异常

错误

---

• 例如

    >>> Print('fdsa');
    Traceback (most recent call last):
      File "<pyshell#0>", line 1, in <module>
    Print('fdsa');
    NameError: name 'Print' is not defined
  

  • NameError被抛出，同时Python还会打印出检测到的错误发生的位置。着就是一个错误处理器（Error Handler）为这个错误所做的事情

---

异常

---

• 输入以下代码，按ctrl+d

    >>> s=input("input ----->");
    input ----->
    Traceback (most recent call last):
      File "<pyshell#1>", line 1, in <module>
    s=input("input ----->");
    EOFError: EOF when reading a line
    >>> 
  

---

处理异常

---

• 我们通过使用try...except来处理异常情况，一般来说我们会把通常的语句放在try代码块中，将我们的错误处理器代码放置在except代码块

• 案例：

    try: 
        text = input("Enter something-->");
    except EOFError:
        print('Why did you do an EOFError on me');
    except KeyboardInterrupt:
        print('You cancelled the operation.');
    else:
        print('You Entered {}'.format(text));
  

---

抛出异常

---

• 可以通过raise语句来引发一次异常，具体方法是：

  • 提供错误名或异常名
  • 抛出异常的对象

• 你能够引发的错误或异常必须是直接或间接从属于Exception（异常）类的派生类

• 案例：

    class ShortInputException(Exception):
        def __init__(self,length,atLeast):
            Exception.__init__(self);
            self.length=length;
            self.atLeast = atLeast;
    
    try:
        text = input("please Enter sth--->");
        if len(text) <3:
            raise ShortInputException(len(text),3);
    except EOFError:
        print('Why did you do an EOFError on me');
    except ShortInputException as ex:
        print('ShortInputException:The input was '+
        '{0}long,expected at least{1}'.format(ex.length,ex.atLeast));
    else:
        print('No exception was raised.');
  

  • warning：我们在except字句中，提及了错误类，将该类存储as(为)相应的错误名或异常名ex；

---

Try...Finally

• 无论是否发生异常，都可以用finally

• 案例：

    import sys;
    import time;
    f = None;
    try:
        f = open("poem.txt");
        
        while True:
            line = f.readline();
            if len(line)==0:
                break;
            print(line,end='');
            sys.stdout.flush();
            print('Press ctrl+c now');
            #为了确保它能运行一段时间
            time.sleep(2);
    except IOError:
        print('Could not find file poem.txt');
    except KeyboardInterrupt:
        print('!! You cancelled the reading from the file.');
    finally:
        if f:
            f.close();
        print("(Cleaning up:Closed the file)");
  

---

with语句

---

• 在try块中获取资源，然后再finally块中释放资源是一种常见的模式。因此还有一个with语句使得这一过程可以以一种干净的姿态得以完成

• 案例：

    with open('poem.txt') as f:
        for line in f:
            print(line,end='');