执行

>>> try:
...     raise KeyboardInterrupt
... finally:
...     print('Goodbye, world!')

复杂示例

def divide(x, y):
    try:
        result = x / y
    except ZeroDivisionError:
        print("division by zero!")
    else: # try代码块正常执行（无异常）时触发
        print("result is", result)
    finally:
        print("executing finally clause")
        
divide(2, 1)


divide(2, 0)


divide("2", "1")

with

自动处理释放资源逻辑

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